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Old 08-27-2008, 10:01 PM   #1
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Question 97 Grand Prix with Intermittent Misfire

Hello Guys,

I hope it is alright to post here, I realize I dont have a Bonneville
but I do have 97 Grand Prix GTP with a L67 3800 supercharged
engine that has issues, and you guys seem to know alot about
superchargers. Here is my problem, about 2-3 months ago the car
started misfiring under heavy acceleration. I scanned the engine and
came up with number 4 cylinder misfire. it appears that this is the only
cylinder misfiring. before taking the car to Pontiac, I changed the plugs,
wires,ignition coils and the the ignition control module. No help, still misfire
present. Took car to Pontiac where they scanned everything, had the car
for 3 hours and they could not figure the problem out. They ran an injector
balance test, ran a compression check all which passed with flying colors.
So I started more troubleshooting, cleaned the throttle body, cleaned the
IAC valve and cleaned the MAF sensor with maf sensor cleaner. Also changed
the air filter and removed and cleaned the EGR valve which appears to be
functioning correctly. Cleaned the PCV valve as well and still no help. The
car is not giving up any codes in memory either. No stored fault codes. I
checked the fuel pressure and it appears to be spot on with the service
manual specs. I also changed the number 4 fuel injector with no help, misfire
still present. Pontiac also informed that the intake manifold gasket was ok and
not leaking. So here is where it gets interesting, I removed the supercharger
belt and the high speed fuel relay for the fuel pump and the misfire is "GONE"
period !!! So what could cause a missfire with supercharger running as opposed
to not running. Any help would be greatly appreciated.

Kind Regards,

Steve

Last edited by srlash; 08-27-2008 at 10:02 PM.
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Old 09-01-2008, 04:48 PM   #2
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Question Bump

Quote:
Originally Posted by srlash View Post
Hello Guys,

I hope it is alright to post here, I realize I dont have a Bonneville
but I do have 97 Grand Prix GTP with a L67 3800 supercharged
engine that has issues, and you guys seem to know alot about
superchargers. Here is my problem, about 2-3 months ago the car
started misfiring under heavy acceleration. I scanned the engine and
came up with number 4 cylinder misfire. it appears that this is the only
cylinder misfiring. before taking the car to Pontiac, I changed the plugs,
wires,ignition coils and the the ignition control module. No help, still misfire
present. Took car to Pontiac where they scanned everything, had the car
for 3 hours and they could not figure the problem out. They ran an injector
balance test, ran a compression check all which passed with flying colors.
So I started more troubleshooting, cleaned the throttle body, cleaned the
IAC valve and cleaned the MAF sensor with maf sensor cleaner. Also changed
the air filter and removed and cleaned the EGR valve which appears to be
functioning correctly. Cleaned the PCV valve as well and still no help. The
car is not giving up any codes in memory either. No stored fault codes. I
checked the fuel pressure and it appears to be spot on with the service
manual specs. I also changed the number 4 fuel injector with no help, misfire
still present. Pontiac also informed that the intake manifold gasket was ok and
not leaking. So here is where it gets interesting, I removed the supercharger
belt and the high speed fuel relay for the fuel pump and the misfire is "GONE"
period !!! So what could cause a missfire with supercharger running as opposed
to not running. Any help would be greatly appreciated.

Kind Regards,

Steve
Hey Guys, someone talk to me, tell me top drop dead, go to hell or
whatever. I see 22 people read my diatribe and no comments or suggestions
simply amazing !!!

No Regards,

Steve
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Old 09-02-2008, 09:25 AM   #3
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Steve...You probably didn't get any responses because you have a "good" problem and have tried all the usual suspects.

On the GP the fuel pump resistors are known for having issues. IIRC #4 is known to run the hottest because of it'* location and that may be why you have the issue with that one. Another thing to consider is if the wire is touching the O2 sensor. Although misfire codes aren't typical.

I would probably try seafoaming the motor in hopes there'* a hot spot or carbon build up on that cylinder. Although the dealer said the gasket isn't an issue, the question would be...which gasket did they test. Their tests could have only been pressure testing the coolant system and spraying TB cleaner at the gaskets or using a smoke machine to check for vacuum leaks. While that should cover the usual bases, it may have missed somethine. If the seafoam didn't do the trick, I'd probably swap the gaskets out to the new style aluminum ones to know 100% that the gasket isn't leaking tiny bit of coolant or vaccum.

Where are you located? Maybe we have someone local that can ponder this in person with you.
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Old 09-02-2008, 08:48 PM   #4
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Question Thanks for the Response !!!

Quote:
Originally Posted by BillBoost37 View Post
Steve...You probably didn't get any responses because you have a "good" problem and have tried all the usual suspects.

On the GP the fuel pump resistors are known for having issues. IIRC #4 is known to run the hottest because of it'* location and that may be why you have the issue with that one. Another thing to consider is if the wire is touching the O2 sensor. Although misfire codes aren't typical.

I would probably try seafoaming the motor in hopes there'* a hot spot or carbon build up on that cylinder. Although the dealer said the gasket isn't an issue, the question would be...which gasket did they test. Their tests could have only been pressure testing the coolant system and spraying TB cleaner at the gaskets or using a smoke machine to check for vacuum leaks. While that should cover the usual bases, it may have missed somethine. If the seafoam didn't do the trick, I'd probably swap the gaskets out to the new style aluminum ones to know 100% that the gasket isn't leaking tiny bit of coolant or vaccum.

Where are you located? Maybe we have someone local that can ponder this in person with you.
I just checked the vacuum once again and wow, was I surprised. At idle
the vacuum is 20/hg and rock steady to boot. I would think that is pretty
decent for a car with 114,000 miles on it. When snapping the throttle wide
open the vacuum drops to about 0/hg and then jumps to 24-25 and then
returns back to 20. I just noticed that the lip seal for the snout is leaking
very bad, I just wonder on the off chance that the oil has been dropping
on the supercharger belt and causing belt slippage which could possibly
be causing my problem. The oil is really slinging all over the tensioner pulley.
Quick question can I just pull the pulley off and change the seal since the
bearings do not have any play in them, or do I need to remove the whole
snout to change the seal? I am located in northern Virginia, about 20 minutes
from George Bush'* house !!!

Regards,

Steve

Last edited by srlash; 09-02-2008 at 08:53 PM.
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Old 12-11-2010, 06:21 AM   #5
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hi m8,,,wouldn't know that type of car from a bar of soap!!due to being in australia but my ride has a 3800 in it not */c though and when i first read your problem i thought it could of been 1 of the coils in the coil-pac that was causeing the mis-fire!!!!!only reason i say this is that it happened to me breaking down at accelaration too,and as it turned out it was number 6,,,,,,,, just food for thought m8 and i would cetainly be getting that seal done,im sure it would be causeing slippage of some sort,,,try chaseing up Danthurs here on the forum i reackon he might know!!!!good luck with it m8!!!!
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Old 12-11-2010, 08:09 AM   #6
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Have the car scanned for Knock retard, (KR)

You have removed all issues and done all tests to track down the misfire. Except one. Misfire is generally caused by bad spark, coil, plug wires, or plugs. You checked all those. Next is fuel. Injectors were checked. Finally is mechanical. You did a compression test. However, that will not show a broken valve spring. Only a visual will show this. Remove the valve cover and inspect the springs.
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