Has anyone ran into a 5.7 Chevy vortec motor that shows a misfire On a cylinder when its not actually misfiring?

100

Asked by Aug 23, 2015 at 10:26 PM about the 2002 Chevrolet Express Cargo 3 Dr G3500 Cargo Van

Question type: Maintenance & Repair

12 Answers

7,325

No, I haven't. You probably do have a misfire, just that is very subtle. The engine computer determines a misfire by the crankshaft slowing down. This is determined by the crankshaft sensor signal. This model year 5.7L was notorious for fuel injector problems. So many problems, as a matter of fact, that GM redesigned the fuel injectors. The fuel injectors are available in a complete kit of 8 injectors, and center pod w/pressure regulator. A new mounting bracket is required for correct installation, and is sold separately. The old injectors had a poppet nozzles. These were eliminated, and a replaced by miniature electric injector, at the ends of the distribution tubes. You can try a bottle of injector cleaner in your tank, such as Techron by Chevron, from your local parts store. It's the same stuff that GM sells at their dealers, and puts a GM label on it. Also, make sure your plugs, wires, cap and rotor, are good. If you have a lot of miles on them, might be time for a tune-up?

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Thanks goodwrench good pointers but this one is a little deeper than that, The engine is a rebuild 0 mile motor ,The motor does not misfire but Snap on scanner picks up a misfire on #2 cylinder , good cap and rotor, try different distribotors, wires plugs, crank sensor and connectors , wires From crank sensor to ecm are ok .03 ohms , Spider is new , have good pulse to All 8 injectors and Also good equal Patterns On scope On All The injector circuits and crank and cam signals are good on frequency test, valves are adjusted correctly, cranking compresiĆ³n ok,running compresiĆ³n ok , leak down ok !!. Only misfire counts under a slight load not heavy and not in gear.!

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64,995

It does happen. Not that often though. It's usually a PCM problem. I'd suspect that this is the case since you have a fresh rebuild and all the new accessories to go with it. I'd have the PCM flashed (programmed) using the latest software. HTH. -Jim

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thanks Jim ,i reflash computer no change , dont think is got a Balance issue either runs so smooth,

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7,325

That's a new one on me. If it's not the PCM, then you must have an issue with #2 not firing evenly, while under slight load. If someone were to tell me that they get a misfire code, while under slight load, I'd be looking for an ignition problem. But you state all is new. Is your scanner capable of viewing live misfire data? I worked for GM, and with the Tech2 scanner, this could be viewed. You could watch exactly when the misfire counts happened/incremented, and the total number of misfires. Other ideas, shot in the dark?:....slight vacuum leak near #2? Um,...since the engine is a rebuild, crank not balanced correctly? You got got me stumped.

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64,995

You're welcome. Glad to try and help. -Jim

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Driving me nuts too! Ignition shows good on scope no interruptions at all so does fuel delivery, triple check for vaccum leaks and verify EGR wasent leaking sinse is Right in front by #2. Yes i have The latest gm scanner MDI also have tech2 but hardly use it anymore. I hate to trow Parts at cars with out accurate diag but looks like i might be trowing a ECM at this one and playing , Will post The results

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Meant to say praying not playing lol sorry guy

7,325

Thanks, please post results. I'm interested in knowing what you find. Sorry I couldn't be of much help.

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Thank you goodwrench any pointers at this time helps , i had 2 Pretty respected techs here in Houston tx look at it and left scraching their heads .i Will for sure post results thanks.!

7,325

Just one more idea, ..... check the the routing of the injector lines on the spider. Any chance you might have 2 of them mixed up? This would not show as a dead miss, since they spray fuel before the intake valve.

100

Done that,you can actually go into injector disable mode and kill individual injectors and watch your misfire live data at same time, i have kill each individual injector and check data at each cylinder make sure The misfire was at place were injector was killed.! Example: if 2 And 8 were swap then when i kill 2 8 would misfire on data.!

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