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In Reply to: Re: Weeked report: hesitation and smells rich. posted by Todd Kenyon on October 01, 2001 at 09:25:52:
Definitely sounds like we have the same problem. From what I can tell, the 1000 rpm down to 700 rpm idle is normal, and always happens if the engine has sat long enough to be "room temperature". It also appears you noticed the fact that when the engine is allowed to cycle to the 700 rpm mode, the car runs fine. It ONLY hesitates if you drive off during the 1000 rpm phase. That should really be a clue here. What causes this higher idle, and what is its purpose? I am guessing that it might be the Idle control module sending a signal to the idle control valve, allowing more air into the intake, and raising the idle. I assume this is supposed to help warm the engine up faster? What is supposed to happen when you press the accelerator and drive off, so it is no longer idling? It is then that something seems out of wack with the mixture. It could be an ICV problem maybe, but if it is, god help us, because from what I can tell the ICV is BURIED beneath the throttles and plenum and impossible to get to. I also tested my TPS switch, and it is fine.
I have also noticed that I get some light backfiring when I shift during the hesitation mode. This is the only time the car does this. You may want to see if yours does the same, turn off radio and open window.
And, get this, I switched back to my stock chip. The problem is still there, but maybe not quite as bad. But it is not caused by the chip. As an aside, the car almost seems to run more smoothly with the stock chip. And the powere difference is very subtle - main difference is you don't quite get the neck-snapping surge at 4500-5000 or so when you step on it.
I think I will post the idle speed thing as a new thread above.
Ok, on Friday morning it ran just fine with no hesitation, Sat. morning it idled at about 1000 rpm for a minute or so, then dropped back to 700 and ran fine. This morning it idled at 700 and ran fine. Not sure why it's different. This morning was colder than Sat. morning, no obvious things that trigger the idle higher!
BTW, my car is chipped with a Jim C chip. My mechanic once casually suggested that it might be due to idle map on chip?? Is your chipped? I still have my original chip, so I could always try switching it. My mechanic is pretty good, has alot of experience with this engine, so I may break down and have him try to find it. I haven't discussed all the symptoms with him yet, so I will do that first and see what he says, but he will probably want me to leave it overnight so he can drive it first thing in the am and experience it himself.
Sheldon-
That is it exactly, like a switch is flipped. My car will drag at say 30-35 mph in second as I leave my neighborhood, and then "the switch is flipped" and it will accelerate rapidly, maybe jump from 3000 to 4000+ rpm, without moving the gas pedal, at which point it drives like a champ. I thought last winter that it was caused by a rich condition because in very cold weather, it actually seemed to be better or nonexistent.
Try the idle thing and see if your car does the same as mine. If you just start the car cold and let it idle, after a few seconds the computer (I assume) lifts the idle speed to 1000-1100 rpm for maybe 30 secs, and then it lowers it to normal (700) range. If I wait for this process, the car drives fine from the get go, even tho temp guage is still in blue. Is yours the same? I am assuming that this high idle thing is normal and caused by the computer to help car warm up. However, maybe it isn't normal - can anyone comment on this? It seem that if the car is driven during this "high-idle" phase, it lags. Anyone have a clue???
Todd, mine does the same thing in the morning. Get it out on the highway and you can feel it hesitating at around 3000rpm. Then about 30 sec. or so it's like the computer flips a switch and instantly it's running like a clock! Any ideas yet?
The one that connects the valve cover to the intake plenum. But it could be caused by an air leak anywhere. I agree however that if it is running rich, then that is not the problem.
What do you mean it smells rich? If you are smelling gasoline, it could be something else. I smelled gas once and it was a loose hose clamp on the end of the fuel rail. Could also be a leaking injector o-ring, etc. Does the exhaust look like it is running rich?
BTW, my problem is not entirely gone, it just improved. I still get some hesitation/stumbling sometimes when engine is cold (instantly, suddenly clears up when temp gauge gets out of blue). Interestingly, it only seems to happen if I start driving immediately after starting. AFAIK, it is normal for the engine to idle at higher rpms (1100) when first started, and then it settles down to 700 or so. If I wait for the computer to lower the RPMs (maybe 30 secs after starting) it runs fine, even tho engine is still cold. See if yours does the same. I would think that this should be a strong clue as to what is causing it, but I haven't heard any good guesses yet.
Todd, which breather hose did you replace?
I have had similar symptoms. It is NOT the cold start injector (which is where Bentley leads you), because there isn't one on this engine. Mine seemed to improve markedly when I replaced the cracked breather hose. I would check for air leaks.
Sorry for the double post. My M5 is hesitant untill completely warm and smells rich. I know what I think it is and have read Benetly, but would like your opinion.
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