C5 misfire help
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C5 misfire help 2000 6sp c5 weird random misfire issue. Misfires randomly across all cylinders, doesn't do it when cold and the hotter it gets the worse it gets. Over 20-30% throttle the car falls on its face and runs lean in every gear. Not a true misfire like unplugging an injector. No codes thrown. Brand new acdelco coils, plugs, wires, and 02 sensors. Intake is clean. New gm fuel filter, 55psi at the rails, same when holding it at 2k rpms in neutral, cleaned injectors. Engine sounds completely healthy. Cleaned maf and ran on speed dens, didn't help. No cats/exhaust restrictions. No unmetered air leaks, no vacuum leaks, no egr/air injection leaks. Checked and cleaned all grounds. Good voltage. Swapped coil wires and same issue. Only thing we can think of is a bad ecm or a bad crank pos sensor but it doesn't throw any codes. Used hp tuners to figure out the misfires. Stfi is 0 to - 3.9 both sides and Lt is - 8.5 to - 10 both sides, idling. Any help would be greatly appreciated!!
TECH Apprentice
Joined: Nov 2016
Posts: 371
Likes: 18
From: switzerland
misfires and showing lean? best idea i can think of is to get somebody with a scope to look at it. make sure he knows his stuff, not many do. maybe something will come up at idle, maybe you need to take it on a dyno.
"runs very lean in every gear"
some additional thoughts
Can you get a scope and look at the waveforms and see that crank waveform is good/solid and your crank/cam correlation is on (no off a tooth)?
Misfires leave air in the exhaust and a side effect is O2's see lean, not sure if that's a cause of your problem or a symptom. But yea I'd chase that lean cause first if it were me
I assume your tune is good as far as VE goes and it's lean in both banks not just one, probably not a small manifold leak because in my experience smallish leaks run lean at idle but become basically insignificant when running it, the opposite of what you're seeing. If it's a leak it's big.
Check MAP and make sure it's 100kpa KOEO. Try to wiggle the MAP cable back there while watching it. Maybe the car is mismeasuring the air (not the MAF because you ran it in SD as a test, I think MAP still is considered in calculations MAF or SD, not 100% sure of that).
If it were me I'd look for large leaks like I mentioned for starters, it's either too much air or too little fuel and if your fuel pressure is up and it's lean on both banks it's probably not related to an injector. Not sure what would make it good afr at idle tho.
Also just look at all PIDs and make sure the values make sense like intake temp and coolant temp.
Just some ideas to put on your checklist
some additional thoughts
Can you get a scope and look at the waveforms and see that crank waveform is good/solid and your crank/cam correlation is on (no off a tooth)?
Misfires leave air in the exhaust and a side effect is O2's see lean, not sure if that's a cause of your problem or a symptom. But yea I'd chase that lean cause first if it were me
I assume your tune is good as far as VE goes and it's lean in both banks not just one, probably not a small manifold leak because in my experience smallish leaks run lean at idle but become basically insignificant when running it, the opposite of what you're seeing. If it's a leak it's big.
Check MAP and make sure it's 100kpa KOEO. Try to wiggle the MAP cable back there while watching it. Maybe the car is mismeasuring the air (not the MAF because you ran it in SD as a test, I think MAP still is considered in calculations MAF or SD, not 100% sure of that).
If it were me I'd look for large leaks like I mentioned for starters, it's either too much air or too little fuel and if your fuel pressure is up and it's lean on both banks it's probably not related to an injector. Not sure what would make it good afr at idle tho.
Also just look at all PIDs and make sure the values make sense like intake temp and coolant temp.
Just some ideas to put on your checklist
Last edited by mk3cn4; Sep 10, 2023 at 11:40 AM.
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Check coils, spark plugs, plug wires... Make sure plugs fully installed... Make sure no cracked ceramics on any plugs... Make sure plug wires are fully snapped on to plugs & coils (have had people before find that plug wires have slipped off, or even plugs themselves have loosened up and come out)... ----- Also, get a can of CRC Mass Airflow Sensor Cleaner and clean the MAF sensor, and make sure it's re-installed facing the correct direction... or if the MAF sensor is bad, get a new stock MAF sensor (not an aftermarket one like Jet or Crapatelli)
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2014 Camaro 2SS/RS - LS3, 6-speed manual, short-throw performance shifter, NPP dual-mode performance exhaust, 1LE front splitter, ZL1 rockers, Z28 Rear Spoiler, all paint-matched Red Rock Metallic, BMR strut tower brace, Cold Air Inductions cold air intake, Apex Motorsports, Inc. catch-can, Custom DiabLew Tune
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06MonteSS / DiabLew Tune
2014 Camaro 2SS/RS - LS3, 6-speed manual, short-throw performance shifter, NPP dual-mode performance exhaust, 1LE front splitter, ZL1 rockers, Z28 Rear Spoiler, all paint-matched Red Rock Metallic, BMR strut tower brace, Cold Air Inductions cold air intake, Apex Motorsports, Inc. catch-can, Custom DiabLew Tune
www.diablewtune.com -- www.diablocustomtune.com -- www.diablocustomtunegm.com
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