Maddening Misfire....

[steveinindy]

My ’87 325is suffers from an aggravating misfire that I have been unable to diagnose. I’m hoping someone here can help. I’ve owned the car for 7 years. It has been rock solid. It has a total of 258,000 miles. I’ve got to add a qt of oil about every 1k miles. The car starts easily, cold or hot, even through the coldest part of the winter (I’m talking -10 and below), but there is sometimes a miss until the car has run for a few seconds. The idle is then a bit lumpy, but steady and solid, and it revs without hesitation from idle. The misfire occurs only at rpms above 4k and generally only during hard acceleration. Here’s what happens: foot to floor; engine revs and pulls strong and fast…no hint of a miss…until there is a sudden, violent engine failure that feels like a backfire. It feels like a single, instantaneous failure. This usually doesn’t happen in first, but happens often, but not always, in second and third. It happens so fast there is no time to lift, and the engine immediately picks up pulling as hard as ever right up to redline, as if nothing happened. This phenomenon is sporadic. While it happens more often than not, there are times the thing runs like new…not a hint of a miss. The miss occurs at randomly different rpms. It might happen at 4,300 during one stretch of multiple accelerations and then at 5,500 during another stretch. It is less likely to occur with slow, gradual acceleration through the full range, but sometimes occurs even then. That is as well as I can describe it.

Today, while driving, I was reminded of another variation from the “normal” in this abnormal phenomenon. Today, the car pulls strongly to 5,400 rpms when it begins to miss, but it will not accelerate beyond that point. It sticks at 5,400 and won’t go beyond. The failure is not as violent as typical and feels much more like it’s hitting the rev limiter. Go figure! This odd, changing, unpredictable behavior is a big part of why I am so stumped.

This is what I’ve done to analyze: just about everything! O2; crank sensor; AFM (have even swapped it);
Temp sensor; throttle position sensor; fuel pressure…. Compression testing: with a max of two compression strokes per cylinder, all cylinders show 130 +/- 5psi. All cylinders come up to 170 +/- 10psi with five compression strokes.

I’ve replaced the fuel filter and even the fuel pump. I’ve changed the plugs. A couple of the old ones were fouled black. One had a broken ceramic insulator. I checked the replacements just the other day. They’ve got maybe 2k on them at most. They have an odd, non-uniform appearance: one side of the plug might look like it’s burning just right, while the other side of the same plug shows black deposits. Photos are attached. You will also see that there is oil all over the threads of all the plugs. Can that possibly be coming from inside the cylinder? I guessed it accumulated from a valve cover leak. It seems excessive for that, however. Is my engine entering a dying stage? I can live with that. I’ve got a replacement that I can swap in. I’m just stubborn about squeezing as much as I can out of the original motor. I get a sense of pride driving a 25 year old car with the original engine and 258,000 miles. Plus, I don’t want to swap motors only to have the new one suffer from the exact same miss. That’s a possibility until I’m more confident about the cause of the miss.

Can anybody help?

[dnguyen1963]

Your symptoms are consistent with having bad AFM and/or TPS. It could also be the DME loosing signal with other components momentarily. What does your tach do when you have problems?

[steveinindy]

Thanks for your response. The tach mirrors what the engine is doing...sort of flutters at the miss: briefly with the instantaneous miss or continuously when the engine sticks at the miss point and won't accelerate beyond that point. I've tested both the TPS and the AFM. I've even swapped out the AFM with no change in symptoms. Plus, the intermittent and variable nature of how it happens sometimes, but not always, how it sometimes accelerates beyond the failure sometimes, but not always, and how it happens at different rpms seems to weigh against either the TPS or the AFM.

[uturn]

How's the cap and rotor lookin? The plastic shield behind the rotor? Insulation on the wires?

[steveinindy]

Swapped the rotor, cap, and wiring set, too. No change. Thanks for your input.

[jlevie]

Thanks for your response. The tach mirrors what the engine is doing...sort of flutters at the miss: briefly with the instantaneous miss or continuously when the engine sticks at the miss point and won't accelerate beyond that point.

If the tach shows a drop on a cutout, that means that there is a loss of timing data to the DME or possibly a loss of power/ground to the DME. A loss of timing data can be from a flaky CPS (only use a new BMW OE part), a flaky CPS input section in the DME, or a flaky engine harness. A corroded fusible link or a flay main relay are possibilities as is a bad ground connection on the right strut tower.

[steveinindy]

Thanks for your suggestion, jlevi!

Interestingly enough, just today I decided to pull the CPS off another engine I have. In trying to figure this out, I had read another thread about a very similar, but not identical, problem. It was on this or another forum. One contributor (it may have even been you, jlevi!) suggested the crank position sensor (cps) could be the culprit. He argued that the symptoms sounded like an ignition timing problem, which could easily be caused by a poor signal from the cps. He insisted that the cps can malfunction even when it tests okay, which mine does. Anyway, I pulled the cps off the other motor just to test the theory. Viola!!! The car pulled to redline without a hitch, which it hasn't done in months. I've only driven maybe 10 miles since the swap, but the difference is like night and day. I'm not quite ready to claim a complete cure yet because I have had a couple of minor hiccups in the few miles I've driven. Nevertheless, the cps made a dramatic difference. It's like a different car. I am very encouraged and am keeping my fingers crossed. Thanks for reading my post and for your input. Best wishes and trouble free driving to all. :-)