Misfire! Help!

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Canon Lansdell

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Hello all. I have a severe misfire on my 98 K1500 that started randomly while driving weeks ago. Misfired on almost every cylinder intermittently, but most on cylinder 6. First thing i changed was spider unit to MPFI setup. Read 55 psi (should read 60-65) fuel pressure. Next changed fuel filter prematurely, ran like a top until I got up to 95 mph where the misfire came back. Tested charging system to make sure everything was getting power, yes it was fine. Cleaned fuel strainer and it ran better but it still had a miss. Next changed fuel pump, misfire persists. Any one experienced this? Also, newer distributor, spark plugs, cap, wires. Tested ignition coil and ICM voltage and it was good. Any ideas?
 

SUBURBAN5

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Hello all. I have a severe misfire on my 98 K1500 that started randomly while driving weeks ago. Misfired on almost every cylinder intermittently, but most on cylinder 6. First thing i changed was spider unit to MPFI setup. Read 55 psi (should read 60-65) fuel pressure. Next changed fuel filter prematurely, ran like a top until I got up to 95 mph where the misfire came back. Tested charging system to make sure everything was getting power, yes it was fine. Cleaned fuel strainer and it ran better but it still had a miss. Next changed fuel pump, misfire persists. Any one experienced this? Also, newer distributor, spark plugs, cap, wires. Tested ignition coil and ICM voltage and it was good. Any ideas?

What does plug 6 look like?
 

Nad_Yvalhosert

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Loading up the parts cannon, huh?
Try-agnostics at 95mph? Hmmm...

If most of the misfire was concentrated on cyl #6, why throw fuel related parts at it. Diag #6 first. Swap the plug with cyl 8, maybe the porcelain is cracked. Swap plug wire 6 with 4 (on the cap too), maybe its arcing... If it stays mostly misfiring on 6, pull the valve cover and check for a fully opening valve
 

Hipster

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Tested coil? How so? Ohms tests and the like are inconclusive. They will tell you it is not bad but won't tell you that it functions under a load. It sure sounds like the coil isn't keeping up.
 

Canon Lansdell

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Loading up the parts cannon, huh?
Try-agnostics at 95mph? Hmmm...

If most of the misfire was concentrated on cyl #6, why throw fuel related parts at it. Diag #6 first. Swap the plug with cyl 8, maybe the porcelain is cracked. Swap plug wire 6 with 4 (on the cap too), maybe its arcing... If it stays mostly misfiring on 6, pull the valve cover and check for a fully opening valve
I’ve figured it’s fuel related due to the low fuel pressure. Another test said i had 52 psi of fuel pressure.
 
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