Weird Misfire Issue
01-26-2019, 02:57 PM
#1
Weird Misfire Issue I need some help/guidance on where to look next to fix my misfire on cylinder #6.
Here's the scenario; I can start the truck cold and it fires on all 8 cylinders. After driving for a while-typically 45+ minutes of drive time, the engine will begin intermittently misfiring on cylinder #6. If I keep driving it-say an hour or more, the misfire becomes more consistent. After 1.5 hours of driving that cylinder is DEAD... If I park the truck and let it sit over-night when I start it the next morning it is firing on all 8 cylinders again UNTIL I have driven it at least 45 minutes. Park it and let it cool off, or park it over-night, then the process starts over. I have confirmed with HP Tuners using the "misfire channels" that once #6 starts misfiring, and is dead, the misfire counter just continues to climb higher and higher while the engine is idling, and even under throttle.
What I have done so far: Changed the spark plug, the wire, the coil, the injector, replaced the PCM, and did a compression test while the engine was hot/misfiring. The compression test showed only 115PSI with the engine running & idling. I then checked #2-same compression. Then #3 on the other side of the motor-same compression. I assume my cam is bleeding off compression and that is why the #'s are so low. This truck has a calculated 11.1+ compression ratio. I can only run 93 in it without it pinging. It has been tuned for the 93 octane and runs really well except for the misfire on 6 after the engine gets hot.
The lifters were supposedly LS7 lifters I bought off E-bay in a plain white box. At this point, since I'm thinking I'm down to a lifter that is not pumping up after it gets hot, or a wiring issue between the PCM and either the coil, or the injector.. Does anyone have any ideas as to what I should be checking next? I'm hoping someone has experienced something similar and can point me in the right direction so I'm wasting the least amount of time & $$ chasing this issue. I built the engine myself with flat top pistons, 799 heads, Fel Pro .040" head gaskets, Elgin 1839P cam, TSP LT headers, TSP Y-pipe cut into true duals with an X-pipe, then 3" Flowmasters in/out. Runs good when all 8 are firing. Really depresses me/dampens my spirits when it starts misfiring.
Here's the scenario; I can start the truck cold and it fires on all 8 cylinders. After driving for a while-typically 45+ minutes of drive time, the engine will begin intermittently misfiring on cylinder #6. If I keep driving it-say an hour or more, the misfire becomes more consistent. After 1.5 hours of driving that cylinder is DEAD... If I park the truck and let it sit over-night when I start it the next morning it is firing on all 8 cylinders again UNTIL I have driven it at least 45 minutes. Park it and let it cool off, or park it over-night, then the process starts over. I have confirmed with HP Tuners using the "misfire channels" that once #6 starts misfiring, and is dead, the misfire counter just continues to climb higher and higher while the engine is idling, and even under throttle.
What I have done so far: Changed the spark plug, the wire, the coil, the injector, replaced the PCM, and did a compression test while the engine was hot/misfiring. The compression test showed only 115PSI with the engine running & idling. I then checked #2-same compression. Then #3 on the other side of the motor-same compression. I assume my cam is bleeding off compression and that is why the #'s are so low. This truck has a calculated 11.1+ compression ratio. I can only run 93 in it without it pinging. It has been tuned for the 93 octane and runs really well except for the misfire on 6 after the engine gets hot.
The lifters were supposedly LS7 lifters I bought off E-bay in a plain white box. At this point, since I'm thinking I'm down to a lifter that is not pumping up after it gets hot, or a wiring issue between the PCM and either the coil, or the injector.. Does anyone have any ideas as to what I should be checking next? I'm hoping someone has experienced something similar and can point me in the right direction so I'm wasting the least amount of time & $$ chasing this issue. I built the engine myself with flat top pistons, 799 heads, Fel Pro .040" head gaskets, Elgin 1839P cam, TSP LT headers, TSP Y-pipe cut into true duals with an X-pipe, then 3" Flowmasters in/out. Runs good when all 8 are firing. Really depresses me/dampens my spirits when it starts misfiring.
01-27-2019, 10:03 AM
#3
It sounds like you have a wire problem, as in a coil pack or injector lead. If it was the wire, plug, coil etc., it would be a steady miss. A wire is breaking down when hot. Check the coil pack lead pinout for voltage, ohms, etc. Clean it with brake fluid and put dielectric grease on the connection. I do not think you should look for a mechanical problem before checking out the circuit. Check it especially when hot and missing.
, coil etc
, coil etc
04-12-2019, 09:21 AM
#4
Just a follow up to put a cap on this. After tracing the IGNITION 6 wire between the PCM and coil pack connector-which tested good-I grabbed another complete set of coils from another core engine I have sitting here and bolted them on. That seems to have fixed the issue. It was a wiring problem-NOT THE COIL. The smaller coil wiring harness that stays attached to the coil bracket was the issue.
04-13-2019, 10:58 AM
#5
Just a follow up to put a cap on this. After tracing the IGNITION 6 wire between the PCM and coil pack connector-which tested good-I grabbed another complete set of coils from another core engine I have sitting here and bolted them on. That seems to have fixed the issue. It was a wiring problem-NOT THE COIL. The smaller coil wiring harness that stays attached to the coil bracket was the issue.