2000 Silverado 5.3 random misfire/cylinder 6 misfire
December 18th, 2022, 6:33 PM
#11
As I under stand it, you have a misfire on #6 cylinder only at idle. No misfires driving, Brake torqueing or off idle and have confirmed no vacuum leak for that cylinder . First thing I would do is a crank variation learn procedure with a scan tool and see what happens. You can also do a compression test and compare the 1ST puff with a known good cylinder and see if it's low or do a cylinder leakage test. Have see leaking valve cause this issue and have also had crank variation learn procedure fix this concern. Just a couple of thoughts.
December 19th, 2022, 4:07 PM
#12
CF Monarch
As I under stand it, you have a misfire on #6 cylinder only at idle. No misfires driving, Brake torqueing or off idle and have confirmed no vacuum leak for that cylinder . First thing I would do is a crank variation learn procedure with a scan tool and see what happens. You can also do a compression test and compare the 1ST puff with a known good cylinder and see if it's low or do a cylinder leakage test. Have see leaking valve cause this issue and have also had crank variation learn procedure fix this concern. Just a couple of thoughts.
December 25th, 2022, 1:17 PM
#13
CF Beginner
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No, this is fixable. We simply need to diagnosis this issue to the Nth degree, 24compact. Give we who are here long-time a chance to solve your issue. There are lots of people here, with even better at Dxing your issues.I wish "tech2" were still here.
The man was a professional tech.
Wherever he is, I hope he returns he comes back here, but we still have really good Techs/Mechs here. Gumby22, Chem_Man, Repairman and OldChevy and many others are my equal, if not beyond, an old truck-driver who first studied in automotive technology, when I was a young man, who knew how to "turn a wrench". We are all here to help you and everyone else. It is what we do, without any thanks or response from.our advice to anyone who asks for help in a repair.
Do not give up so soon. Diagnosis of the issue is 80 to 90 percent or higher, in regards to repairing a vehicle, 24compact.
The man was a professional tech.
Wherever he is, I hope he returns he comes back here, but we still have really good Techs/Mechs here. Gumby22, Chem_Man, Repairman and OldChevy and many others are my equal, if not beyond, an old truck-driver who first studied in automotive technology, when I was a young man, who knew how to "turn a wrench". We are all here to help you and everyone else. It is what we do, without any thanks or response from.our advice to anyone who asks for help in a repair.
Do not give up so soon. Diagnosis of the issue is 80 to 90 percent or higher, in regards to repairing a vehicle, 24compact.
December 29th, 2022, 7:40 PM
#14
CF Veteran
I don’t have time to go into detail right now, so these are Just my observations - consider the implications before jumping into more advanced diagnosis and/or performing unnecessary scan tool procedures.
1) For a misfire to be isolated to one cylinder only (regardless of engine speed/load) means that anything which can affect all cylinders (or only one bank) is immediately ruled out. Things such as fuel quality, Overall fuel pressure, intake leaks upstream of the specific cylinder port, EGR getting stuck open, PCV sucking excess oil, and more can be ruled out because these are things that would affect ALL cylinders, not just one.
2) You didn’t state whether or not you checked fuel pressure at key on, engine off, and what those results were. It should hold the specified pressure for at least 2 minutes without losing more than 5 PSI. Immediate pressure drop off and/or a gradual decrease indicates a faulty fuel pump, pressure regulator, leaking fuel line, or leaking injector.
3) Given that you’ve replaced the pump, and fuel pressure, although low at idle, is still adequate to run the engine, I think the 2 most likely culprits are a leaking fuel injector or a head gasket leak.
Fuel pressure and/or compression testing will determine the next steps.
1) For a misfire to be isolated to one cylinder only (regardless of engine speed/load) means that anything which can affect all cylinders (or only one bank) is immediately ruled out. Things such as fuel quality, Overall fuel pressure, intake leaks upstream of the specific cylinder port, EGR getting stuck open, PCV sucking excess oil, and more can be ruled out because these are things that would affect ALL cylinders, not just one.
2) You didn’t state whether or not you checked fuel pressure at key on, engine off, and what those results were. It should hold the specified pressure for at least 2 minutes without losing more than 5 PSI. Immediate pressure drop off and/or a gradual decrease indicates a faulty fuel pump, pressure regulator, leaking fuel line, or leaking injector.
3) Given that you’ve replaced the pump, and fuel pressure, although low at idle, is still adequate to run the engine, I think the 2 most likely culprits are a leaking fuel injector or a head gasket leak.
Fuel pressure and/or compression testing will determine the next steps.
December 31st, 2022, 7:45 PM
#15
Just for information, no disrespect intended. PCV pull over on later(07 and later)LS engines is common causing misfires on cylinders 1 or 7 due to oil fouling the spark plugs. There is an updated left valve cover that has a different baffle. Even on engines with the up dated valve cover have been found to leak also.
Had a 2017 Ram in the shop with one cylinder misfire. compression test "normal"(as per the Guy working on it). and the 1st puff was 30psi lower than the rest and cylinder leak test was 10%. Pulled the head and found exhaust valve with extremely loose guide and leaking when psi testing the head.
If you have a bidirectional scan tool already attached to the vehicle, performing a crank variation learn only takes seconds to do.
I would agree, a injector balance test would be of value.
Had a 2017 Ram in the shop with one cylinder misfire. compression test "normal"(as per the Guy working on it). and the 1st puff was 30psi lower than the rest and cylinder leak test was 10%. Pulled the head and found exhaust valve with extremely loose guide and leaking when psi testing the head.
If you have a bidirectional scan tool already attached to the vehicle, performing a crank variation learn only takes seconds to do.
I would agree, a injector balance test would be of value.
January 1st, 2023, 8:22 PM
#16
CF Veteran
Just for information, no disrespect intended. PCV pull over on later(07 and later)LS engines is common causing misfires on cylinders 1 or 7 due to oil fouling the spark plugs. There is an updated left valve cover that has a different baffle. Even on engines with the up dated valve cover have been found to leak also.
Had a 2017 Ram in the shop with one cylinder misfire. compression test "normal"(as per the Guy working on it). and the 1st puff was 30psi lower than the rest and cylinder leak test was 10%. Pulled the head and found exhaust valve with extremely loose guide and leaking when psi testing the head.
If you have a bidirectional scan tool already attached to the vehicle, performing a crank variation learn only takes seconds to do.
I would agree, a injector balance test would be of value.
Had a 2017 Ram in the shop with one cylinder misfire. compression test "normal"(as per the Guy working on it). and the 1st puff was 30psi lower than the rest and cylinder leak test was 10%. Pulled the head and found exhaust valve with extremely loose guide and leaking when psi testing the head.
If you have a bidirectional scan tool already attached to the vehicle, performing a crank variation learn only takes seconds to do.
I would agree, a injector balance test would be of value.
2) While it’s been found that certain engines (GM or otherwise) can develop single-cylinder misfires from the PCV system, that’s not a situation that applies to the OP.
I can give more specific details if you want, but the primary reason for the updated valve cover is because the LS engines use a metering orifice for the PCV system whereas the OP’s engine uses a vacuum actuated valve.
3) For me it’s not about how long a relearn procedure will take but whether or not there’s any actual value in doing so and/or if it will affect my ability to diagnose the problem.
Most manufacturers do not recommend performing a cam-crank relearn unless a specific service procedure (typically valve train or timing component replacement) requires it. Likewise, there have been plenty of service bulletins from many manufacturers that advise doing it in certain cases, so who knows?
It’s been my experience that a misfire that can be felt (as the OP states) is unlikely to be caused by anything that a cam-crank relearn is likely to resolve.
I’m happy to continue discussing this but for now there’s some chocolate cake trying to escape my refrigerator.
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