Request help: L31 5.7 Cylinder 5 Misfire

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Looking for help with ideas to diagnose and fix my 1998 Chevy K1500 5.7L L31 misfire. The situation and actions I have taken to date appeaser below. Any thoughts on where to go next would be appreciated.

Symptom - Cylinder 5 misfire at all engine speeds - P0300 DTC; no other DTCs. Diagnosed with Snap On Solus Pro scan tool.

New GM L31 replacement engine (old engine had a crack in cylinder 5 and the same P0300 misfire in cylinder 5 only)

New GM multi-port fuel injection (MPFI) meter replaced central sequential port fuel injection (CSFI) meter

New crankshaft position sensor. Ran crankshaft position sensor system variation learn procedure.

Ignition system inspections and tests
  • New spark plugs with correct gap.
  • New distributor cap and rotor.
  • Ignition wires and coil wire passed resistance test.
  • Tested for arcing, cross-firing, and electro-magnetic interference with no issues found.
  • Swapped spark plugs and ignition wires to different cylinders. Misfire remains on cylinder 5.
  • Tested spark on cylinders 1, 3, 5, and 7. Adequate spark on all cylinders.
  • Distributor and camshaft position sensor. No defects apparent in visual inspection. Replaced with known good distributor and camshaft position sensor from another truck. Misfire remains on cylinder 5. Cam retard offset on both distributors set to zero and confirmed with scan tool.
  • Ignition coil and ignition control module. Replaced with known good units from another truck. Misfire remains on cylinder 5.
Fuel system inspections and tests
  • Fuel pump delivers 60 psi and system holds 55 psi after shutting off pump.
  • New fuel filter.
  • Fuel injector balance test. Pressure 1 = 55 psi on all 8 cylinders. Pressure 2 = 28 psi on all 8 cylinders. Average pressure drop 27 psi on all 8 cylinders.
  • Fuel injector coil test. Steady reading on all 8 injectors. Readings range from 6.9 to 7.1 volts.
  • Inspected connectors at the vehicle control module and fuel injectors. No apparent deformities or other problems affecting the connection.
  • Checked injector ignition feed circuits with a test light. Test light illuminated for all circuits.
  • Checked fuel injector circuit with an injector test lamp. Flashed correctly for each injector.

Other inspections and tests
  • Searched and tested for vacuum leaks. None found.
  • Inspected and cleaned all grounds. No corrosion or other issues found.
  • Monitored all engine sensor data with scan tool (engine in open and closed loop). Idle air control (IAC), throttle position (TP), engine coolant temperature (ECT), manifold absolute pressure (MAP), exhaust gas recirculation (EGR), all within typical values.
  • Long term fuel trim on Bank 1 was 154 counts. Typical value is 127 counts.
  • Heated oxygen sensor Bank 1, Sensor 2 voltage varied widely just like Sensor 1. Bank 2, Sensor 2 was steady as expected for the H02S downstream from the catalytic converter. This variation indicates to me that there is too much unburned fuel in the exhaust in Bank 1, probably from the misfiring cylinder 5.
Thanks for any suggestions,
George
 

Orpedcrow

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What engine related parts were reused from the old engine?
Have you checked the valve lash?
 

Orpedcrow

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Compression test?
What does plug 5 look like after running, compared to the others?
 

Komet

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Leakdown test on cyl 5 might be good. Also pop the valve cover and have a look to see if a cam lobe is wiped or something.
 

Orpedcrow

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If you have fuel and spark, the only thing missing is air (and compression)
 
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Old parts reused during new engine install

  • Upper and lower intake manifolds (new gaskets for lower intake)
  • All sensors attached to intake manifold and throttle body (inspected for vacuum leaks; sensor data outputs reviewed with scan tool)
  • Distributor and camshaft position sensor (replacing with known good units from another truck did not fix the problem, so I ruled out the distributor and CMP)
  • Coil and ignition control module (replacing with known good units from another truck did not fix the problem, so I ruled out coil and ICM)
  • Ignition wires and coil wire (these are four years old; all passed resistance test and visual inspection; also switched wires and plugs to different cylinders and rerouted wires to see if the misfire moved with any of them; misfire remained on cylinder 5)
  • Exhaust manifolds (new gaskets)
  • Heated oxygen sensors
  • Knock sensor
  • Accessories (generator, air conditioning compressor, power steering pump)

Compression #1 = 174; #3 = 173; #5 = 175; #7 = 174. Did not check 2-8.

Did not check valve lash on the new engine. No broken valve springs, bent valves, or worn cam lobes on new engine. Misfire symptom was exactly the same on the old engine.

Pictures of spark plugs follow. Where there is more than one plug they are #1, #3, and #5 from left to right. First photo of a single plug on this post is #1. Second photo of a single plug on this post is #3.
 

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Spark plugs continued.

Both photos of a single plug on this post are #5. Photo of three plugs shows #1, #3, #5, left to right.
 

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b454rat

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I have a similar misfire with my Yukon. But mine seems to be at idle/low RPM. I was told an intake leak....Maybe your gasket got boogered on install. Haven't had a chance to fix mine yet....
 

Orpedcrow

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Now I’m not a spark plug reading guru, but 5 doesn’t look like it’s firing.
At the same time it doesn’t look like it has unburnt fuel on it either.
(Don’t be offended by this next question)
Does it run like it has a misfire?
Or is it just a code?
Was the misfire there from the beginning with the new engine?

I would still crank it over with the valance cover off and watch your rockers.
And reset lash just to confirm.

Is there a possibility that there is something stuck in the intake runner or in the injector hole?

Is there a big coolant/carbon booger in the exhaust runner on 5?
 

termite

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Check the ceramic for cracks. I've chased a misfire before to discover that one new plug had cracked ceramic resulting in weak spark and noticable misfire.
 
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