We're talking about my 89 Seville STS. 4.5 TBI which I posted a long repair history in the Reports, Reviews & Articles forum. After all that the car still idles high but it's different now. It idles high until you stop then like one second after the car isn't moving anymore the idle goes down. I still don't think it's as low as it should be but it's the best it's been in two years, around 900 in park and 750 in gear.

I tested this theory yesterday. I'm driving coming up to a stop and I put the car in neutral and watched the tach. It's funny because it almost holds the RPM at whatever point I put in neutral, but not really. It slowly drops to about 1500 then I come to a stop and a second or two later the idle goes down.

Does anyone have any idea what I should be looking at? Keep in mind the new parts include, Engine control computer, IAC, TPS, TBI Base gasket.

I am not knowledgeable of Caddy engines, but it sounds like it might be a vacuum leak, or possibly part of the emissions system. Try turning the AC on while the idle is high and see if that drops it some. Good luck!

Well unfortunately, my local mechanic, Ken, also at some point but a bad belt tensioner which managed to destroy all the accessories on the front of the engine. The AC compressor being one of them. All have been replaced except that due to cost.

Does anyone feel my pain with mechanics? :Bang

yeah, the belt tensioners are known for going bad on most GM products. I just caught that problem on my Monte Carlo

you don't have an IAC (Idle Air Control solenoid) on your car, it uses an ISC (Idle Speed Control motor) and they are notorious for going bad. They are also heavily dependant on a "key" adjustment called "min-air" which involves adjusting both the tip of the ISC plunger, the TPS, and the throttle stop screw. If you're not familiar with the ISC and it's adjustments etc, let me know, I have the whole procedure on PDF available to anyone who needs it.

you don't have an IAC (Idle Air Control solenoid) on your car, it uses an ISC (Idle Speed Control motor) and they are notorious for going bad. They are also heavily dependant on a "key" adjustment called "min-air" which involves adjusting both the tip of the ISC plunger, the TPS, and the throttle stop screw. If you're not familiar with the ISC and it's adjustments etc, let me know, I have the whole procedure on PDF available to anyone who needs it.



Kevin that PDF should be posted in the tech section .... that ISC is probably the least understood and the most mis-adjusted item of GM fuel injection ... I agree with your diagnosis

yeah dude post that bad boy. Thats good info.

As far as the mechanics, i wouldnt use them. Humans make mistakes and tend to make 55 times more mistakes when it comes to taking my money. Screw that ill take a class and fix it myself.

Hey I, for some reason, am just reading this. Can you pass this along. That would be great.

Thanks

you don't have an IAC (Idle Air Control solenoid) on your car, it uses an ISC (Idle Speed Control motor) and they are notorious for going bad. They are also heavily dependant on a "key" adjustment called "min-air" which involves adjusting both the tip of the ISC plunger, the TPS, and the throttle stop screw. If you're not familiar with the ISC and it's adjustments etc, let me know, I have the whole procedure on PDF available to anyone who needs it.

Would that procedure apply to most GM FI systems? My friend has a 92 Skylark with a very high idle condition (~1500-1800 in gear) and if some adjustment/replacement of parts will fix it that would be nice.