LT5 missing, runs rough when hot
07-31-2004, 08:09 PM
#1
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LT5 missing, runs rough when hot
Any suggestions out there. I just changed the plugs only to find I still have the same problem. From first start up and things are warm ( not hot ) the engine performs just fine. Once things have heated up I experience the engine missing or stumbling in both the normal and full power position. Are there possible vacuum leaks or air intake problems.
I have had it backfire from first start up a couple of times. When it does the hose just undernaeth the MAP sensor blows off. Any help will be appreciated. Thanks Jeff
I have had it backfire from first start up a couple of times. When it does the hose just undernaeth the MAP sensor blows off. Any help will be appreciated. Thanks Jeff
07-31-2004, 09:55 PM
#2
Safety Car
This may sound kindof dumb but try this first Start car up let it warm up til the peoblems start then go to a very dark dark area make sure all lights are off and look under your hood for any arcing sparks from the wires or plugs. I kindof had the same thing and found out after puting in about 3 grand of stuff (injectors, coils, fuel pumps) that the problem was just a arcing spark plug. I hope this will help you find it.
08-01-2004, 07:26 AM
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Originally Posted by jhong321
This may sound kindof dumb but try this first Start car up let it warm up til the peoblems start then go to a very dark dark area make sure all lights are off and look under your hood for any arcing sparks from the wires or plugs. I kindof had the same thing and found out after puting in about 3 grand of stuff (injectors, coils, fuel pumps) that the problem was just a arcing spark plug. I hope this will help you find it.
08-01-2004, 07:28 AM
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Originally Posted by 90 Corvette ZR-1
Does the car sit alot?
The car gets plenty of use and when I take it out, there are quite a few roads that I am able to open it up on.
08-01-2004, 01:45 PM
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Originally Posted by Jeff Begor
The car gets plenty of use and when I take it out, there are quite a few roads that I am able to open it up on.
08-02-2004, 04:53 AM
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Hey guys - Finally resolved the problem which had to do with spark plug gap. I did purchase the correct plugs. My maintenance manual called for .050 gap ( for a plug orginally called for in 1990 ). Furhter investigation out on AC Delco found that the new plugs needed to be .042 as opposed to .050..Big difference with the proper gap...
07-29-2017, 02:00 PM
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Having the same problem with my 90.
Hey guys - Finally resolved the problem which had to do with spark plug gap. I did purchase the correct plugs. My maintenance manual called for .050 gap ( for a plug orginally called for in 1990 ). Furhter investigation out on AC Delco found that the new plugs needed to be .042 as opposed to .050..Big difference with the proper gap...
07-29-2017, 09:59 PM
#11
Race Director
So you changed the plugs after initially having an issue with the car running rough after it got hot ? I'm having that same issue, and have gone through all the vacuum leak scenarios and eliminated them. Just curios if you noticed anything wrong with the plugs that were initially removed, fouling etc ? My car has 15k on it and has just started doing this.
07-30-2017, 08:32 AM
#12
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I ain't ready to buy 0.008" of plug gap (alone) was the make/break for the issue. The fact it was running rough before installing new plugs, could be the new (clean) plugs have no carbon tracking (yet) and for now are performing better than the old ones.
"After it warms up" it ran poorly mitigates the plug issue somewhat. After it warms up, the ECM switches OFF the secondary fuel pump. A failing primary pump could cause fuel pressure to drop from mid 40 psi to below 40 which typically results in misfire AND backfires - resulting in the MAP hose being blown off - which you are apparently familiar with...
I'd want to consider testing the fuel pumps current draw (using the pump diagnostic connector, the red wire jutting out of the wire loom, next to the wiper motor).
If it is a weak primary pump, the symptoms would crop up (I would think) quite suddenly: motor running fine one moment, and missing/stuttering/backfireing the next. However, if it s a gradual condition, then it smells like electrical: injectors, coils, etc.
Anywayz... I don't believe the plug gap was the ticket
. I'd not want to leave it there, but do some more checking to be sure. One way to prove the spark plug theory: change the gap back to .050 and see if the problem returns! (I'll bet a ham sandwich it doesn't - as long as the plugs were good at 0.042.
) But, in any case, you do want to know if the fuel pressure is as it should be. Otherwise a lean condition could burn a hole in a piston, or burn a valve (ask me how I know..).
Just sayin.
"After it warms up" it ran poorly mitigates the plug issue somewhat. After it warms up, the ECM switches OFF the secondary fuel pump. A failing primary pump could cause fuel pressure to drop from mid 40 psi to below 40 which typically results in misfire AND backfires - resulting in the MAP hose being blown off - which you are apparently familiar with...
I'd want to consider testing the fuel pumps current draw (using the pump diagnostic connector, the red wire jutting out of the wire loom, next to the wiper motor).
If it is a weak primary pump, the symptoms would crop up (I would think) quite suddenly: motor running fine one moment, and missing/stuttering/backfireing the next. However, if it s a gradual condition, then it smells like electrical: injectors, coils, etc.
Anywayz... I don't believe the plug gap was the ticket
. I'd not want to leave it there, but do some more checking to be sure. One way to prove the spark plug theory: change the gap back to .050 and see if the problem returns! (I'll bet a ham sandwich it doesn't - as long as the plugs were good at 0.042.) But, in any case, you do want to know if the fuel pressure is as it should be. Otherwise a lean condition could burn a hole in a piston, or burn a valve (ask me how I know..).Just sayin.
Last edited by Paul Workman; 07-30-2017 at 08:38 AM.
08-16-2017, 09:39 AM
#13
Le Mans Master
Any suggestions out there. I just changed the plugs only to find I still have the same problem. From first start up and things are warm ( not hot ) the engine performs just fine. Once things have heated up I experience the engine missing or stumbling in both the normal and full power position. Are there possible vacuum leaks or air intake problems.
I have had it backfire from first start up a couple of times. When it does the hose just undernaeth the MAP sensor blows off. Any help will be appreciated. Thanks Jeff
I have had it backfire from first start up a couple of times. When it does the hose just undernaeth the MAP sensor blows off. Any help will be appreciated. Thanks Jeff
08-20-2017, 12:43 PM
#14
Drifting
As Paul noted, fuel pump is always suspect. If all else checks out, as others have suggested, I would also second the injectors as a possible cause. Even if injectors test out electrically OK when cold, they sometime change resistance when they are hot (and should be checked then as well.) Just a thought. --Bob
