C4 high and surging idle
Thread Starter
1st Gear
Joined: Feb 2023
Posts: 1
Likes: 0
C4 high and surging idle
I’ve got a 1990. No engine light, no codes. It’s got a high idle which usually sits at about 1600 rpm, sometimes at 2000 though. Sometimes it will surge a little bit between 1600-2000 rpm. Never idles below 1600. IAC valve has been checked and looks new/clean. Replaced the coolant temp sensor underneath intake which didn’t change anything. Really stuck, any suggestions?
Racer
Joined: Mar 2020
Posts: 387
Likes: 148
From: Langton, Ontario, Canada
2025 C4 of the Year Finalist - Unmodified
Not sure if this will help you with your 1990, but it worked for me and my 85 Hoping some parts of it will apply.
https://www.corvetteforum.com/forums...dle-speed.html
https://www.corvetteforum.com/forums...dle-speed.html
Racer
Joined: Feb 2004
Posts: 427
Likes: 86
From: Rockwall TX
Check the throttle position sensor, located on the side of the throttle body near the ac compressor. Once I left that unplugged and then had a 2000rpm idle as you describe, plugged it back in and the idle normalized. Verify the wires and connection, replace it. 
The Lars article that was referenced, I think does not apply to 90.
The Lars article that was referenced, I think does not apply to 90.
Oil Producer
Joined: Apr 2014
Posts: 9,373
Likes: 2,736
First, go through the minimum idle set procedure as linked above. Advise inspecting your tps while you are there with a resistance meter attached. ensure no bad spots.
if that doesn't fix it. Go through the min idle set procedure again to ensure you did it correctly.
if that doesn't work, you have a vac leak. Most of the time, you don't get lucky finding a leak, but you need to try. Tape over the throttle body. Grab a cigar, remove the vacuum hose from the brake booster. Blow smoke hard into the vacuum hose. This is the best/easiest smoke machine i have found.
Have a friend help you with a flashlight looking for tendrils of smoke escaping. I have fouind doing this in the dark, with a flashlight, helps detect the smoke tendrils. You have to blow fairly hard to pinpoint.
Usual suspects are: gaskets, pcv valve hose to intake manifold, throttle body bushing at hinge.
the one i found for my 1990, was the darn IAC was the incorrect part number for the 90. There is a thread on here somewhere I documented it. The pintle shape is slightly different. Even with the pintle fully closed, it was not wide enough to plug the IAC port. Hence, vac leak.
scanning the car to see where things like the IAC and TPS are at helps but you can get through it without.
if that doesn't fix it. Go through the min idle set procedure again to ensure you did it correctly.
if that doesn't work, you have a vac leak. Most of the time, you don't get lucky finding a leak, but you need to try. Tape over the throttle body. Grab a cigar, remove the vacuum hose from the brake booster. Blow smoke hard into the vacuum hose. This is the best/easiest smoke machine i have found.
Have a friend help you with a flashlight looking for tendrils of smoke escaping. I have fouind doing this in the dark, with a flashlight, helps detect the smoke tendrils. You have to blow fairly hard to pinpoint.
Usual suspects are: gaskets, pcv valve hose to intake manifold, throttle body bushing at hinge.
the one i found for my 1990, was the darn IAC was the incorrect part number for the 90. There is a thread on here somewhere I documented it. The pintle shape is slightly different. Even with the pintle fully closed, it was not wide enough to plug the IAC port. Hence, vac leak.
scanning the car to see where things like the IAC and TPS are at helps but you can get through it without.
