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gmax1

98 Chevy 5.7 Misfire

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My 98 K3500 with 161k was throwing a lot of codes. Replaced all 4 O2 sensors. Then Mass air flow sensor, new AC Delco platinum plugs, new Auto Zone plug wires, New NAPA distributor, tried switching NAPA cap and rotor with new GM brand. Mechanic (Jeff- former Honda, Toy, Subaru dealer mechanic) found cracks in intake manifold gasket and replaced with new upgraded Felpro brand. Still running rough. Jeff unable to find cause.

 

Brought truck to Dick- former GM dealership mechanic. Dick went through all of the diagnostic proceedures Jeff went through. Truck runs okay on start up but within 1-2 minutes starts running rough at idle and gets worse when put into gear. This has been problem from beginning. Number 4 and 8 plugs skipping the worst. All plugs on that side badly sooted up. Other bank of plugs (odd #) looks good. Sometimes misfire carries over to #1 plug. Truck is blowing black smoke and burning excess fuel. Compression in all cylinders is 175 to 185 with one cylinder showing 160.

 

Dick pulled O2 sensor before catalytic on passanger side and engine quieted right down. Says truck has plugged catalytic so new cat and muffler assembly for that side ordered from GM and installed. Ouch! Slight improvement but still running very rough, no power, misfires on 4 and 8, and spark as before advanced by 22 degrees.

 

Pulled NAPA distributer with the new GM cap and rotor button and switched it with a used GM distributor and again used the new GM rotor and cap. No significant change. Replaced central port fuel injector unit with one from a junk yard that supposedly came off a running engine. No significant change. Fuel pressure is 54 or 55 psi.

 

Headed back to junk yard tomorrow to switch used CPFI with another used one they have that is also supposedly off a running engine. If this doesn't work my next step will be to bring truck to GM dealership and have them diagnose it for possibility of installing new GM CPFI as a cure but that's a $500 - $600 fix and currently I have almost $2000 into labor and parts with Dick. Dick thinks the CPFI may be the problem. If it isn't he does't know what to do next as he has tried everything in his bag of tricks and has consulted with other mechanics he knows. We're both frustrated. Nothing anyone has done has resulted in significant improvement.

 

This thing is beating my wallet hard. Any suggestions would be appreciated.

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My 98 K3500 with 161k was throwing a lot of codes. Replaced all 4 O2 sensors. Then Mass air flow sensor, new AC Delco platinum plugs, new Auto Zone plug wires, New NAPA distributor, tried switching NAPA cap and rotor with new GM brand. Mechanic (Jeff- former Honda, Toy, Subaru dealer mechanic) found cracks in intake manifold gasket and replaced with new upgraded Felpro brand. Still running rough. Jeff unable to find cause.

 

Brought truck to Dick- former GM dealership mechanic. Dick went through all of the diagnostic proceedures Jeff went through. Truck runs okay on start up but within 1-2 minutes starts running rough at idle and gets worse when put into gear. This has been problem from beginning. Number 4 and 8 plugs skipping the worst. All plugs on that side badly sooted up. Other bank of plugs (odd #) looks good. Sometimes misfire carries over to #1 plug. Truck is blowing black smoke and burning excess fuel. Compression in all cylinders is 175 to 185 with one cylinder showing 160.

 

Dick pulled O2 sensor before catalytic on passanger side and engine quieted right down. Says truck has plugged catalytic so new cat and muffler assembly for that side ordered from GM and installed. Ouch! Slight improvement but still running very rough, no power, misfires on 4 and 8, and spark as before advanced by 22 degrees.

 

Pulled NAPA distributer with the new GM cap and rotor button and switched it with a used GM distributor and again used the new GM rotor and cap. No significant change. Replaced central port fuel injector unit with one from a junk yard that supposedly came off a running engine. No significant change. Fuel pressure is 54 or 55 psi.

 

Headed back to junk yard tomorrow to switch used CPFI with another used one they have that is also supposedly off a running engine. If this doesn't work my next step will be to bring truck to GM dealership and have them diagnose it for possibility of installing new GM CPFI as a cure but that's a $500 - $600 fix and currently I have almost $2000 into labor and parts with Dick. Dick thinks the CPFI may be the problem. If it isn't he does't know what to do next as he has tried everything in his bag of tricks and has consulted with other mechanics he knows. We're both frustrated. Nothing anyone has done has resulted in significant improvement.

 

This thing is beating my wallet hard. Any suggestions would be appreciated.

 

You never said about anyone checking the EGR valve? Sounds like maybe a piece of carbon is keeping it stuck open a bit, it will run ok on start up and then when idle down run rough and then uses alot of fuel creating black smoke and using lots of fuel. Yes the CPFI on those motors are bad about injectors sticking open and flooding out cyclinders but if you changed the unit and no change you should be good there. But asuming you have good power while driving most likely it wont be the CPFI!! First thing I would chack if it came in my shop would be the EGR it has ALL the right symtoms!

 

Sometimes thinking outside the box always helps and one mechanic is stuck on thinking one thing its hard to find the real problem, we've all been there i hate it. Hopefully this helps you out, let us know!

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Id go with the EGR valve too, you already replaced just about everything but it!

 

 

I had a similar problem on my older pickup too, rough Idle, would die in gear. A new EGR valve fixed it.

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My 98 K3500 with 161k was throwing a lot of codes. Replaced all 4 O2 sensors. Then Mass air flow sensor, new AC Delco platinum plugs, new Auto Zone plug wires, New NAPA distributor, tried switching NAPA cap and rotor with new GM brand. Mechanic (Jeff- former Honda, Toy, Subaru dealer mechanic) found cracks in intake manifold gasket and replaced with new upgraded Felpro brand. Still running rough. Jeff unable to find cause.

 

Brought truck to Dick- former GM dealership mechanic. Dick went through all of the diagnostic proceedures Jeff went through. Truck runs okay on start up but within 1-2 minutes starts running rough at idle and gets worse when put into gear. This has been problem from beginning. Number 4 and 8 plugs skipping the worst. All plugs on that side badly sooted up. Other bank of plugs (odd #) looks good. Sometimes misfire carries over to #1 plug. Truck is blowing black smoke and burning excess fuel. Compression in all cylinders is 175 to 185 with one cylinder showing 160.

 

Dick pulled O2 sensor before catalytic on passanger side and engine quieted right down. Says truck has plugged catalytic so new cat and muffler assembly for that side ordered from GM and installed. Ouch! Slight improvement but still running very rough, no power, misfires on 4 and 8, and spark as before advanced by 22 degrees.

 

Pulled NAPA distributer with the new GM cap and rotor button and switched it with a used GM distributor and again used the new GM rotor and cap. No significant change. Replaced central port fuel injector unit with one from a junk yard that supposedly came off a running engine. No significant change. Fuel pressure is 54 or 55 psi.

 

Headed back to junk yard tomorrow to switch used CPFI with another used one they have that is also supposedly off a running engine. If this doesn't work my next step will be to bring truck to GM dealership and have them diagnose it for possibility of installing new GM CPFI as a cure but that's a $500 - $600 fix and currently I have almost $2000 into labor and parts with Dick. Dick thinks the CPFI may be the problem. If it isn't he does't know what to do next as he has tried everything in his bag of tricks and has consulted with other mechanics he knows. We're both frustrated. Nothing anyone has done has resulted in significant improvement.

 

This thing is beating my wallet hard. Any suggestions would be appreciated.

 

You never said about anyone checking the EGR valve? Sounds like maybe a piece of carbon is keeping it stuck open a bit, it will run ok on start up and then when idle down run rough and then uses alot of fuel creating black smoke and using lots of fuel. Yes the CPFI on those motors are bad about injectors sticking open and flooding out cyclinders but if you changed the unit and no change you should be good there. But asuming you have good power while driving most likely it wont be the CPFI!! First thing I would chack if it came in my shop would be the EGR it has ALL the right symtoms!

 

Sometimes thinking outside the box always helps and one mechanic is stuck on thinking one thing its hard to find the real problem, we've all been there i hate it. Hopefully this helps you out, let us know!

 

 

Thanks for the reply. Thanks to ZZ327 also. Forgot to mention Dick tried another (good used I think) crank sensor. I also tried a Duralast MAP sensor but that also made no difference so changed back over to my old MAP. Dick removed the EGR valve and cleaned it a little with a knife and reinstalled but this didn't seem to make a difference. Maybe I need to try a new EGR? Can an EGR be cleaned. EGR is not giving any codes.

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Thanks for the reply. Thanks to ZZ327 also. Forgot to mention Dick tried another (good used I think) crank sensor. I also tried a Duralast MAP sensor but that also made no difference so changed back over to my old MAP. Dick removed the EGR valve and cleaned it a little with a knife and reinstalled but this didn't seem to make a difference. Maybe I need to try a new EGR? Can an EGR be cleaned? EGR is not giving any codes. Truck runs like on 6 or 7 cylinders going down the road but doesn't stall. Hills are a strain.

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Thanks for the reply. Thanks to ZZ327 also. Forgot to mention Dick tried another (good used I think) crank sensor. I also tried a Duralast MAP sensor but that also made no difference so changed back over to my old MAP. Dick removed the EGR valve and cleaned it a little with a knife and reinstalled but this didn't seem to make a difference. Maybe I need to try a new EGR? Can an EGR be cleaned? EGR is not giving any codes. Truck runs like on 6 or 7 cylinders going down the road but doesn't stall. Hills are a strain.

 

Yes EGR can be cleaned, just look at it and check to see if its seating all the way closed, EGR codes are hard to throw it has to be not working at all. The EGR is a pretty pienny to buy see if someones got one to try.

 

When the upper intake pulled is anything shinny like leaking fuel into the intake somewhere.

 

When in idle how does it do when you give it full throttle a couple of times smooth, real rough, has hard time getting up to it.

 

Also when key on engine off does it hold good fuel pressure or does it bleed off?

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Don't know about leaking fuel, will ask Dick tomorrow. I would hope he would notice if that were so. Heading for bed so I can make run to auto recycler early tomorrow. Leaving by 0700 and it's almost a three hour round trip! Thanks for all your help and if anyone thinks of another angle I'm very interested. I hate to keep throwing parts at this stubborn beast but I don't know what else to do. It's no good to anyone the way it's running at this time.

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Will ask Dick if he looked at fuel pressure bleed off when engine is not running. What effect would that have on the engine when it is running? Could it cause my symptoms?

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Will ask Dick if he looked at fuel pressure bleed off when engine is not running. What effect would that have on the engine when it is running? Could it cause my symptoms?

 

Yes if the engine can get pressure but cant get the flow it needs it could becausing no power issues and missing it makes they run funny sometimes each mocar acts differently to the issue.

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UPDATE: Delivered another used central port fuel injector unit to the 3 mechanics. They said the unusual thing about my problem is that the poor running is all on one bank of cylinders- 2,4,6,8 with 4 and 8 being the worst. They can watch them get worse on their engine analyzer as engine warms up. They have never seen a problem so pronounced concentrated in only one bank with no apparent cause. Nothing they have tried has resulted in any significan improvement. They have also done a leak down test and that was good. They do not think it is an EGR issue as my EGR is located center of both cylinder banks and should distribute evenly to both banks.

 

This is part of the problem a lot of things that could cause a running problem like this would affect cylinders on both banks. For instance if it was a fuel filter or fuel pressure regulator problem it should affect the way all cylinders run.

 

Can't find a used EGR to try. Most recycling yards don't bother with them as they get a lot of bring backs on this part. New EGR is 190.00 so it would be good to be pretty sure that is problem before installing same.

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I think that's a problem.... there's no way for anyone to be "pretty sure" from a forum. I know it's frustrating given the amount of time and money that you've already put in - but also keep in mind that you're replacing wearing parts with other USED wearing parts! There's no way to rule out the possibility that a previous part isn't compounding the problem, or having the same issue.

EGR seems like a good bet though - good luck!!!

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Well now that we know its the whole even bank messing up and we know its using too much gas not a lean condition that will elimate a intake leak again with new gasket, a rich condition would be a pluged exaust but that is replaced and it uses a central fuel injection so that rules out a computer telling the injectors wrong. Ignition doesnt choose just one side to misfire, It has to be getting too much fuel from somewhere lines going in to the injector assembly dumping gas on the even bank flooding them out. The whole injection assembley is being replaced right poppets and all?

 

If its all on one bank EGR would like not be the problem but if u dont have a EGR to try you can lift the egr and put a soild gasket in between it to block the entire ERG off and that will tell u quickly if its the problem or not.

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UPDATE: Delivered another used central port fuel injector unit to the 3 mechanics. They said the unusual thing about my problem is that the poor running is all on one bank of cylinders- 2,4,6,8 with 4 and 8 being the worst. They can watch them get worse on their engine analyzer as engine warms up. They have never seen a problem so pronounced concentrated in only one bank with no apparent cause. Nothing they have tried has resulted in any significan improvement. They have also done a leak down test and that was good. They do not think it is an EGR issue as my EGR is located center of both cylinder banks and should distribute evenly to both banks.

 

This is part of the problem a lot of things that could cause a running problem like this would affect cylinders on both banks. For instance if it was a fuel filter or fuel pressure regulator problem it should affect the way all cylinders run.

 

Can't find a used EGR to try. Most recycling yards don't bother with them as they get a lot of bring backs on this part. New EGR is 190.00 so it would be good to be pretty sure that is problem before installing same.

 

2 mechanics said the same thing to me awhile back. It wasnt putting out any codes, and most didnt think it was the problem. I sprayed carb cleaner in it, it would still open and close. But when I checked a new one, that little piston moved up and down a lot easier and faster so I replaced it myself. I think it only took a half hour. 2 bolts and a gasket if I remember right. mine wasnt even close to 190 bucks, it was $55

 

Silveradobobby is right, try blocking it off.

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After messing with the crank sensor, I'd bet this is a candidate for a "crank relearn" procedure. I chased a random misfire code for a week once and a crank relearn fixed it. Just a thought.

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I think that's a problem.... there's no way for anyone to be "pretty sure" from a forum. I know it's frustrating given the amount of time and money that you've already put in - but also keep in mind that you're replacing wearing parts with other USED wearing parts! There's no way to rule out the possibility that a previous part isn't compounding the problem, or having the same issue.

EGR seems like a good bet though - good luck!!!

 

 

Good point about the wearing parts. Was hoping that a used CPFI with poppets and fuel pressure regulator would either cure issue or show some difference in symptoms. With a new part one would have a higher confidence level that that issue is likely eliminated.

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Well now that we know its the whole even bank messing up and we know its using too much gas not a lean condition that will elimate a intake leak again with new gasket, a rich condition would be a pluged exaust but that is replaced and it uses a central fuel injection so that rules out a computer telling the injectors wrong. Ignition doesnt choose just one side to misfire, It has to be getting too much fuel from somewhere lines going in to the injector assembly dumping gas on the even bank flooding them out. The whole injection assembley is being replaced right poppets and all?

 

If its all on one bank EGR would like not be the problem but if u dont have a EGR to try you can lift the egr and put a soild gasket in between it to block the entire ERG off and that will tell u quickly if its the problem or not.

 

I wondered about removing EGR and covering hole somehow if that would show some difference. Thanks for that suggestion.

Also no evidence of fuel leak under phlenum.

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