High Idle on my 96' Buick Riveria SC

GMCSierra04 · March 7, 2005

Hi GMTech, I have a 1996 Buick Riveria with the 3800 Series II SC and the problem is that the car idles really high. At start up the car races up to about 3,000 rpm's and when you put it into drive it goes down to about 1,200 rpm's. What should i look for? Could excess carbon build up cause this on the TB, leaking gaskets causing unmetered air to enter the engine (intake, throtle body?)? Thanks so much for any help!!!

Zackm

GM-Tech · March 7, 2005

Idle speed is controlled by varying the amount of air allowed to enter the intake. So, either the IAC (Idle air control) is whacked and stuck in an open position or you're getting air from somewhere else... big vacuum leak, etc.

A coolant sensor that is giving an indication that the engine is really cold when it's not could also drive the idle high, but I doubt it would be 3000 rpms.

If you have a scanner available, look at the IAC counts. If there are zero counts, the PCM is trying to drive the IAC closed because it recognizes the high idle. In other words, no other sensor is giving false info to the PCM, causing it to command the idle high. Of course this doesn't tell you if there's a vacuum leak or if the IAC is simply just not responding to the PCM's commands.

GMCSierra04 · March 7, 2005

Thanks GMTech! For the IAC counts, 0-40 would be normal with no problems so what you saying is that the closer to 0 indicates unmetered air into the engine b/c the IAC port is trying to fully close off? This could be verified by checking the LTFT, higher than +/-0 would indicate some sort of vacuum leak right b/c it has to add fuel for the lean condition right? Could a leaking TB gasket cause this (depending upon size of leak) or intake manifold gasket(s) cause this without setting a MIL? Could a vacuum leak cause this big of a RPM change without making the LTFT go out of its 20% +/- range and set a DTC (seeing its happened on more than 2 occasions)? Thanks for all the input, deffinetly appreciated on this side!!

GM-Tech · March 7, 2005

If the idle is that high, I'd say the IAC counts wouldn't be closer to zero, they would be zero. And yes, I would expect fuel trim to be way positive if you have unmetered air entering.

We've had some problems with the upper plenum on the 3800, but more with coolant leaks than leaks that would allow air to enter. Here at Chevy, we haven't had the supercharged 3800 until just recently, so I'm not familiar with any particular problems with those.

GMCSierra04 · March 7, 2005

No problem, you have been very helpful on this! Unfortunatly, I'm away at school in NC and the car is back in NY so the whole diagnosing over the phone doesnt work well, especially on late model vehicles... Could excess carbon build-up behind the throtle plate be causeing the idle to flare up and be so high too? Only thing is that there would have to probably be a lot to do this but not sure.

Thanks again, Zack