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Gary
Gary, ASE Certified Technician
Category: Chevy
Satisfied Customers: 878
Experience:  Senior Technician
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1996 impala ss cylinder 6 misfire

Customer Question

1996 impala ss cylinder 6 misfire
Submitted: 1 year ago.
Category: Chevy
Expert:  Tool Hut replied 1 year ago.

Hi, My Name is ***** ***** for posting your question to just answer. I am a independent contractor for just answer, and ASE Certified Master Auto Technician with 20 years experience. I am confident we can resolve your problem together.Please remember I don't know your skill level. I can't see, smell, hear, or touch the vehicle. It may take a few back and forth replies to get to a satisfactory resolution.There will probably be some equipment needed for a proper diagnosis. If you are not comfortable with this or do not have any test equipment please let me know ASAP. I also try to give all of the information you may need expecting a positive rating. I am not compensated for my time until you leave a positive rating. Your rating should be based on the service you received.

Do you have a vacuum gauge?

Customer: replied 1 year ago.

we was working on this a few weeks ago and i gave up to finish my jeep. no i am back to finish.

i already did vacuum, compression, leak down, and switched injectors

Expert:  Tool Hut replied 1 year ago.

What is your vacuum reading and is it steady?

Expert:  Tool Hut replied 1 year ago.

Do you have a noid light to confirm that the PCM is opening injector?

Customer: replied 1 year ago.

15psi steady and yes checked noid light on injector and spark plug

Expert:  Tool Hut replied 1 year ago.

MY suspicion is that it is mechanical in nature.

Customer: replied 1 year ago.

2 things

it only happens after car runs for 5-10 min

ant the last thing i did before i got this code was take timing cover off to change gasket and seals

Expert:  JOHN aka Eauto replied 1 year ago.

has a compression or leak down test been performed?

Customer: replied 1 year ago.

both and good

Customer: replied 1 year ago.

yes

Expert:  Gary replied 1 year ago.

Hello Sal,

Does your engine have AFM or DOD, V8 runs in 4 cylinder mode?

If so, cylinder 6 is an AFM cylinder and probably has a rotated valve lifter.

Gary

Customer: replied 1 year ago.

they do that in 1996?

Expert:  Gary replied 1 year ago.

Not sure if your has it, but you have exhausted all other testing paths.

Compression will test average, valves operate while cranking.

Can you run the engine with the valve cover off please?

You are the second customer I have had today with same problem.

Gary

Expert:  Gary replied 1 year ago.

GM started this concept in 1981.

Gary

Customer: replied 1 year ago.

i just took it for a ride to get code to come back on and do leak down test while hot.

when i hook air up and cran engine over by hand there is no air coming out either valve??

now code is random misfire

will take valve cover off tomorrow

Expert:  Gary replied 1 year ago.

Hi Sal,

So you had a P0306 (Cylinder 6 misfire) and now you have P0300 (Engine misfire detected).

Your engine is a V8, isn't it?

This code was not present prior to taking the timing cover off, is that correct?

Did you drop some gasket residue down the intake ports by accident?

Gary

Customer: replied 1 year ago.

yep yep.

how bout opti vent check valve being clogged

Expert:  Gary replied 1 year ago.

Is the P0306 definitely gone now, and is only a P0300?

I would say you possibly had some residue under some valve seats and it is clearing out now, it may come good.

Did you have the intake manifold off as well as the timing cover?

Gary

Customer: replied 1 year ago.

No. I believe that check valve closer to the optispark I put on backwards I just drove it around for 20 minutes no check engine light so far

Expert:  Gary replied 1 year ago.

Ok, it may have been a combination of things, but ultimately was still a vacuum leak, hence the last code of P0300, versus the cylinder 6 specific of P0306.

Great you found that with the check valve.

Is it a good system, but fraught with problems as they age.

Hope you all good now.

Go out and give her a good run now.

Hopefully good from cold start as well.

Gary

Customer: replied 1 year ago.

well i was wrong went to drive to work today after 5 min p0300.

left it home all day cleared codes let it run til code came back now p0306

idling between 620-680rpm

any chance new opti is damaged or i ruined it

Expert:  Gary replied 1 year ago.

Hi Sal,

If this only happens once it warms up, does the misfire stay there while it is idling?

We need to be sure whether the injector pulse is dropping out, the spark is dropping out, or it does have a valve not operating.

What does the spark plug look like after it is has been missing like this?

Gary

Customer: replied 1 year ago.

black.

exhaust is sucking in air on both sides. valves?

Expert:  Gary replied 1 year ago.

Yes, sounds like exhaust valve not seating.

Black from fuel, but poor combustion due to lack of compression.

Customer: replied 1 year ago.

taking a break from this for a few days to finish my jeep.

then i will resume the repair

Expert:  Gary replied 1 year ago.

Ok.

I hope my advice has been of assistance to you.

Wishing you all the best.

Regards

Gary