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?
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?
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