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91 intermittent rough running

#1
Hi,

any clues what this might be, no codes thrown.

Starts up and idles at the turn of the key, runs fine oil pressure steady. (started 1st turn after spending 5 months untouched since October last year)

As the engine warms up it begins to run less smoothly, the oil pressure gauge rises up against the stop and the needle bounces around, engine hunts and runs unevenly, then back to smooth running, oil pressure gauge drops back steady, centre of the gauge, while it is rough running the rev counter drops to zero, it always reads low anyway.

If I accelerate/drive it, it runs fine, oil pressure gauge steady and rev counter rising and falling as I accelerate/decelerate., but stop at the lights, on tick over, runs rough then smooth then rough.

There is also an interference/buzzing on the speakers which isn't there when the engine is running up from cold.

Not sure how all these are related, just passed its MOT, emissions are low so running fine most of the time.

I suspect an electrical glitch, but where do I start looking, any suggestions ?

Some sort of electrical glitch, breaking down when hot, but not all the time

suggestions/experience/words of wisdom greatly appreciated.

pete
 
#3
Thanks,

not dumb at all, I know the answers to most of the points, and know that they are extremely unlikely because of the replacement bits and checks that have been done over recent time.

It is the intermittent nature of the rough running, it starts and runs fine, under power it pulls like a train, its emissions are low, fuel consumption not excessive(for a corvette) and the fact that when it starts running rough the oil pressure gauge dances and rev counter drops which points to some sort of electrical gremlin, maybe insulation failing on a wire somewhere when it gets hot or a sensor breaking down, but then it returns to normal for a while.
 

teamzr1

Well-known user
#4
Have you checked out the alternator as to outputting correct voltage when at idle
Also the belt to pulley to see at low RPMs is slipping ?
Possible battery has a weak cell effecting voltage at low RPMs, can you check that or use a battery
load tester to test it ?

How old is the gas in tank, as if too old would effect low engine RPMs
Possible fuel injectors are gummed up a bit, need cleaning

You own or have access to a OBD-I scanner to look at feedback sensors such as O2 sensors, fuel AFR, voltage reading, etc ?
 

FelixP

CCCUK Member
#6
Yeah, if it gets worse when it's warm then I'd say electrical or ignition problem. Have you changed the ICM under the dizzy cap before? When was it last serviced? If it was a while ago, I'd do plugs, leads, cap, rotor, coil and ICM.
 
#7
Thanks for all the replies and suggestions,

Pretty much everything has in last few years been replaced but not the Coil or ICM so I will try those next.

Haven't replaced the injectors but I run it on Super, regularly add injector cleaner and also some Xylene which ups the octane and also cleans the injectors, I did check resistance readings and they were all well within expected readings.

The voltage output reads high on the dash gauge and the serpentine was replaced recently so is also not slipping.

The bouncing oil pressure gauge and rev counter, which happen when it starts rough running, so I still think that its electrical, maybe the coil and ICM will sort it.

Thanks again
 
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