I've got a 89 iroc-z with a 305 tpi, it's given me problems since i bought it, but i expected as much. I've got most of the issues fixed up except for one. The car likes to idle at 2500, and when when letting off the gas the RPM's don't drop. The car shifts fine, but when moving through the rpm curve if i let off the gas the rpms stay where they are and the car won't start to slow down until I hit the brakes. The high rpms is causing a real heat issue since i'll be sitting at a light at 2500 rpms, and I"m not even going to mention how much gas the car eats up.

I've had the car at the shop for the past 3 weeksand my mechanic is completely stumped, I trust my mechanic, i know him personally and he's restored his own 88 camaro with the same engine. He's hooked the car up to his scanner to check the codes and there is a couple coming up but they all relate to the egr system. He's replaced a couple off the control modules and we've cleaned up the throttle body to make sure it's working smoothly. He's to the point where he thinks all we can do is throw parts at it until something fixes it.

Is there anyone out there that can offer any suggestions of what might be wrong? Me and my mechanic are at the ends of our ropes and my wife's fustrated to the point where she just wants to get rid of it. I need help.

 

i had a problem like this. it ended up being the throttle body was sucking air through the shaft/ seal

 

My 87 v6 had an EGR code last year that turned out to be a minor vacuum leak. High idle is normally very indicative of some kind of vacuum leak because your mass air flow sensor only thinks so much air is getting to the engine, but your O2 sensor is telling it that its running too little fuel so your engine steps up the fuel to compensate for having more air than expected.
Anyways what I would do is a thorough inspection of all of your vacuum lines, and charcoal canister. Basically anything with vacuum (possibly even your brake booster or heater controls as they run off vacuum too) Then failing that i would run oil over the places where your intake seals to the heads, plenum or throttle body to see if the idle drops because if oil blocks the leak, there won't be any excess air (this however is not a permenent fix because eventually the oil gets sucked through). Then you'll know where your leak is, whether it's a vacuum line or intake gasket leak.
Try that.
Also you could try cleaning the TB, sometimes carbon on the plates cause them to stick open a little, and Voila! vacuum leak.

 

Vacuum leak or a TPS sensor reading wrong (too high). Those would be my usual go-tos in that situation.

Check to make sure it's reading down around .5V at idle with your scan tool.

 

Ok, so we spent the weekend looking for a vacuum leak but couldn't find anything. We took the car over to another shop that my mechanic deals with. The new shop hooked the computer up to their scanner and it returned with every single error code possible. So now we're thinking that it may not only have a vacuum leak but also a bad computer. Does anyone know if a bad computer would cause my RPM issue and should I keep looking for a leak or concentrate on the computer?

 

its entirely possible that the computer could do that, computers have really weird issues sometimes. What kind of error codes did you get?

 

I was working at the time they hooked the car up but they called me and said that ever code in their book was returned.

 

The usual rule of thumb is that any more than 4 codes simultaneously and you should start to suspect the ECM itself may be bad.

I usually clear the codes first, see if it helps. Drive it around for a while and see how many return before I start throwing parts at it.

Obviously, if you have reason to beleive those codes are REAL faults (say the wiring harness looks like a train wreck, for instance) you have a much longer road ahead of you.

I've brought back some real "train wrecks" from the dead. In those cases you usually find it's a combination of many things all adding their little piece to the mystery (or interacting with eachother- even worse!). No choice but to start tracking down the biggest faults first and then move on to the more minor stuff. ONE THING AT A TIME or you stand a good shot at making it completely undrivable.

 

Yea, that's why i've pretty much left it in the hands of this new shop we took it to. I know w decent amount about repairs but once it starts coming done to the ECM's and sensors and error codes I usually start relying on the professionals. Speaking of which Í just heard back from my friend who spoke with the shop the car is at and the answer I got was "they've been working on it all day and can't find the problem". This answer really doesn't sit well with me but what can I do, I hate not being able to stick my nose in when it comes to my car.

 

RPM's are now acting normally, it took replacing the idling module, idling motor?, and finally the idling sensor. No vacum leaks were found, the computers not reporting any trouble codes, got the car home and had to dump a gallon of tranny fluid into it, later that night it spewed about 2 quarts worth all over my driveway. So i dumped more fluid into it and got it back to the shop that replaced the tranmission, still waiting word on status.