Extensive Tune Up Misfires

[jamesdking]

This sounds a lot like the threads I’ve been reading but I’m lost and defeated…

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. Makes living in the mountains tough

parts replaced in order:
Plugs
Wires
Fuel filter (had black gas in 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
Cap and Rotor again!
Fuel Pump (cut a hole in floor) tank looked plastic and clean through the opening anyway.

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.

fuel pressure at idle 51
Fuel pressure at koeo 55
Seems low and didn’t change with new pump even a little bit.


Thoughts:
Fuel filter plugged again from **** in the lines
Multiple bad replacement parts
Timing is set to damn near 0 CPM
Haven’t touched cam or crank sensor
Haven’t cleaned MAF yet
Is it time to compression test for stuck valves or floating?

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All 8 plugs looked like this after 6 months of this problem.

 

[sync]

All your grounds cleans? Clean all the "power" line connections? (from battery/alternator/etc)

 

[jamesdking]

All your grounds cleans? Clean all the "power" line connections? (from battery/alternator/etc)

I’ll have a looksie. The rust is real up here in Montana. But honestly the bay is pretty clean.
battery cable, fuse box, and engine strap grounds seem like likely culprits. What else? Where is the fuel pump grounded? It should be coming off the distribution fuse box under the hood right?

 

[east302]

Have you checked the pressure upstream of the catalytic converters?

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[jamesdking]

Have you checked the pressure upstream of the catalytic converters?

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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. I updated the post since I forgot to include that. The only work I didn’t do myself because I couldn’t get the bolts out myself.

 

[Hipster]

I don't see a coil on your list. Ohms test are inconclusive. A weak coil can fail to keep up under load.

 

[jamesdking]

I don't see a coil on your list. Ohms test are inconclusive. A weak coil can fail to keep up under load.

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 could easily swap it back and go for a drive tho.

 

[Hipster]

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 could easily swap it back and go for a drive tho.

Ok, so the icm wasn't new? Icm is another piece of the ignition that can test ok and fail to perform under load.

 

[jamesdking]

Ok, so the icm wasn't new? Icm is another piece of the ignition that can test ok and fail to perform under load.

Noted. Care to elaborate? Otherwise I’m off to search and google more about testing it. I know when replacing it you need to use thermal paste to install it like a PC component and that’s the extent of my knowledge of it.

 

[jamesdking]

Noted. Care to elaborate? Otherwise I’m off to search and google more about testing it. I know when replacing it you need to use thermal paste to install it like a PC component and that’s the extent of my knowledge of it.

Also that wouldn’t relate to my low Fuel Pressure after the pump replacement. Though my gauges could be cheep and from harbor fright…