erratic idle
Thread Starter
2nd Gear
Joined: May 2017
Posts: 2
Likes: 0
From: Wolfeboro New Hampshire
erratic idle
I have a 1989 C4 corvette and when I start the car the rpm's vary. It takes a while to idle down to 700 rpm. When I drive and come to a stop the idle stays high for a little bit maybe 5 seconds so I have to make sure I hold the brakes until it idles down to 700 or so. When I shut the engine off and start after only a few minutes it will race up high again and I wait for it to idle down before I proceed. So idle varies and is dangerous
Le Mans Master
Joined: Oct 2004
Posts: 6,314
Likes: 508
From: South-central Missouri
My 90 ZR-1 uses the same basic ECM, IIRC, and your characteristics sound similar to a 90 Z: At start-up, the idle will run up to 1800 or so for a second or two before ramping down to 850. And, after coming down to stop from highway speed, the idle is a high - maybe 1500 for again - a couple or 3 seconds before coming back to idle. Furthermore, if for any reason the engine stalls while I'm rolling and I re-start it, the car has to come to a complete stop before the idle finally settle down.
I'm told it is typical for a 90 ECM. My wife's 91 has NONE of those "features". In certain circumstances, it can be annoying...VERY annoying in fact. But, it has always done that for the 10 years I've owned it. So, I accept that's the way is is. But, I do make an effort to avoid stalling the motor while rolling - or having to immediately move the car the instant the engine fires.
Misery loves company, I guess. No biggie. Just thought I' share in case you're experiencing the same things.
I'm told it is typical for a 90 ECM. My wife's 91 has NONE of those "features". In certain circumstances, it can be annoying...VERY annoying in fact. But, it has always done that for the 10 years I've owned it. So, I accept that's the way is is. But, I do make an effort to avoid stalling the motor while rolling - or having to immediately move the car the instant the engine fires.
Misery loves company, I guess. No biggie. Just thought I' share in case you're experiencing the same things.
Pro
Joined: May 2012
Posts: 520
Likes: 9
my 89 idle is up and down when cold for the first minute or two why I have no idea why can't seem to fix it so I live with it. I had the same problem as you when coming to a stop .I installed a new throttle body and this went away, I believe the bushings were warn in mine.
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
I have a 1989 C4 corvette and when I start the car the rpm's vary. It takes a while to idle down to 700 rpm. When I drive and come to a stop the idle stays high for a little bit maybe 5 seconds so I have to make sure I hold the brakes until it idles down to 700 or so. When I shut the engine off and start after only a few minutes it will race up high again and I wait for it to idle down before I proceed. So idle varies and is dangerous
Last edited by billschroeder5842; Jul 27, 2017 at 12:11 PM.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
I looked at the ECM code and it sets the startup idle to 1200 RPM (in my car). That then decays down to the specified idle as the coolant warms up. When the coolant is hot, the decay to lower idle speed is very fast (maybe a second).
When you're driving the IAC goes into "throttle follower mode". That means that the IAC raises the idle as the gas pedal is mashed down. The purpose of this is to keep the engine from dying if you take your foot off the gas and the throttle plates slam shut. Sometimes it takes a second or two to get down to the lower idle speed. This is especially true for manual transmission cars. The ECM watches the vehicle speed and idles down the IAC as the speed decreases. It should go down to idle speed at zero MPH.
When you're driving the IAC goes into "throttle follower mode". That means that the IAC raises the idle as the gas pedal is mashed down. The purpose of this is to keep the engine from dying if you take your foot off the gas and the throttle plates slam shut. Sometimes it takes a second or two to get down to the lower idle speed. This is especially true for manual transmission cars. The ECM watches the vehicle speed and idles down the IAC as the speed decreases. It should go down to idle speed at zero MPH.
