1992 has Fluctuating Idle problem..
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Instructor
Joined: Jun 2012
Posts: 111
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From: Mariposa, CA
1992 has Fluctuating Idle problem..
This is NOT a rough idle problem .....Engine has a new IAC valve, ICM and coil and PCV valve and hoses. 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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
Any ideas ? Could a problem with the EGR system cause something like this...
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
Likes: 1
From: Mariposa, CA
This is NOT a rough idle problem .....Engine has a new IAC valve, ICM and coil and PCV valve and hoses. 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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
Any ideas ? Could a problem with the EGR system cause something like this...
Module 1 H72 and H74
Module 4 --- None ---
Module 9 H83
1.0
Can anyone translate those for me ??? And maybe shed some light on what I should think about testing first to fix this idle surging issue ..... Thanks....
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Joined: Apr 2002
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From: Anaheim CA
Last edited by Cliff Harris; Dec 30, 2014 at 12:45 AM.
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
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From: Mariposa, CA
Codes don't help
Thread Starter
Instructor
Joined: Jun 2012
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From: Mariposa, CA
Vacuum leak
BPC9MM, That's what I am thinking.. took a look for an obvious leak but could not spot anything. Because it only surges or loops after warm up I was thinking EGR since the solenoid does not send vacuum to the EGR valve until after warm up,, or that's what I have been told ... One diagnostics book I have suggested it could be the TPS messing up and the effect would only be happening after warm up and system goes closed loop...
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
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From: Mariposa, CA
Yes, the engine running fine until it warms up is whats throwing me off thinking its the Optispark. I will have to try to diagnose whether its the Opti or not before I go that route...... that's about a $450 part, right ????
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Race Director
Joined: Apr 2002
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From: Anaheim CA
One thing that can cause this symptom is a problem with the ignition module, especially if it doesn't have thermal grease under it. It gets very hot and will go into thermal shutdown if it overheats. When it cools down it recovers and starts working again.
Intermediate
Joined: Feb 2011
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From: Eureka Calif.
check ECM ground
BPC9MM, That's what I am thinking.. took a look for an obvious leak but could not spot anything. Because it only surges or loops after warm up I was thinking EGR since the solenoid does not send vacuum to the EGR valve until after warm up,, or that's what I have been told ... One diagnostics book I have suggested it could be the TPS messing up and the effect would only be happening after warm up and system goes closed loop...
Thread Starter
Instructor
Joined: Jun 2012
Posts: 111
Likes: 1
From: Mariposa, CA
Thanks Cliff Harris and BPC9MM ..... I am away and not able to do anything on the Vette this week but will check out those two items as soon as I can. The ignition module was put on by someone else, so a check of whether or not the thermal grease is there is a good thing to do ..... One question on that... from your experience, if I take it off and there is NO thermal grease there... does that mean that the module is toast and should be replaced with a new one,, or do you think just putting some grease there and bolting the existing one back on will work ? Just planning ahead.. not sure how much that part costs. Doesn't look like much when you look at it .... Thanks !
Race Director
Joined: Apr 2002
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From: Anaheim CA
Thanks Cliff Harris and BPC9MM ..... I am away and not able to do anything on the Vette this week but will check out those two items as soon as I can. The ignition module was put on by someone else, so a check of whether or not the thermal grease is there is a good thing to do ..... One question on that... from your experience, if I take it off and there is NO thermal grease there... does that mean that the module is toast and should be replaced with a new one,, or do you think just putting some grease there and bolting the existing one back on will work ? Just planning ahead.. not sure how much that part costs. Doesn't look like much when you look at it .... Thanks !
The ignition module has a fail-safe circuit inside it that will cause it to shut down if it gets hot. The symptoms are that the car runs OK when cold but shuts off when hot. This will not normally damage the ignition module. You can have the ignition module tested at most auto parts stores. The test should be done several times to make sure it's OK when it heats up.
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Instructor
Joined: Jun 2012
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From: Mariposa, CA
Haven't had much time to work on the Vette lately .... I did take a short trip in in the other day, long enough to get it nice and warmed up.... Amazingly, it ran fine ... no idle speed up and slow down,,, even after I let it sit and idle for about 10 minutes. I did have the check engine light come on .... I checked for codes .. I now have a 43 in Module 4 ....... Knock sensor circuit ?? But the idling probelm has temporarily gone away ..... Could a bad Knock sensor cause idle speed to cycling between 500 and 1500 rpm or thereabouts ?? Where are these mounted ?? And how difficult is the replacement procedure ??
Thanks ............
Thanks ............
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Joined: Apr 2013
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From: New Port Richey Florida
This is NOT a rough idle problem .....Engine has a new IAC valve, ICM and coil and PCV valve and hoses. 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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
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 ......
Would a bad Opti cause fluctuating BUT smooth idle issues ?
Any ideas ? Could a problem with the EGR system cause something like this...