C5 random misfire, help!
Thread Starter
5th Gear
Joined: Aug 2023
Posts: 5
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C5 random misfire, help!
2000 c5 manual 140k miles. All stock except exhaust. Has a weird random misfire that jumps between (usually) cyl 3/5/7/6/8. New spark plugs and wires. Swapped coils and didnt help. No unmetered air leaks, nor on the air injection either. Good fuel, fuel filter is new. Good fuel pressure at the rail. For the first 2-5 minutes of the car while cold it runs great until open loop activates. Thinking my issue might be with the front 2 02 sensors. When driving at low rpm it's 'decent' but anything over 20/30% throttle and anything higher than 3/4krpm and it's a dog and leans out hard. Chugs and spits then pops as I let off the throttle. I've done just about everything besides throwing parts at it. There's NO check engine light on the dash nor does it throw any codes at all. Any help would be greatly appreciated 
Thread Starter
5th Gear
Joined: Aug 2023
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Race Director
Joined: Oct 2004
Posts: 12,861
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2000 c5 manual 140k miles. All stock except exhaust. Has a weird random misfire that jumps between (usually) cyl 3/5/7/6/8. New spark plugs and wires. Swapped coils and didnt help. No unmetered air leaks, nor on the air injection either. Good fuel, fuel filter is new. Good fuel pressure at the rail. For the first 2-5 minutes of the car while cold it runs great until open loop activates. Thinking my issue might be with the front 2 02 sensors. When driving at low rpm it's 'decent' but anything over 20/30% throttle and anything higher than 3/4krpm and it's a dog and leans out hard. Chugs and spits then pops as I let off the throttle. I've done just about everything besides throwing parts at it. There's NO check engine light on the dash nor does it throw any codes at all. Any help would be greatly appreciated
Instructor
Joined: Nov 2019
Posts: 162
Likes: 80
From: Central NJ
A good scanner would help. Plot out your Fuel Trims and 02 sensors at idle, acceleration, and 3k rpm cruise. A mode 06 scanner will also be able to plot misfires per cylinder as well.
If you have exhaust pops on deceleration, you might have a high positive fuel trim from an air leak causing a lean condition that the car wants to add more fuel. Check your injectors a well.
If you have exhaust pops on deceleration, you might have a high positive fuel trim from an air leak causing a lean condition that the car wants to add more fuel. Check your injectors a well.
Le Mans Master
Joined: Feb 2006
Posts: 5,267
Likes: 40
From: Phoenix AZ
Get a scanner that shows the O2 sensors on a graph doing the up and down wave at idle. Get a propane torch that is NOT LIT but on and "spray" propane around the injectors or other areas to check for intake leaks. IF there is a leak you will see it instantly go full rich on the graph., In my case it took me years to figure out the injector orings were hard as glass and leaking air.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
Likes: 4,023
From: Cape Coral, Florida
With your Dad being a Master Tech it should be fairly easy for him to diagnose it…fuel, spark, or mechanical…he just needs to get into the right diagnostic “funnel”…if he has a labscope he has everything…is it a misfire that is felt or just shows up on the scan tool ??…being that it runs good in open loop the fuel trims are not involved in the fueling “schedules…he should see if the front O2’s are stuck “rich”…around 700-900mv’s.
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Thread Starter
5th Gear
Joined: Aug 2023
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With your Dad being a Master Tech it should be fairly easy for him to diagnose it…fuel, spark, or mechanical…he just needs to get into the right diagnostic “funnel”…if he has a labscope he has everything…is it a misfire that is felt or just shows up on the scan tool ??…being that it runs good in open loop the fuel trims are not involved in the fueling “schedules…he should see if the front O2’s are stuck “rich”…around 700-900mv’s.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
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From: Cape Coral, Florida
What makes you think it’s a bad PCM…what checks have been done ??…does your Dad have a labscope ???
Thread Starter
5th Gear
Joined: Aug 2023
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As far as checks Go, all grounds have been cleaned, new acdelco wires and coils and plugs, swapped coil wire harness (no change), 55psi fuel at the rail at idle and at 2k rpm. All injectors have been cleaned. Holds pressure when off. New gm fuel filter. Runs good when cold and after a little warm it's a random slight misfire across all cylinders, not a full misfire like unplugging an injector if that makes sense. No codes being thrown, runs lean when over 20-30% throttle. The hotter the car gets the worse it runs/idles. Motor sounds good via stethoscope. No unmetered air leaks, maf was cleaned and still runs the same on speed dens. Bypassed egr/air injection, same. No vacuum leaks. Besides the ecm we're stumped. I'm sure he does have a labscope somewhere
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
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From: Cape Coral, Florida
With multiple cylinder random misfires this could be a bad crank sensor if you don’t feel a misfire (not a full misfire as you say) or possible bad torque tube bushings…if you ruled out fuel and ignition there is not to much left.
Thread Starter
5th Gear
Joined: Aug 2023
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Torque tube bushings were replaced in February, I'll pull the crank sensor and see if it's bad or just replace it and see if that helps
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
Likes: 4,023
From: Cape Coral, Florida
You can’t tell if it’s bad by looking at it…just use a new OE sensor and I’d check the wiring…12 volt reference, signal, and reference ground…if he has a labscope he can check the signal wire before replacing it.