Hey 308 guys, I've got an issue with fast idle (1500+ rpm) at rest at lights etc. with the motor good and warm. Also tends to run-out pretty free rather than de-cellerate when I get off the throttle driving hard (6-7,000 rpms, 80 to 100 mph). Sticky TB butterfly? EFI issue? Vacuum leak? My car has a new Mercedes Benze EFI brain installed by the conversion company for California EPA compliance. Any thoughts? Thanx. Ciao, Luigi
Thanx Guys, But it's not a floor mat issue. It is not consistent and the idle drops if I load the engine by releasing the cluth a bit with the brakes on at stop lights, etc. Kinda odd. Input? Luigi
Okay Luigi.... I know your car is rather different now but I had a similar problem with my 328. The fast idle seemed kind of random. I mean the engine could be fully warmed up, I shut it off for only a minute or two and when I restart I get high idle again (around 1500 rpm). Would also happen at a stop light when I take it out of gear etc. I verified that the aux. air valve worked properly when heated and cooled and I did not find any vacuum leaks on the engine. Recently I removed the coolant bridge (in the photo) that connects the two coolant manifolds from the heads. I found that the small pipe, on the bridge, that runs to the coolant expansion tank was plugged up with "gunk." I would say it was at least 50% blocked. The rest of the cooling system looked very clean. For some reason, probably the small tube diameter, it likes to collect in there. Anyway, this tube allows coolant to circulate through the expansion tank. The coolant continually goes in and out of the tank and this means the coolant temperature in the tank should be more or less representative of the coolant temperature in the block. This is important because the aux. air valve is mounted to the bottom of the coolant tank and the water temperature in the tank dictates how much the air valve closes (reducing idle). Now.... if the small tube going to the expansion tank is not passing as much water as when it was new (because of the blockage) the average temperature in the expansion tank will probably be lower (colder) than it should be in comparison the majority of the coolant in the block. If the tank temperature is abnormally low because of poor circulation the aux. air valve will be open to a greater degree, resulting in a higher idle, because it "thinks" the engine is cold (or colder than it really is). Unfortunately I cannot verifty that cleaning this coolant bridge tube on my car solved the problem because I have removed the aux. air valve, vacuum valve, cold start injector etc. from my car along with other parts. I'm hoping these parts are not essential but I will not know until I finish the major + on my car and see how it starts/runs. Also, I know there are some variations between models. I don't have a temp. switch on my coolant tank, a lambda sensor, frequency valve etc. Erich Image Unavailable, Please Login
I have the same issue from time to time, & I load w/clutch or when coming to a stop, or leave in gear till tach hits idle rpm and it stays down. I noticed that you originally posted this a couple of years ago, what did you find to be the cause?
I finally traced the problem to small cracks in the vacuum line that goes from the intake plenum to the ignition module mounted under the trunk floor on the drivers (left) side of the car. It was old and dry, and had developed cracks and vacuum leaks at both the plenum end and the ignition ECU connection. Hard to pinpoint, but an easy fix. I suggest you go over your vacuum lines with the engine warm and at idle, wiggle or flex the lines at the connection points, especially the ones near the hot areas of the engine. Listen for changes in idle speed or the hiss of a leak, you'll find it.
Is this the hose you're talking about? What does this do anyway, is it like a vacuum advance? Image Unavailable, Please Login
WOW! My '83 QV has been doing this intermittently too. I will check these lines. You guys are great on info for this car!! Thanks.
Did you have ignition problems also? Wouldn't/couldn't this cause ignition problems with a vacuum leak so close to the vacuum module? I mean it could have been selecting the wrong map because the vacuum was lower than it should have been due to the leak.
Yes those, out of sight and out of mind. Actually vacuum retard, I believe. I never experienced any performance issues, just idle rpm.
Thanks for your speedy reply. My lines are of a cloth braid nature (cracks won't be vis.) so I wiggle and listen and watch for idle variation. I'll repost if something else is causing the issue.
