98 Yukon 5.7 vortec 200,000 miles
p0300 and rough misfire under load or on hills at highway speed. Won’t accelerate past 2000rpm when it starts happening.
Is your scan tool capable of showing individual cylinder misfire history?
What is the misfire history of each cylinder?
parts replaced in order:
Plugs
Wires
Fuel filter (had black gas in it)
That can't be good. How old was it?
ICM
Rotor and Cap ( timing light was erratic when placed on any wire)
Distributor
Rotor and cap again
Replaced cat
Spider MPFI (modern)
Intake gasket (upper)
Wires and plugs checked again
Sprayed down with brake clean looking for vac leak
DO NOT use "brake clean" to check for vacuum leaks.
1. Not all aerosol brake cleaners will burn. If it doesn't burn, you don't get an indication of RPM rise from the addition of fuel. Some folks use propane, I use aerosol CARB cleaner.
2. The chemical formulation of some brake cleaners leads to the production of Phosgene gas when burned. That crap can **** YOU UP, sometimes for years or even permanently. High enough concentration of Phosgene will outright kill you.
Cap and Rotor again!
Fuel Pump (cut a hole in floor) tank looked plastic and clean through the opening anyway.
Cutting an opening in the floor is NUTS. Say you're in a rollover collision, and the gas tank ruptures. The spilled gasoline drains into the passenger compartment, you and your loved ones burn alive. That's one reason GM didn't provide an opening to begin with.
Weld that hole shut, or at least use solvent-resistant sealer and enough reinforcement that the hole
won't distort open in a collision that bends the body.
runs better than it ever has. Until you get to a hill. Still throws a code p0300 or sometimes a specific cylinder 0305 and 0304 but always accompanied by 0300.
What is the cranking compression of #4 and #5? (At that mileage, I'd test all eight.) If you find compression problems, use a cylinder leakdown tester to find where the compression is going.
fuel pressure at idle 51
Fuel pressure at koeo 55
Seems low and didn’t change with new pump even a little bit.
Verify that the gauge on your fuel pressure tester is accurate.
Almost no-one verifies their test equipment.
Thoughts:
Fuel filter plugged again from **** in the lines
Maybe.
Multiple bad replacement parts
Very possible. Not my first guess.
Timing is set to damn near 0 CPM
That's NOT ignition timing. That's just the synchronization of the cam sensor waveform to the crank sensor waveform.
Haven’t touched cam or crank sensor
Haven’t cleaned MAP
MAP sensors don't get cleaned. MAF sensors sometimes get cleaned.
Is it time to compression test for stuck valves or floating?
Yes, perform a cranking compression test.
At 2000 RPM, if the valves are floating, it's because the valve springs are broken.
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All 8 plugs looked like this after 6 months of this problem.
Looks good.
I have not. But the shop I took it to told me the cats were plugged and replaced them with new ones I purchased. They likely reused the o2 sensors tho.
There's other reasons an exhaust system could be restricted beyond "plugged catalyst(s)".
Test the back-pressure.
Use the scan tool to look at the O2 sensor activity. If you can PROVE that they're working properly, you can leave 'em alone. If the sensors have more than ~50K miles on them, or there's
any question about their age, or the speed that they flip from high voltage to low voltage you might as well just replace 'em all. The sensors ahead of the catalyst(s) are more important for engine performance than the ones behind the catalyst(s).
ICM was meant as the coil. I didn’t replace the little computer but just swapped in the part the coil wire hooks to and goes to the dizzy.
I have no idea what you're talking about.
Also that wouldn’t relate to my low Fuel Pressure after the pump replacement. Though my gauges could be cheep and from harbor fright…
As said, verify your pressure gauge, and re-check the fuel filter.