P300 Random Misfire

[rlpitts]

I have been troubleshooting this for some time now. I have replaced plugs, wires, fuel pressure regulator (I confirmed it was bad with fuel coming out of the vacuum line), and fuel filter. I checked the coils with a spark tester and all checked good.

 

I just purchased an Exhaust Back Pressure gauge and am confused by the results. The right side shows 0 psi at idle and when accelerating the engine. When hooked up to the left side the gauge fluctuates from 0 to 2 psi at idle. At steady 2000 rpm the gauge fluctuates wildly from 0 to full scale. It moves so fast from 0 to full scale that you can't see hardly see the needle.

 

The freeze frame data is listed below.

Fuel Sys 1 Open Loop

Fuel Sys 2 Open Loop

Calculated Load 39.2%

Engine Coolant 125deg F

STFT Bank 1 -10.9%

LTFT Bank 1 -2.3%

STFT Bank 2 14.8%

LTFT Bank 2 8.5%

Intake Manifold Press 23.9 inch hg

Engine RPM 3117 rpm

Speed 45mph

Mass Air Flow Sensor 12.69 lb/min

Throttle Position 42.3%

 

I am guessing the left cat is clogged but why is the needle fluctuating so wildly?

 

Any suggestions would be GREATLY appreciated.

[Stevens03]

What mods do you have?

[8.1HD/ALLISON]

Judging from your data I would say the left side cat is clogged. Also I don't like the fuel trims. Left bank is running rich, probably due to cat clogged, right side running rich, not sure if that is related. I would definately look into replacing the cat and check the fuel trims again after repair.

[rlpitts]

It's a completely stock truck.

 

I'm going to get a new set of cats in a week (working out of state) and see what happens. I drove it today and it barely moved. I'll post the results when I replace them.