Category Archives: Cars

Sanity Check: 0-60 Times

Cars, Math, Tech
by Mike Clements

With electric cars, the classic performance metric of 0-60 time has gotten much faster, approaching the theoretical limits of available traction. Yet some cars show 0-60 times even faster than this, which seems impossible. Accelerating faster than available traction requires thrust that doesn’t depend on traction, like jet or rocket engines.

I suspect that the 0-60 times being quoted for some of these cars are not real, but just theoretical projections based on power to weight ratio. Here’s a way to sanity check them.

Braking is already traction limited. So when acceleration is also traction limited, the car should accelerate 0-60 in the same distance and time it takes to brake from 60-0. These might be slightly different, due to the car’s uneven front-rear weight distribution and different sized tires front and rear. But it’s still a good rough guide and sanity check.

Braking 60-0 is usually given as distance rather than time. But assuming constant acceleration (not exactly true but a decent approximation) it’s easy to convert. Remember our basic formulas:

v = a*t
d = 1/2 * a * t²

The best street legal tires have a maximum traction of about 1.1 G. You can get up to about 1.3 G with R compound racing tires, but most are not street legal and the ones that are, don’t last more than 1,000 miles.

Here’s how we compute this for 1.1 G with English units:

60 mph = 88 fps
1 G = 32 fps/s
v = a*t --> 88 = 32 * 1.1 * t --> t = 2.5 secs
d = 1/2 * a * t² --> d = 1/2 * 32 * 1.1 * 2.5² --> d = 110 feet

Braking from 60 to 0 at 1.1 G takes 2.5 seconds and 110 feet. If you look at the highest performance cars, this is about equal to their tested braking performance. So, that same car cannot accelerate 0-60 any faster than 2.5 seconds because no matter how much power it has, that is the limit of available traction.

Some cars claim to do 0-60 in 2 seconds flat. This is 1.375 G of acceleration and takes 88 feet of distance. It might be possible with R compound racing tires, but not with street tires. Any car that actually does this in the real world, must be able to brake 60-0 in 88 feet. If its 60-0 braking distance is longer than 88 feet, then it takes longer than 2 seconds to go 0-60.

Note: there’s rule of thumb for cars whose 0-60 time is power limited (not traction limited). Divide weight in lbs. by power in HP, then take half that number. For example, a 3,000 lb. car with 300 HP has a ratio of 10, and will do 0-60 in about 5 seconds. This of course is only a rough approximation, but it’s usually close; it works because acceleration depends on power to weight ratio.

Airplane Engines

Cars, Flying
by Mike Clements

Introduction

Most small airplanes are powered by piston engines. Car engines are sometimes used for kit or experimental airplanes. It seems like a logical thing to do since most car engines are reliable and less expensive than aviation engines. Yet while some car engines have performed well in aviation, they are the exception that proves the general rule to the contrary.

Here I’ll discuss some of the important ways in which airplane engines are different from car engines.

Rotational Speed

A typical prop for a small airplane has about a 76″ diameter (more or less). That’s a circumference of about 20′, which is how far the tips move in each revolution. The speed of sound is about 1100′ per second (sea level standard conditions), so that’s 55 revolutions per second, which means at 3,300 RPM the tips of the prop are moving at the speed of sound.

When the tips move faster than about 85% of the speed of sound, they start to lose efficiency. The airflow changes and they start making more noise & turbulence, and less thrust. And it creates unnecessary stress on the prop. So we need to limit the prop to about 2800 RPM. But we need to limit a bit more than that, because the airplane flies at high altitude where air is colder and sound travels slower. So typical small airplanes like this have a prop redline of 2700 RPM, plus or minus (lower for bigger props).

Power moves the car, or the airplane, or anything else that moves. In an engine, power is torque * rotational speed. Cars have a transmission enabling the engine and wheels to spin at different speeds, so they can rev up the engine to make good power, then gear it down to the wheels to maximize performance. To avoid the complexity, weight and reliability issues of a geared engine, in most airplanes the prop is bolted directly to the engine crankshaft. Thus, limiting the engine to 2700 RPM limits the power it can produce.

Consequently, most aviation engines don’t make much power for their displacement (for example the popular Lycoming O-360 which makes 180 HP from 360 ci), but they are designed to produce their rated power continuously while being lightweight and reliable.

Duty Cycle

Cars spend a lot of time in traffic constantly changing speeds. And cars rarely use their full rated power, but spend most of their time producing only a small part of it. For example, it takes about 30 HP to move a typical car down the freeway at 60-70 mph. For a car with a 150 HP engine, that’s only 20% of its rated power. A car engine is optimized for this duty cycle: to be efficient and reliable while producing a low % of its rated power.

Airplanes spend most of their time in cruise flight moving at a constant speed. The engine is running at a constant speed at or near wide open throttle, producing a high percentage of its rated power. For example, cruising at 70% power is typical. Airplane engines are designed to operate efficiently and reliably while generating their full rated power.

Lightweight

The value of light weight in an airplane engine is obvious. Consider the Lycoming O-360 mentioned above. It is a large displacement 360 ci engine that weighs only 260 lbs. A typical car engine of similar displacement weighs more than twice as much.

Of course, that 360 ci car engine would produce more than 180 HP. So for a fair comparison consider a modern car engine making 180 HP, like the Mazda Skyactiv 2.5. It produces 180 HP and weighs 260 lbs. In power and weight it’s similar to the Lycoming. But that Mazda is not designed to produce its rated power continuously. If you ran it constantly at wide open throttle at 6000 RPM it would not last very long.

It’s not easy to produce a lightweight engine that can operate reliably while continuously producing its full rated power. From a power / weight / reliability perspective, the Lycoming O-360 is comparable to modern car engines in 2022. This is especially notable when one considers that the Lycoming is a design from the 1950s.

Efficiency

Modern car engines are fully computer controlled. The driver applies a certain amount of throttle, and the engine computer determines the valve timing, spark timing, air/fuel ratio, etc. and constantly changes or adapts these settings to the conditions.

Airplane engines are manual. The pilot sets the throttle, RPM, and mixture manually. How can a human compete with a computer? Pretty well, it turns out, because the airplane spends most of its time in cruise flight, running at a constant power level, RPM, and altitude. This gives the pilot time to carefully optimize these settings and leave them there for hours.

One way to measure efficiency is miles per gallon. That Mazda gets about 40 miles per gallon on the freeway. A Cessna 172 in cruise gets about 18 miles per gallon. The Mazda wins, right? Well, it’s not really a fair comparison because the Cessna is going twice as fast. If you drive that Mazda twice as fast (say 130 miles per hour), it’s going to get about 1/4 the fuel economy, which is 10 miles per gallon (or less). So at the same speed, the airplane is almost twice as efficient. Indeed, other airplanes like Mooneys are far more efficient than the Cessna.

Yet this method of measuring efficiency is more about air resistance or drag, than the engine. Airplanes are just inherently more efficient than cars. What if we ignore that and focus on the engine itself?

Another way to measure efficiency is BSFC: brake-specific fuel consumption. That is, how much fuel does the engine consume to do a certain amount of work? One way to measure this is horsepower per gallons per hour.

Let’s estimate this for the Mazda. Suppose it’s getting 40 miles per gallon at 65 miles per hour. Each hour it burns 65/40 = 1.625 gallons of gas. Traveling that fast takes about 30 horsepower, so it produces 30 / 1.625 = 18.46 HP per gallon per hour.

Now consider the Cessna 172. It’s getting 18 miles per gallon at 130 miles per hour. Each hour it burns 130/18 = 7.2 gallons of gas. But how much horsepower is it generating? That is about 65% power, which is .65 * 180 = 117 horsepower. It produces 117 / 7.2 = 16.25 HP per gallon per hour.

So here the Mazda engine is about 13% more efficient (18.46 versus 16.25). However, keep in mind that this is when producing only 30 / 180 = 17% of its rated power. The Lycoming was producing 65% of its rated power. When you open the throttle to make the Mazda produce 65% of its rated power, its efficiency drops significantly, well below the Lycoming.

Note that each engine, car or airplane, is more efficient than the other when operating within its typical duty cycle.

Reliability and Durability

If an aircraft engine fails in flight, the airplane stays in the air but not for long; it becomes a glider that is going to land somewhere very nearby, very soon (within minutes), and most likely off-airport. It is an emergency situation that can lead to injury or death. If a car engine fails, you coast down and simply pull over to the side of the road. It’s an inconvenience, not an emergency.

Airplane engines are designed for reliability. Their spark plugs are powered by magnetos, so (unlike a car) the engine keeps running even if the electrical system fails. Each piston has 2 spark plugs, so if one fails, the piston still produces power. They have 2 separate magnetos and half the plugs are fired by one magneto, half by the other, so if one magneto fails, the engine keeps running. They are air cooled, so there is no water pump that can fail, no radiator that can leak. Also, they spend most of their time in cruise operating around 2500 RPM, so they have static spark timing optimized for that speed – no need for timing advance means simplicity and reliability.

Plenty of historical examples demonstrate the problems using car engines in airplanes. In the 1980s, Mooney made a plane that was optionally powered by a Porsche engine. It had so many problems, the changes needed to make that engine reliable in aviation were so extensive, Porsche gave up and discontinued it. Thielert had a similar situation building Mercedes diesel engines for aviation use. You can google the details on these and other examples.

Yet how do we reconcile this history with the fact that aviation engines use technology that is more than half a century old? A pilot’s pet nickname for Lycoming is “Lycosaurus”!

Consider how any engine becomes reliable: start with a good design, then tweak a little it every year to address any problems discovered in the field. Cars follow this pattern. They come out with a new engine, the first year has some issues, each year it gets a little better, then 5-10 years down the road, just when the engine is reaching its peak, they scrap it and start all over with a new design incorporating new technology. Imagine how reliable car engines would be if they never scrapped it, stuck with the design and continued that incremental improvement for 50 years. The engine would be “low tech” for sure. And may not be as efficient. But reliable? You betcha – they’ve seen just about every failure there is and incorporated changes to address it.

This is what a typical Lycoming or Continental certified aircraft engine is: the result of more than 50 years of incremental improvement on a design that was pretty good to begin with. It’s ancient technology, yet it’s highly optimized and adapted in an incremental, evolutionary way.

Production

Last year, Mazda built more than half a million engines. Lycoming produced about 4,000 engines. Yet this difference of more than 100:1 understates the difference, because there are many car manufacturers while there are only two manufacturers of certified aircraft engines: Lycoming and Continental. For each aircraft engine built, more than 1,000 car engines are built.

To produce reliable engines at such low volumes, aircraft engine manufacturers use completely different production methods. Each engine and all the parts in it are individually hand-built, inspected, and tested before leaving the factory. Visit a modern car engine factory and it looks like a scene from a sci-fi movie where robots have taken over the world. Visit an aviation engine factory and it looks like you’ve gone back in time to a boutique hot rod custom engine building company.

Conclusion

Cars and airplanes are completely different applications with different requirements. It should be no surprise that engines optimized for one are not well suited to the other. High technology is not and end, it is a means to an end. The end or goal is meeting the requirements for the application. Pilots building their own kit / experimental airplanes can use any engine they want. Yet most of them still prefer certified aviation engines from Lycoming or Continental, despite the high cost and low technology compared to car engines. This is not irrational, but backed by some of the reasons discussed above.

All that said, much of the reason aviation engines are so low tech and expensive, is certification. The cost to certify an aircraft engine is so high, and production volume is so low, they can never break even on a new engine design. Over the years, this forced them to differentiate and improve their products with incremental tweaks to existing designs. One can view this as an unintended consequence of over-regulation: certification rules that were intended to promote safety, led to technological stagnation. Or, one can view it as a beneficial outcome that optimizes for reliability in their intended application, which is crucially important with aircraft engines.

Mazda 3 Racing Beat Exhaust

Cars
by Mike Clements

Last year I upgraded my Mazda 3’s suspension, making it much more fun to drive. The only springs I could find that were made for performance, not for looks or lowering, were from Racing Beat. I used Racing Beat parts back in the 1990s autocrossing my 3rd gen RX-7, so I knew they were top quality.

I also wanted a tuned exhaust. Sure there are plenty of aftermarket exhausts for the Mazda 3, but most are just loud; they are not tuned. I didn’t expect big gains because most cars are well tuned by the factory – but at least some gains would be nice. The Racing Beat exhaust meets both requirements: it’s barely louder than stock, and is tuned providing marginal gains. They dyno tested it and published the results. It’s not much of a gain, but still more than I expected.

1665788489806.png

This is the same exhaust that Mazda provided as an OEM part on the limited “club” version of the car. Power gains through tuned exhaust are achieved through increasing volumetric efficiency, so one can expect slightly better fuel economy too. But it was unavailable – out of stock. I signed up to get notified and waited…

Finally, in Oct 2022 it became available and my order went through. It took just a few days for it to arrive here in Seattle. It’s a giant size box that cost over $100 to ship via UPS. Fortunately, Racing beat doesn’t charge tax outside CA state. So the total price was $696.

Installation

The exhaust comes with new nuts, bolts & washers, and a new seal, to attach to the car’s exhaust pipe. Quality is top notch, better than OEM. Installation took less than an hour, including the time to jack up the car and clean up afterward. I sprayed liquid wrench on the original exhaust pipe nuts, but they were not frozen in place. Summary of installation:

  • Jack up the rear of the car safely with jack stands.
  • Remove the 2 bolts securing the muffler to the tailpipe.
  • Spray the muffler’s 4 rubber hanger joints with WD-40 to lube them.
  • Jack up the old muffler in place so it doesn’t fall when you unfasten it.
  • Unfasten the 4 rubber hanger joints and remove the old muffler.
  • Hang the new muffler on the 4 rubber hanger joints.
  • Secure the new muffler to the tailpipe using the new sealing washer, bolts, washers & nuts.
  • Ensure the muffler is properly centered and secured.

Fit and Finish

The new muffler is better than OEM quality. It weighs about the same. The tips are larger diameter and fill the circular bumper curves with about 1/2″ of clearance, just enough that they won’t touch as it vibrates over bumps. The fit is perfect. When the muffler’s circular metal hanger studs are properly inserted into the rubber hangers, it’s perfectly positioned and centered without any finagling or tweaking. The muffler’s connection to the car’s existing tailpipe is perfectly positioned and angled. Its exterior dimensions are similar to the original so it doesn’t conflict with any of the hardware under the car.

Sound

I’ve installed aftermarket exhaust systems on several bikes & cars in the past. This is the quietest that I have seen or heard. It’s almost indistinguishable from stock when putt-putting around town. When you apply full throttle, it’s just a touch louder than stock but only slightly. Many other cars are louder than this with their factory exhaust.

Yet what is a bit different is the tone quality or timbre. This exhaust suppresses the higher frequencies, producing a lower pitch with a touch of rumble. I say just a touch because you are never going to get much rumble with a 2 liter 4 cylinder engine.

What is pleasantly missing from the sound is drone. There is none.

That said, the stock exhaust doesn’t sound bad. It is clean with no rattle and, unlike most other economy cars, sounds like the engine wants to be revved. This Racing Beat exhaust adds some depth to the sound without being loud or obnoxious. It’s a subtle tuned exhaust for adults.

Performance

Shown above, the performance gains are marginal. The gains are smooth and consistent through the entire RPM range from 1500 RPM to redline. The shape of the curve is not changed, with peak torque between 4000 and 5000 and peak power at 6000. The peak increase in torque is about +8 ft.lbs. at 3000 RPM, or about +6%. The peak increase in power is about +5 HP from 5500 to 6500, or about +3%. That’s small enough, any difference you think you feel is placebo. But with small engines like this, I’ll take any advantage I can get.

Conclusion

With a subtle sound that enables you to get on the throttle without blushing, the Racing Beat exhaust is suitable for daily driving. The marginal gains in torque & power, and the appearance and sound add a bit of fun. With quality better than OEM it’s a lifetime part. No doubt this is the best aftermarket exhaust for the Mazda 3.

However, the pragmatic view is that even a well tuned aftermarket exhaust doesn’t make much difference in performance. If you’re autocrossing or racing this isn’t going to improve times by any appreciable amount. And it’s expensive. When it comes to adding fun to your car, or improving your autocross or track times, the biggest bang for your buck is suspension upgrades. Do that first. After that, the Racing Beat exhaust is for when you still want that extra smidge of fun and are willing to pay top dollar for it. Or, if for some reason you need to replace your OEM exhaust (maybe it started to rust), you might as well upgrade to this one.

Postscript

What to do with the old/OEM exhaust? You could keep it, but you’ll probably never need it again so it’s just another large thing to fill up your garage. You could throw it away as trash, but it’s big & bulky enough that’s going to cost you. And, what a waste that is. I thought of two better options:

  • Give it someone who needs it. The Mazda 3 is a popular car, somebody, somewhere, needs an OEM exhaust in serviceable condition.
  • Take it to a salvage or scrap yard. They’ll pay you for it. And it will either be recycled, or put back into service on another car.

I called around and none of auto salvage yards in my area wanted a muffler – they only accept entire cars. So I took mine to Schnitzer scrap metal recycling in Woodinville. They pay about $3.50 per 100 lbs. so the muffler is only worth about a buck. But that beats paying $30 or more just to dump it in the trash.

Mazda 3 Suspension

Cars
by Mike Clements

Summary

The Mazda 3 is a popular car and here I’ll describe a suspension setup that really transforms and improves its handling.

Background

I like to tweak things, but I also believe in “if it ain’t broke, don’t fix it”. Last year I installed a stiffer rear swaybar in my 2014 Mazda 3 (3rd generation) to reduce understeer and improve handling. It works nicely. Later, when replacing the tires I discovered the left rear wheel well had a film of greasy oil. This often indicates the shock has failed and is spewing its internal oil. That was all the excuse I needed to replace the shocks & springs

Research

The Mazda 3 has lots of suspension options: shocks, springs, coilovers and sway bars. Many of them are not focused on performance, but appearance — e.g. lowering. For example, the Eibach springs sold by Tire Rack are actually softer than OEM! I wanted the car to be firmer than stock. By that I mean less body roll, and to a lesser extent, less squat and dive. But I did not want to change the car’s front to rear balance. I already had a stiffer rear sway bar that effectively reduced oversteer. If the car ended up a bit lower, that is OK if the difference is an inch or less, but lowering is not my goal.

Springs & Shocks

Racing Beat has a set of springs that +20% stiffer than OEM, front and rear. From what I gather, the OEM spring rates are about 138 in-lb. front and 174 in-lb. rear. The Racing Beat springs are 162 front and 211 rear. And they are bright red in color. That may be a pro or con, depending on your perspective.

I opted for Koni Yellow shocks, which are adjustable. The fronts can be adjusted with a simple knob turn while installed. To adjust the rears, they must be removed from the car, fully compressed and rotated. The softest setting on these shocks is firmer than OEM. I set them “blind” at 1/3 of the way up from the softest setting, so just below the halfway point. This turned out to be perfect.

The car sits about 1/2 inch lower in the rear, 3/4 inch lower in the front. Not a big difference, but you can see it if you look for it.

Installation

The job took all day but it wasn’t hard. The only special tool you need is a spring compressor. These aftermarket parts fit perfectly like OEM, and their quality was OEM or better. After completing it, I took the car in for an alignment check. It was still perfect – swapping the shocks and springs did not change the alignment.

Test Drive

I quickly realized that the car was transformed. It wasn’t as stiff as the RX-7 and Panoz Roadster that I used to race in SCCA. At high speed, like cruising down the freeway at 80 mph, the car feels as solid as a brick house, like a Lexus or Mercedes sedan. In the corners, it’s a precise steering flat-tracker with eager quick turn-in. Typical of FWD cars, it has throttle-off oversteer and throttle-on understeer. It’s very street-able yet with more responsive handling.

I set the adjustable rear swaybar back to its stiff setting (3x the stock rate), and it was even better. With the OEM shocks & springs, this gave the car an unsettled dartiness so I kept the swaybar on the soft setting (2x the stock rate). But with the new suspension this was completely gone and the car is absolutely perfect on the stiff setting.

Conclusion

If you have a Mazda 3 this is a great suspension setup for performance.

Car Performance & Handling: Swaybars!

Cars
by Mike Clements

It’s been several years since I autocrossed or owned a high performance car. I still like fun to drive cars, I just don’t have the time for it anymore. I finally got around to doing the first performance upgrades that I’ve done in years. Back in my SCCA days, the first 2 mods anybody did was (1) tires, and (2) swaybar.

Swaybar 101

When a car has a swaybar, in order for the body to roll L or R it must twist the swaybar. Stiffer swaybars reduce L-R body roll without affecting the spring or shock rates. If both wheels hit a bump and move together, the swaybar does nothing. It only kicks in when the L and R sides try to move differently. When one side (L or R) tries to move up or down, the swaybar forces the other side to also move up or down too. How much, depends on the swaybar’s stiffness.

A stiffer swaybar reduces body roll, which reduces weight transfer, which reduces overall traction. Yet at the same time, it improves response and agility. So it’s a trade-off. Put differently, you need to allow some body roll to get good traction, but too much of it reduces response.

Adjustable Swaybars

When the car’s body roll twists the swaybar, it does so through connecting arms. Longer arms give the body roll forces a longer moment arm, making it easier to twist the bar, making a softer bar (lower rate) with less resistance to body roll. Adjustable swaybars usually have several points along their arms where you can connect the end links.

Must both of the swaybar arms have the same length? Imagine if one arm is longer than the other; that is, connecting the end links to different mounting points on each arm. Newton’s 3rd law says the torques exerted are always equal and opposite, which seems to suggest that an asymmetric setup could give symmetric roll response. In this case, an adjustable bar with 2 holes actually has 3 different rates, or with 3 holes has 5 different rates.

However, while the torques are always equal and opposite, even when connected asymmetrically, the moment arms are not. And the torque on the bar exerts a force on the opposite side through its moment arm. So connecting the swaybar asymmetrically would create asymmetric roll rates: stiffer to the L than to the R, or vice versa.

The conclusion: always connect the end links to the swaybar using equal length arms.

Tuning the Response

Most cars are designed to understeer: that is, under most conditions the front slides before the rear does. This is easy to control, especially for unskilled drivers. But skilled drivers find excessive understeer to be less fun and even annoying. Excessive understeer makes a car less responsive. As a general rule:

  • A stiffer rear swaybar reduces understeer, increases oversteer.
  • A stiffer front swaybar reduces oversteer, increases understeer.

This is all relative. Most factory cars are too soft overall and also understeer, so a stiffer rear swaybar is ideal. But if the car is too stiff overall and also understeers, you might use a softer front swaybar.

Mazda 3

My 2014 Mazda 3 is actually pretty fun to drive, for a FWD economy car. At 36000 miles I finally had to replace the OEM tires. While I was doing this I figured I’d also install a stiffer rear swaybar.

This is such a common car there are many options. I used a 22mm Progress bar. It was relatively inexpensive and came with new bushings and brackets to handle the larger forces. The OEM rear swaybar is 18mm diameter with a rate of 334 in-lbs. The Progress is 22mm diameter with rates of 772 in-lbs (about twice as stiff), and 1,015 in-lbs. (about 3 times as stiff). This bar has excellent build quality and perfect fit with the end links pointing straight up and down just like they do with the OEM bar.

The soft setting made a noticeable yet not a huge reduction in body roll. I quickly shifted to the stiff setting which was completely different. Less body roll, quicker turn-in and more precise handling. But, it got twitchy. The swaybar was too stiff for the rest of the suspension. So I kept it at the soft setting (still twice as stiff as stock).

A few months later, I replaced the shocks & springs all around. With the stiffer shocks (Koni yellows) and springs (+20% rates from Racing Beat), the swaybar’s stiff setting was better matched to the rest of the suspension. The twitchiness was gone, now it was just a precise, sharp, flat tracking, great handling car.

Subaru Forester

My wife’s 2018 Forester is a lot softer than our 2004 Forester. It drives more like a bus than a car. Over the years, Subaru softened the suspensions. Subaru has OEM swaybar replacements in 2 sizes: 19mm or 20mm. Stock is 16mm, so the 19mm is about twice the rate. I got this bar from Subaru Online Parts, cost about $100 and included new brackets and bushings.

This makes the new Forester handle more like the old one. It’s less like a bus, more like a car. It feels tighter and more controlled. But not too tight. If you go too stiff with an AWD vehicle it can impair traction off road. It’s perfect for my wife, who wanted less body roll but is not a performance car enthusiast.

NOTE: the end links on this Subaru were quite nearly frozen. The end link attachment bolt was corroded to the nut. And the car is only 2 years old, 4700 miles, and has not been driven on salted roads. I removed the end links from the car, so I could remove the entire bar with end links attached to it. Soaked the end link bolts in liquid wrench and moved the nut back and forth chasing the threads until it finally came free.

More generally, a stiffer swaybar applies greater forces to the end links. So if you more than double the rates, don’t be surprised if the end links eventually break. Keep an eye on them and be ready to replace them with more robust aftermarket end links.

Fixing Intermittent Car Problems

Cars, Tech
by Mike Clements

A few months ago Michelle’s car (2004 Subaru Forester), which has been solid & reliable since we bought it new almost 13 years ago, acquired an intermittent problem: it would not start when warm. Cold starts were always good, but after you drive it 5-10 miles, just enough for the engine to warm up, then turn it off, then come back 15-30 mins later, it would not start. The problem was intermittent, happening only about 10-20% of the time. When it did fail to warm start, remove the key from the ignition and try again. It would almost always start the 2nd try. The start failure was: engine would crank like normal, but would not actually start. If you modulate the gas pedal it would start and run smoothly but it wouldn’t idle. No check engine light, and no OBD-II codes were ever thrown – not even when it was refusing to start. When the problem started, the car was about 12 years old with about 78,000 miles. It had always been well maintained – oil changes, air filter, clutch, tranny, brake & diff fluids, belt tension, etc. and was still getting about 20 mpg in around town driving, same as when it was new.

I do all our car maintenance because it’s fun problem solving, I trust myself to take the time and do the job right, and it saves a lot of money. Intermittent problems can be frustrating, but the challenge to fix them can be fun.

Since the problem only affected idle, and was electronic and intermittent, the obvious culprit was the Idle Air Control Valve (IACV). But this is a $350 part, and if it fails the engine is supposed to throw codes – but it wasn’t. There are several far less expensive parts that could be causing the problem, and I’d feel like an idiot replacing a $350 part only to find that the real problem was an $8 set of spark plugs or a $25 sensor.

Here’s what I did, in order… after each step I gave it a week or so to see if it had any effect.

  • Replace the front O2 sensor (the rear ones had been replaced a few years ago).
  • Replace the spark plugs (new ones gapped to spec). The old ones were clean but gap was about 4 times higher than spec. It ran smoother but didn’t fix the problem.
  • Re-teach the ECU idle (disconnect battery, ignition OFF then ON pattern, etc.). This improved the idle but didn’t fix the problem.
  • Clean the IACV – idle air control valve. It was pretty clean to start with, but cleaned it anyway. Also tested its function – OK.
  • Check & clean the crankshaft & camshaft position sensor. Upon removal they were surprisingly clean, but I measured the proper impedance, cleaned & re-installed them anyway.

That last item is what fixed it.

Correction: Dec 2016 – no it didn’t fix it – problem returned!

Since the sensors were operational, I can only surmise that the problem was an intermittent or poor electrical connection to the sensor, that got cleaned when I removed & reinstalled it.

Since the problem came back – next steps on my list below. Since the engine has never thrown a code or lit up check engine light, I wondered if the OBD-II system was even working. When testing the IACV I unplugged it while the engine was running. It immediately threw 4 codes, one for each wire pin. So the OBD-II system and my code reader are both working.

  • Replace the fuel pump relay: sometimes with age, the point contacts get corroded and don’t provide enough power to the fuel pump. When my 15-year old Honda Civic developed a similar problem, this was the root cause.
    • Replaced in Nov – did not fix the problem.
  • Main relay: probably not the problem; everything else on the car works fine – radio, headlights, etc.
  • Clean throttle body: no. A dirty throttle body would cause problems all the time.
  • Clean/replace the MAF: this engine – 2004 2.5 liter Subaru flat 4 – has no MAF.
    • It has a TPS – throttle position sensor
      • Inspected OK – operates smoothly and measures 190 Ohm – 5 kOhm
    • It has a MAP – manifold pressure/vacuum sensor

Update: Jan 2017

Finally, I decided to do what the original symptoms suggested: replace the IACV. By this time I had replaced every other cheaper part that could be causing the problem, to no avail. I found an IACV on Amazon for $250, which is still ridiculous but about $100 cheaper than the local parts place wanted, has a warranty, and is probably the exact same part from the same manufacturer. Took all of 10 minutes to install it, and the difference was instantaneous and obvious. First start-up, engine spun up to 2,700 RPM (which is unusual but this is a brand-new sensor the computer is learning how to control) then slowly ramped down to a normal idle speed. Next morning’s cold start (ambient temp 31* F) engine fired right up, spun initially to 1,700 RPM then slowly ramped down to 750 as it warmed up.

Ah, give me the good old days when an engine’s idle was adjusted by cracking the throttle open a smidge with a simple set screw. There’s a reason airplane engines don’t use all these electronic controls.