'92 LT1 Rough Idle issue
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
Likes: 1
From: Mariposa, CA
'92 LT1 Rough Idle issue
I have a '92 with 140K miles. Recently the idle has become erratic. Seems to be worse after the car has warmed up. When in drive ( auto trans.) it can drop under 400 rpm and the engine will sometimes stall, but will start back up. Also seems to surge sometimes when in drive with foot on the brake, trying to increase the rpms...... Ant ideas as to possible or likely causes based on this vintage LT1 ? Thanks !!!!
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Possible IAC motor is defective.
The opti can cause this effect as well. It may also cause the tach to bounce around at idle.
When in high gear and accelerating, does it buck or kick after warm up. This is a opti symptom.
The opti can cause this effect as well. It may also cause the tach to bounce around at idle.
When in high gear and accelerating, does it buck or kick after warm up. This is a opti symptom.
Instructor
Joined: Feb 2008
Posts: 170
Likes: 8
Bad gas?
I just experienced the same symptom with my 93, and it turned out to be bad gas (due to age). I could not get past 3rd gear before it bucked. I syphoned it all out, and dumped it into my 1954 John Deere which made the tractor run better. It totally cleared up with a 5 gallon can of premium 93 octane, but I still had to use the 10% ethanol.
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
Likes: 1
From: Mariposa, CA
Some more info ... No lights lit up on dash, no check engine or anything. Runs fine on the highway. I did have to put some regular gas in it a week ago, maybe 4 gal,,, since station was out of premium,,, but I filled it with premium at another station right after that and it ran fine, so I don't think its gas related... that would be too easy !!!!
Where is the ICM located [ that's ignition control module, right ?] and how expensive and hard to change is it ??
What is the IAC motor and what does it do ? How do you test it ?
OPTI is the distributor assembly ??? How is that tested ?
Sorry for the basic questions but I am new to this car,, had a '95 Z28 LT1 and never had any issues.
These modern electronic nightmares always worry me .... My '71 Chevelle 454 never gives me any grief .... I have had that ride for 15 years with minor issues that I have easily fixed..... easy to work on !!!!
Thanks,,,,
Where is the ICM located [ that's ignition control module, right ?] and how expensive and hard to change is it ??
What is the IAC motor and what does it do ? How do you test it ?
OPTI is the distributor assembly ??? How is that tested ?
Sorry for the basic questions but I am new to this car,, had a '95 Z28 LT1 and never had any issues.
These modern electronic nightmares always worry me .... My '71 Chevelle 454 never gives me any grief .... I have had that ride for 15 years with minor issues that I have easily fixed..... easy to work on !!!!
Thanks,,,,
Racer
Joined: Jun 2011
Posts: 283
Likes: 5
Try cleaning your IAC and the housing. They're located under the throttle body. You could also take off the throttle body and clean it also. If the IAC is really dirty, a new one is about $100, but try cleaning it first with throttle body cleaner. You are going to have to reset the IAC also. Good luck.
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
Likes: 1
From: Mariposa, CA
Engine has a new IAC valve, ICM and coil. Opti was replaced about 30K miles ago, and that was 10 yrs ago.... Idle is fine up until about 20 minutes after starting and engine has warmed up... Then the idle starts to bounce between 500 and about 1000 rpm, but runs smooth, doesn't misfire.... If in Drive, it will try to surge to 800 or 1000 rpm and you have to keep good pressure on the braked to stay put !
Finally, when these conditions are present it will sometimes stall when taking off from a stop, not always ...... but starts back up. When these low rpm problems are present, if you get on the highway and cruise at 60 to 70 mph,, runs fine ......
Any ideas ? Could a problem with the EGR system cause something like this...
Finally, when these conditions are present it will sometimes stall when taking off from a stop, not always ...... but starts back up. When these low rpm problems are present, if you get on the highway and cruise at 60 to 70 mph,, runs fine ......
Any ideas ? Could a problem with the EGR system cause something like this...
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
A check of the EGR when cold or slightly warm and pushing on the valve should give you an idea if the response is correct when it closes.
If it does it when it's warm, I would like to get a reading on fuel pressure at the time of the occurrence.
There are a few things you can still check and see if all is good. But I would not take the opti off the list yet.
If it does it when it's warm, I would like to get a reading on fuel pressure at the time of the occurrence.
There are a few things you can still check and see if all is good. But I would not take the opti off the list yet.
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