Drive-by-wire (EML) throttle system crash

Maintenance/Repairs
1

This is not a question but rather an account of problem resolution that Car Talk readers may find interesting.

Drive-by-wire throttle system crashes are mentioned occasionally in threads on different forums, but few have posted their resolutions.

Drive-by-wire throttle systems are great for reducing manufacturing costs and parts counts on cars with cruise control, traction control, stability control, etc. They can be a pain, however, to diagnose when they fail.

All functions in an EML have redundant components, and the system is designed to fail closed when it detects ANY problem. This is because manufacturers live in fear of charges of unintended acceleration.

On a BMW, a soft EML crash gives you a limp-home mode in which you have about a 40 mph top speed. More commonly, however, the EML crashes completely which gives you a fluttering throttle butterfly in a nearly closed position, 1100 RPM and zero power. Shutting off and restarting the engine resets the computer and generally allows you to drive normally again.

Solutions I have found posted included failed throttle body, failed throttle pedal, and failed wiring harness connections making it appear to the computer that the throttle or throttle body had failed.

In MY case, however, ('04 325iT, 140k mi) I got a P16A7 code, which is defined as “MAF signal implausible”.

The reason my MAF signal was implausible was because my bank 2 catalytic converter was completely plugged up, and my bank 1 catalytic converter was not too good. The only reason that cyl 4 through 6 were running at all was because their exhaust was going up through the secondary air passage and coming out in the front exhaust manifold.

When the computer detected that the MAF was not reporting nearly enough air flow for the throttle opening and RPM, it shut down the drive-by-wire throttle system. When that happened, I just turned the key to the off position for a half second and then back on. The car bump started and continued to drive as if nothing had happened, other than the Service Engine Soon and sometimes the EML lights illuminated.

It took me a while to figure this out because my test for an obstructed exhaust is to put my hand next to the exhaust opening when the car is idling. If I can feel individual bursts of exhaust, it is OK. This does not work when you have two cats feeding a single exhaust. I did not identify the problem for certain until I dropped the exhaust pipes and put my hand at the opening of the downpipes, below the cats. Then it was obvious that one was blocked.

2

Yeah. I can’t wait 'til they apply DBW to brake systems.

Tester

3

Have you seen the new cruise control systems that maintain your distance from the car ahead of you? Can’t wait to diagnose one of those when it fails.

My cars already have functions that I don’t even try to fix. Rain-sensing wipers and light-sensing headlight functions are simply disabled at the first hint of a problem.

4

The only issue the DBW system presented was the propensity to blame it or something more complex than the actual problem. Human nature to do that. It must be this new fangled expensive hard to diagnose stuff! Been guilty of that myself.

Good news is it worked exactly as intended and as one would hope it would.

You also were fortunate to get a code that would make you think twice and then look elsewhere. I can see doing exactly same thing without considering the complex way the exhaust can find its way around. Impressive diagnostic reasoning on your part!

Now, why did those cats plug up? You may have yet to uncover the real root cause…

5

I wish I knew why these cats plugged up.

Buying a used car is a roll of the dice. This is my fourth used BMW, and the first three have been great cars. I fear, though, that I bought more mechanical history than I bargained for when I bought this one at 100k miles. It looks pristine, but for all I know, the P.O. may have tun nitrous through it.

This car consumes about a quart of synthetic oil every thousand miles. It has been doing that for the 40k miles I have driven it, and perhaps for some time prior to that. I first suspected the front cat had failed because the #1 cylinder has compression 18% lower than the other five.
All plugs and exhaust ports look uniform, and have no buildup to speak of. I have cleaned the PCV system and replaced the PCV valve, which is the most common reason for these cars to consume oil.

No reason to suspect it, but I think I am going to switch this car from Costco fuel back to Chevron, which is what I burn in my other cars. If nothing else, the Techron will keep the injectors cleaner.

6

Odds are the oil consumption is what clogged it due to oil caking onto the leading edge of the honeycomb in the converter.

I’ve cut a few converters open, knocked out chunks of the substrate for a look-see, or eyeballed a straight converter against a light source and it can be easily seen.

7

Soak in lacquer thinner.
Nothing to lose…

8

Revisiting a very old thread to conclude the story:

I ended up replacing the engine with a junk yard engine with about the same miles. Have driven 60k miles on the replacement engine with virtually no oil consumption.

If I had it to do over, I would have tried aggressive solvents on the piston rings before junking the engine. I had nothing to loose. I have since read that these BMW engine have a tendency to get their oil rings stuck in their grooves. Some Nissans and GM engines apparently suffer from the same problem. The admin assistant at my office had to have the pistons pulled from her Buick V8 to free the rings. That Buick was a terrible car.

When my '04 330i BMW started consuming oil this year, I ran the passenger side up on ramps to try to make the cylinders vertical, and poured a couple of ounces of fuel injector cleaner into each cylinder and let it soak for a couple of days. Have driven about 5k miles since then, and can report that it reduced oil consumption significantly.