1997 C1500 P0307 Misfire

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blackburb

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I know another misfire thread… lots of details

I could use some help with my truck that has had a history of misfires that I have always been able to resolve, with the experience from GMT400 owners from this site. I’ve followed the factory service manual to diagnose the misfires. Found several issues but none of the repairs have resolved the P0307 misfire that started in November of last year. Neighbor is a mechanic (Cummins but rebuilds late model wrecks for spare cash) and he can’t figure out the problem either. See below for related service history.

Current issue.
In late November the truck started running rough and displaying a flashing CEL with P0300 & P0307 misfire codes. Cylinder 7 is misfiring so bad that it’s practically a dead cylinder. Cylinder 2 is also misfiring but not as bad and no code. I believe the cat is damaged due to all the misfiring. There is an occasional “rattle” coming from the catalytic converter. I believe unburned fuel is igniting in the cat. PS this truck is a 49 state truck and only has 1 cat after the Y pipe.

For diagnostics I have a Snap-On Solus pro with 2016 update and the GM specific personality key (needed for antilock brakes) that I am using to read codes, view live data, run tests etc. I also have Blue driver and Dash command with the GM module available.

I have a few ideas on what to do next…
  • Potential fuel pump issue 62psi KO/EO, 50psi running. Fuel pressure is 50psi at idle and at almost all other engine speeds no fluctuations up or down with acceleration… do I have a bad fuel pump? Previous fuel pump issues happened below 45psi.
  • Possible exhaust issues, back pressure from clogged cat? How is this measured and diagnosed?
  • Possible engine mechanical issues, leaking valve? Compression check is good, all cylinders within 3%.
  • Need to replace air intake adapter / throttle body gasket… part discontinued, suggestions?

Relevant Service History for reference:
2008 Mileage 218,250
P0305 & P0306 Misfires, low power after warm up, no compression on cylinder 5 or 6, dealer diagnosed cracked cylinder heads, I confirmed. My BIL and I installed a Chevrolet Long Block Crate Engine($1600 delivered!) & A1 Cardone Remanufactured Distributor, runs like new. This engine has approximately 85k miles on it now.

2014 Mileage 254,141
P0307 code, no compression, bent valve, exhaust valve struck top of piston. Son was driving truck swears it happened just off idle from a red light… right. Cylinder head passed Magnaflux inspection, R&R cylinder head with new valve guide, exhaust valve and spring. Machine shop tested cylinder head; all other valves, guides, springs and pushrods passed inspection. Upgraded CSFI with ACDelco 217-3029 GM Original Equipment MPFI Conversion Kit, Tune up: Cap, Rotor, Spark Plug, Plug Wires, Fuel Filter, 180° thermostat, runs better than new

2015 Mileage 258,541
Failed Smog Test, No codes but engine is running rich, smells like unburned fuel at tail pipe. Computer temp sensor reading low all the time, replaced. Retested failed smog again… Observed live data, potential issues with TPS. Installed ACDelco 213-912 GM Original Equipment Throttle Position Sensor, & original ACDelco 214-1080 EGR Valve. CMP is -9 can only get to -5 before cap hits intake manifold, Distributor gear severely worn, Oreily’s replaced A1 Cardone Remanufactured Domestic Distributor under warranty. Failed smog again. At the suggestion of the Smog Station manager replaced 49 state cheap $99 cat(installed @150k) with CARB compliant Magnaflow Performance Catalytic Converter & Bosch Oxygen upstream sensors, x2. Passes smog

2017 Mileage 269,425
P0300 misfiring and low power, low fuel pressure, installed Delphi Fuel Pump & Sending Unit, Cleaned distributor, cap and rotor.

2021 Mileage 286,225
Smog Due: Tune-up: Plugs, Wires, Cap, Rotor, Fuel filter, Air Filter, Cleaned rusty distributor, Reset CMP to -1.7, cap hitting intake manifold. Passed Smog.

2023 Issues/Events/Repairs
Current Mileage 303,179
  1. February: Discovered multiple exhaust leaks due to loose exhaust manifold nuts on left and right cylinder heads as well as loose donut fittings
    • Torqued all nuts and exhaust fittings around donut gaskets. Exhaust leakes cleared up but returned three days later at passenger donut gasket
    • Replaced passenger side donut gasket. Fixed
  2. April: noticed truck has rough idle and running very rich after warmed up, no codes
    • Fuel pressure checked 62psi
    • Coolant Temp never gets above 170-180F (180° thermostat installed 2014) Truck runs hot with the 190° thermostat runs “cool”with 180° thermostat.
    • Checked & cleaned cap and rotor
    • Added Lucas fuel system treatment to three consecutive tanks seems to resolve issue temporarily
  3. May: EGR Valve code, Cleaned EGR Valve & reinstalled with new gasket, code cleared on it’s own
  4. July: Passed Smog check… barely Nox was at max allowable reading. Technician ran the truck more than once to get it to pass.
  5. July: Developed coolant leak on driver side from plastic radiator tank, replaced radiator and OE upper and lower hoses
  6. December: CEL Flashing, codes P0300 & P0307
  • Check cap and rotor: noticed corrosion on cap but no rust. Cleaned and reinstalled cap and rotor, no change still misfiring on cylinder 7
  • Checked cylinder 7 spark plug and notice significant gap with center electrode eroded flush with ceramic.
  • Re-gaped plug and reinstalled temporarily, no change still misfiring on cylinder 7
  • Replaced all 8 plugs with AC Delco 41-993(not from Ebay this time) , AC Delco cap and old rotor(new AC Delco rotor was molded poorly, could not be installed), also swapped an old set of ACDelco wires in cylinder 5 & 7, no change still observing misfire live data on cylinder 7 and now cylinder 2, but much lesser extent, no code for cylinder 2 misfire
  • Checked CMP offset -7.6, cannot adjust cap hitting intake manifold
  • Checked secondary ignition system with HEI spark tester. All 8 plugs & Coil verified working normally
  • Ohm’d coil, passed… didn’t trust so replaced with Original coil. No change
  • Replaced Ignition Control Module same time as coil
  • Checked compression on all 8 cylinders, Cylinder 7 is 185psi with all other cylinders within 3%.
  • Fuel pressure KO/EO peaks at 62psi but bleeds down immediately
  • Performed Fuel Injector Balance Test. All injectors start at 60psi and drop to 43psi
  • Performed fuel pressure test procedure, per the GM manual. Test indicated fuel pressure leak down is caused by fuel pressure regulator
  • Replaced fuel pressure regulator. Noticed mostly clean lower intake runners behind MPFI injector housing, lots of oily carbon build up in front of central injector housing. No change, still running rough and misfiring on cylinder 7 and sometimes cylinder 2. CEL starts flashing within 2 minutes of clearing codes.
  • Check fuel pressure and new from O’Reilly’s Standard Motor Products fuel pressure regulator is regulating fuel between 55-30psi. Ugh.
  • Suspect leaking lower intake gasket so decided to R&R lower intake while replacing Standard Motor Product regulator with ACDelco fuel pressure regulator and new fuel filter.
  • Felpro MIK98000T lower gasket kit with metal backed lower intake manifold gasket, later read here that the paper gasket is preferred
  • During lower intake R&R notice intake manifold bolt between #7 intake runner and water jacket is loose enough to turn by hand.
  • Observe minor failure of intake gasket near #7 intake runner
  • Last nut nearest #7 exhaust port on driver side is loose again, retorqued all exhaust nuts
  • Installed AC Delco 9718Q Spark plug wire set
  • Installed United Motor Products 9362 Distributor, Blue Cap & Rotor A1 Cardone Distributor gear was severely worn
  • Installed new AC Delco PCV Valve Hose Pipe(cracked)
  • Installed AC Delco MAP Sensor (gasket failed)
  • Dorman Vapor Canister Purge Valve(mounting clip broke during re-assembly)
  • Bench tested MPFI and injectors with Fuel Injector Balance Test. Observed all injectors shoot a stream of fuel vs an atomized spray. Is this normal? All injectors start at 62psi and drop to 30psi and all injectors metered the same volume of fuel (observed) used a small clear container, marked a sharpie line after cylinder 1 test, repeated for all injectors, all metered the same relative volume of fuel
  • P0307 Code is still present, returns within seconds after clearing codes, still running rough and misfiring on cylinder 7 and sometimes cylinder 2.
  • Truck is driven on occasion with constant misfires in cylinder 7, but no longer daily for last two months
  • March 2024 recent fuel pressure check 62psi KO/EO immediately drops to 50psi and holds when pump shuts off
  • Inspected #5 & #7 plug #5 looks new with slight tan color, #7 is moist and black.
 
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Schurkey

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In late November the truck started running rough and displaying a flashing CEL with P0300 & P0307 misfire codes. Cylinder 7 is misfiring so bad that it’s practically a dead cylinder.
Cylinder 2 is also misfiring but not as bad and no code.

I believe the cat is damaged due to all the misfiring. There is an occasional “rattle” coming from the catalytic converter.
Seems reasonable. Don't fix the cat until AFTER the misfire is handled.

  • Potential fuel pump issue 62psi KO/EO, 50psi running. Fuel pressure is 50psi at idle and at almost all other engine speeds no fluctuations up or down with acceleration… do I have a bad fuel pump? Previous fuel pump issues happened below 45psi.
Fuel pump affects ALL cylinders, not one or two.

  • Possible exhaust issues, back pressure from clogged cat? How is this measured and diagnosed?
A single catalyst affects ALL cylinders, not one or two.

  • Possible engine mechanical issues, leaking valve? Compression check is good, all cylinders within 3%.
Consider a leakdown test, and verifying that the valves are opening and closing as they should on #7 and #2.

2023 Issues/Events/Repairs
Current Mileage 303,179
  1. April: noticed truck has rough idle and running very rich after warmed up, no codes
    • Fuel pressure checked 62psi
    • Coolant Temp never gets above 170-180F (180° thermostat installed 2014) Truck runs hot with the 190° thermostat runs “cool”with 180° thermostat.
Get the proper 195 degree thermostat installed.

  1. July: Passed Smog check… barely Nox was at max allowable reading. Technician ran the truck more than once to get it to pass.
NOx is caused by high combustion chamber temps. EGR is the usual cure. Verify EGR system.

  1. December: CEL Flashing, codes P0300 & P0307
  • Checked cylinder 7 spark plug and notice significant gap with center electrode eroded flush with ceramic.
I'm curious why that cylinder has such an eroded spark plug. Were the others worn, too? Or just #7?

  • Replaced all 8 plugs with AC Delco 41-993(not from Ebay this time)
Wisdom. Lots of counterfeit parts on eBay and Amazon. I got burned on "Iridium" spark plugs from an Amazon seller in Forney, Tx.

  • , AC Delco cap and old rotor(new AC Delco rotor was molded poorly, could not be installed), also swapped an old set of ACDelco wires in cylinder 5 & 7, no change still observing misfire live data on cylinder 7 and now cylinder 2, but much lesser extent,
Misfire that bad "should" be easy to track down.

  • Checked CMP offset -7.6, cannot adjust cap hitting intake manifold
SOMETHING is wrong. Distributor position, worn distributor/cam gears, slack timing chain. This should be addressed.

  • Checked secondary ignition system with HEI spark tester. All 8 plugs & Coil verified working normally
The spark tester does not check the spark plugs. I have some concern about the spark actually jumping the plug gap on #7 and #2. Consider moving those two plugs to other cylinders. See if the misfire follows the plugs or stays with #7 and #2.

This DOES seem to verify the distributor cap, though. I'd have expected failure to spark at the spark-tester when connected to #7, and perhaps #2 if the cap or plug wires were the problem.

  • Ohm’d coil, passed… didn’t trust so replaced with Original coil. No change
  • Replaced Ignition Control Module same time as coil
Coils and modules affect ALL cylinders, not one or two.

  • Checked compression on all 8 cylinders, Cylinder 7 is 185psi with all other cylinders within 3%.
Within 3% of 185psi?

If the exhaust valve doesn't open, the compression pressure can still be good--but the cylinder won't run.

  • Fuel pressure KO/EO peaks at 62psi but bleeds down immediately
  • Performed Fuel Injector Balance Test. All injectors start at 60psi and drop to 43psi
  • Performed fuel pressure test procedure, per the GM manual. Test indicated fuel pressure leak down is caused by fuel pressure regulator
  • Replaced fuel pressure regulator. Noticed mostly clean lower intake runners behind MPFI injector housing, lots of oily carbon build up in front of central injector housing. No change, still running rough and misfiring on cylinder 7 and sometimes cylinder 2. CEL starts flashing within 2 minutes of clearing codes.
Bummer. Seemed like a good potential cause of misfire.

  • Check fuel pressure and new from O’Reilly’s Standard Motor Products fuel pressure regulator is regulating fuel between 55-30psi. Ugh.
Bummer. I'd have expected 60+ psi.

  • Inspected #5 & #7 plug #5 looks new with slight tan color, #7 is moist and black.
Swap, or replace #7 plug. Maybe #2 as well.
 

blackburb

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Seems reasonable. Don't fix the cat until AFTER the misfire is handled.


Fuel pump affects ALL cylinders, not one or two.


A single catalyst affects ALL cylinders, not one or two.


Consider a leakdown test, and verifying that the valves are opening and closing as they should on #7 and #2.


Get the proper 195 degree thermostat installed.


NOx is caused by high combustion chamber temps. EGR is the usual cure. Verify EGR system.


I'm curious why that cylinder has such an eroded spark plug. Were the others worn, too? Or just #7?


Wisdom. Lots of counterfeit parts on eBay and Amazon. I got burned on "Iridium" spark plugs from an Amazon seller in Forney, Tx.


Misfire that bad "should" be easy to track down.


SOMETHING is wrong. Distributor position, worn distributor/cam gears, slack timing chain. This should be addressed.


The spark tester does not check the spark plugs. I have some concern about the spark actually jumping the plug gap on #7 and #2. Consider moving those two plugs to other cylinders. See if the misfire follows the plugs or stays with #7 and #2.

This DOES seem to verify the distributor cap, though. I'd have expected failure to spark at the spark-tester when connected to #7, and perhaps #2 if the cap or plug wires were the problem.


Coils and modules affect ALL cylinders, not one or two.


Within 3% of 185psi?

If the exhaust valve doesn't open, the compression pressure can still be good--but the cylinder won't run.


Bummer. Seemed like a good potential cause of misfire.


Bummer. I'd have expected 60+ psi.


Swap, or replace #7 plug. Maybe #2 as well.
Thank you Shurkey. I have a free weekend to do the leak down test and inspect the valve train to make sure the valves are opening.

I’ve swapped plugs with no change to the misfires.

I got plugs from the ebay seller in TX as well. I suspect they were fake AC Delco. Every plug was equally eroded. FWIW I was also buying from Victory Lane Parts on Ebay. All of the parts I received were shipped from Rockauto.
Not sure if they are directly related or if they are just drop shipping.

In regard to the thermostat the 180 was installed in 2014 because the engine was overheating with old and new 195°. I’ve since replaced the radiator and the water pump. I’ll try a 195° thermostat again. Any recommendations on brand?

Here’s the compression test results
 

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Schurkey

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They're pretty-much all made by the same company, in Israel. (MotoRad)

Be prepared to get more than one before you find one that works properly.
 

blackburb

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Leak down test resulted in less than 5% on cyl 7 same on cyl 5. I checked valve train and noticed the exhaust was tight. Loosened about 1/8 turn. In the process of swapping plugs I noticed #7 had oil/fuel seeping around the metal and outer ceramic insulator. I snapped the plug and noticed what looks like internal corrosion. CMP offset was -7.6. It looks like I didn’t tighten the nut fully. Replaced #7 plug with new and misfire on # 7 and #2 appears to have been resolved. Still a slight misfire on #4. Dive in to that next weekend.
 

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