3.4 and what I think is a misfire... - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-17-2007, 12:21 AM   #1
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Default 3.4 and what I think is a misfire...

So I have installed a 3.4 MPS and Autolite 104'* @ 0.55.

I seem to have this misfire that is getting more apparent. I notice a popping in the exhaust and a little bit of jerking randomly after 3000 RPMs. It has not thrown any sort of code.

Tonight I just tanked up with 94 octane instead of the usual 91. It killed my KR by a good amount (peak 3.5 degs @ 90 F IAT) whereas it used to be 5-7 degrees at that temperature. The weird thing is, it made this misfire a little worse.

Is this a misfire I am dealing with? I did notice plug wire # 6 was touching the O2. I will reroute that tomorrow and see if it helps, but is there anything else you can think of?

Thanks
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Old 07-17-2007, 01:04 AM   #2
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are you sure its poping from the exhaust, or could it be coming from the intake
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Old 07-17-2007, 01:05 AM   #3
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Pull the plugs and check them out. I've tried using Autolites but they don't seem to last quite as long as NGKs, but then again you haven't had the 3.4" on very long either.

Ed
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Old 07-17-2007, 03:41 AM   #4
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Have you checked your fuel pressure? I'd start looking there after you move that plug wire.
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Old 07-17-2007, 07:46 AM   #5
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Quote:
Originally Posted by b18jason
are you sure its poping from the exhaust, or could it be coming from the intake
This isn't overly likely.. The Supercharger is int he way.
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Old 07-17-2007, 09:13 AM   #6
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3.5 degrees of KR on pump gas is too much, much less 5-7. 3-5 degrees on 94 octane is also too much. Go back to a 3.5" pulley.

If you think you're misfiring, get a scantool on the car and see.
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Old 07-18-2007, 01:44 AM   #7
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So this is what I have figured out.

The misfire is causing the KR. The misfire is happening only at running temperature.

When I left work tonight, I did a WOT run a minute after the car had been started. I had 0.5 degrees of KR the whole time, no sign of a misfire, and insane amount of power.

Then 10 minutes later, I did another WOT run (intake air temp was the same), and I had massive KR and a stumble that could be felt and heard through the exhaust.

The only difference between the two runs is the fact the car was at operating temp.

What is going on here?
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Old 07-18-2007, 01:46 AM   #8
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When your car isn't warmed up, it over-fuels to get it there. That'* why you don't have KR.

If you have more than 1-2 degrees of KR AFTER you're warmed up, you need to FIX the KR with something other than the normal octane fuel you run daily. This means exhaust or valvetrain mods, or Pully UP.
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Old 07-18-2007, 01:55 AM   #9
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I have been talking with DHP about getting a PCM for the '96. A new PCM should help kill KR correct?

Other then that looks like I will pulley up for the time being...is the misfire an isolated issue or is this part of the too much KR thing?
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Old 07-18-2007, 01:57 AM   #10
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KR is fixed mechanically by preference.

You need to do as much of that as possible. Tuning it out is a last resort ALWAYS.

Troubleshoot the misfire as you would on any other car.
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