Idle Wild Problem - 92 318i Aloha, Forum Members! A while back, I began having a wildly-fluctuating idle problem (well, my car did, that is). Manual trans/110k miles, mine since Jan 93. I lived with it for a while, eventually took off the idle control valve (ICV) at the rear of the engine and sprayed it out w/brake-cleaner. Seemed to help a little, but not for too long. Lived w/it for a while longer! Eventually figured the electrical part of the valve must be "bad" so bought new part and installed it (yes, correctly, I am sure - very difficult to mess it up, although I DID see an on-line reference saying that the arrow on the part has to point to the block, whereas in MY case, this is not possible, so I cannot but conclude that reference was not correct). Problem remains. What else did I do? Glad you asked! Checked all vacuum-related and intake hoses and boots (some I had replaced not too long ago), no obvious leaks. Checked the throttle position sensor w/multimeter and it seemed to be w/in specs. The old ICV appeared to work OK, insofar as, when I removed the electrical connector, engine revs changed, and when connector replaced, revs changed again, but I replaced it because I could not find anything else that seemed to be a culprit. I cleaned the air-filter and the plugs. I have also been experiencing some oil on the plugs and black fouling (no pitting and otherwise, plugs look fine). I was suspecting a leaking valve-cover gasket. Replaced the (original: 100+k on car now) O2 sensor not long ago during this problem, due to check engine light being on, Peake code-read on O2 sensor, and fact that it WAS the original sensor! Still getting check-engine light, sometimes stays on for a while, or comes back on when at stop and idle goes way down (ocasionally, engine has stalled at idle). Was checking the vacuum hoses under the manifold, where there is a control valve (forget what it's called right now, but a tiny "flying saucer shape) and a nearby "one-way" or non-return valve, a tiny item with rubber hoses entering (or leaving) from bottom and exiting (or entering!) from top. These small hoses WERE a little aged and loose. I temporarily tightened the connection from the top with a twist-tie (all I had!). Perhaps the hoses and that valve need replacement, but not sure these are critical points in ths system, and I saw no improvement in the idle at any rate. A neighbor threw this at me: he has "seen many idle probs before" (appears to be a reasonably decent shade-tree mechanic - and a pro welder) and my spark-plug problem, and more often than not, he finds that due to "bad fuel" (his claim), the catalytic converter gets "junk" in it, which in turn can cause all the other symptoms I have w/my car. he takes the cat off and shakes, beats it or whatever, getting all kinds of crud out, and after replacement, problem is cleared-up. My exhaust system is the original, so there is no doubt that the cat COULD be at the end of it's useful life. And problems in the cat would give me O2 sensor codes, even w/a new sensor, I suspect. I DID replace the o2 sensor myself, a pretty simple job and I do not think it is possible to mess up its electrical connection. And when I did that, the exhaust system looked pretty good (of course, what's inside couldn't be seen). The guy suggested a compression test and fault-code analysis, and I am leaning toward that, since there seems to be little else to do right now. One other real oddity: a week or so ago, I was driving on a freeway, maybe 10 minutes or less after starting the car, when I heard a distinct "thud" seeming to come from the right side or maybe under the car. I had not noticed anything in the road that I may have hit, tho it was dark, so who knows. It almost sounded like a soccer-ball or something had maybe hit the lower right side - but this was freeway, with no playing areas or residents adjacent. I saw no dents or anything. Yesterday, after moving the car only 100 yards to an area with more daylight, in order to continue my troubleshooting (e.g., the car was virtually completely "cold", and I ran it one other time for a minute or two to check things under the hood, then off again), out of nowhere, I heard the "thud" again! Seemingly same area (right side, maybe underneath). I was on left side of engine bay. Car was not running or moving when this happened, unlike the first time. Anybody have ANY idea/theory/experience as to what the heck THAT was/is? I thought, OK, something exhaust-related? Cat converter, maybe? But what could cause this "thud"?! Some sort of pressure build-up that randomly releases?