v8inmk2

Hi, guys, My 89 Corvette 6-speed car already 1000 mile after stock engine rebuild, it runs ok at first, but after a 300 miles road trip, the idle raise to 1300-1500, I already replace the IAC and adjust IAC and TPS. it runs perfect first time after adjust, idle is at 750, then I run for 30 mins, and turn the engine off. the second day, when I start the engine, the idle came to 1500 again. after I drive it for couple minutes the and reach the normal temp, the idle still really high.

And I don't know what should I do now, after high idle, car runs good. but I don't want to drive a 1500rpm idle manual car. So is there any solution for the high idle problem? Thanks!

BTW, the last owner put a stage 2 chip in the car, will that be the problem?

aklim

How did you adjust the idle? Using the FSM method or using the scanner by IAC counts? When you say you replaced the TPS and IAC, it makes me wonder why you did that. Did you clean the Throttle Body? I usually clean it off the car. I take it off, take the IAC off, take the IAC housing off and the cover plate off. Clean all passages and reassemble with new gaskets. Then I set the idle via the IAC counts. In your case, you have to also set the TPS. Is the timing right?

Do you have a scanner and an infra red thermometer handy?

Those aftermarket junk chips are an unknown quantity. Do you happen to have the stock chip?

v8inmk2

When I have the high idle problem first time, I drive my car to a local shop, and they told me I need a new IAC, and they replaced it, then they tell me it fixed. But actually not, then I followed the a thread "how to adjust Iac and TPS on early c4" to set idle. And it works at first time, I got a nice idle, drives perfect, just like it should, but next time I start the engine, high idle again. I didn't have a scanner, but when I ground the pin A and.B, I found there is no code. I didn't check the timing yet, I will check it tomorrow

BTW, I don't have a original chip, but I will get one and throw these aftermarket chip away

aklim

Check timing or we can be chasing our tail all day long. You are going to need a scanner or it will be a long day. We need to see what the ECM sees and not what you think it should see.

Gunk builds up in the TB. Take it out and apart and wash it in brake cleaner. All the passages and reassemble again with new gaskets. You don't reuse condoms so use new gaskets.

Turn the adjustment screw out and when you maxed it out, if the IAC is still high, we have a leak somewhere. You want it to be at 20 to 30 counts with everything but the motor off. Rev and see what it does. Adjust and repeat.

Also check that the4 CTS is transmitting the right information to the ECM

v8inmk2

Cool! Thanks for these info, but what kind of scanner should I buy? And what is the4 CTS? I will clean the TB first.

aklim

SNAP ON MT2500 or any other that can read output. Just have to play aropund a bit. The coolant temp sensor is another issue. Beam it so you know what it should be, See what the ECM thinks it should be. Compare your thermometer vs what the ECM syas it shoudl be reading.