My 86 328 GTS (US) is not consistently returning to idle. Car has about 39k miles. When I start it from cold, the idle increases as expected to around 1500 RPM. As it warms, the idle settles to around 900 RPM. This is without touching the throttle. If I depress the throttle any amount, the idle increases to around 3000 RPM. I can kick it down, but it should not increase like that. Once the engine is at operating temp, about 180f, the idle becomes erratic. Between shifts, upon depressing the clutch, the RPM increases without throttle input. When slowing to stop, the idle remains around 3000 RPM. However, I can "kick" the throttle and the idle will almost always return to around 900 RPM. This manual intervention is almost always requires. If I'm aggressive with throttle operation, it normally returns to normal idle. This problem has just recently developed. The engine runs great otherwise. What I have done: I removed and cleaned the throttle body. Shaft seems within tolerances. Replaced throttle position sensor. Removed fuel distributor (found broken O Ring at base. Did not rebuild) Found stiff/sticky pivot points on the airflow sensor arm. I cleaned and replaced. Thanks in advance for your thoughts and comments!
Was it: 1) working perfectly and then this problem suddenly appeared, or 2) the problem has been present since the last Mechanic made adjustments? To run at 3000 RPM requires a large amount of air to be entering the engine so your checking the throttle plate is working properly makes sense. IIRC, the only other path for a lot of air to enter the intake tract on a US 328 is the AAV system -- wouldn't hurt to inspect the hoses, and you can always just pinch, or block, the AAV air hose(s) closed during warm-running to simulate a properly closed AAV as a first test. Otherwise, you must have some huge, intermittent air leak somewhere.
Thank you Steve for your reply! Yes, this issue recently appeared. However, a few months ago, a hose "popped" off the AAV. I reattached it and checked the other connections and tightened clamps as necessary. I am going to re-check and disconnect/block while the issue is occurring. I need to eliminate that. What about the fact that I can "kick" the throttle and it will return to idle? I use "kick" in the sense that in a carburated car to get the fast idle cam to release the choke setting and thus fast idle. That's why I went into the airflow box and fuel distributor but that seems to be in good shape. Like you said, there's a bunch of air getting in to raise the idle. BTW, I've operated the throttle cable on and off the throttle body and it works smoothly.
Agree, that that seems very "mechanical"/"shifting around"/"something is unrepeatable"-- like a sticky advance weight in a distributor -- but you don't have that . I believe yours is the first report of something like this on an injected 308/328 so it isn't something that we've discussed before, and the cause and fix will probably be quite unique and unusual.
Sticky throttle cable? There have been threads of people having this problem with 355s. Fresh cable, lightly lubed resolved the issue.
Update - I Went back to the basics. I rethought this after all the advice I received in this chat and re-verified all the air/vacuum side of the intake system. That included verifying all the hoses and connections were tight and leak free. I re-checked the throttle linkage and throttle valve for smooth operation, full closing and no excess wear on the shafts. I also re-cleaned the shafts and external linkage. I usually don't put lube in these areas if wear is normal. Then, I mentioned I replaced the throttle position sensor previously. Well, I looked it over and noticed the sensor adjustment was blocked by a fuel injector line. I adjusted the line so the TPS would have its full range of adjustment. I started the car with the TPS loose, let it warm to operating temp, adjusted the TPS and immediately heard a difference. I further adjusted the idle speed and TPS and Viola!!! I took the car out for testing and it looks like it's now resolved!!! Thanks to all that helped me get my thoughts together and develop a strategy that succeeded!!!
