Random misfire - with codes

[freak007]

My truck is a '97 Mounty, 155K miles, Gibson cat back exhaust and gutted rear cat.

Has ran great for the last 75K miles that I have had it (50K with the exhaust mods). Developed a random misfire about 12 months ago... Seemed to happen most often at light loads, but was impossible to reliably duplicate...

6 months ago CEL came on, a scan brought up P0420 (Catalyst System Efficiency Below Threshold (Bank 1)), wasn't worried about it... Misfire still random, and unable to duplicate (intentionally)

Last week it stalled out - again unable to duplicate... But this time a scan shows (in order):

P0420 (Catalyst System Efficiency Below Threshold (Bank 1))
C341F (I know it is a Chassis code, but cannot find any info on it)
P0546 (Exhaust Gas Temperature Sensor Circuit High (Bank 1))

Not sure where to go from here...

Wires seem to be fine, plugs were replaced ~50K ago, I pulled a couple today and gaps were .054 -.060, plugs looked pretty good. Wires seem fine also. Fuel filter has 50K on it also...


Any suggestions?

 

[Turdle]

You could not possibly get a p0546 code on a 97 mounty as there is no exhuast temp sensor.

I think it is an erroneous code, possibly should be a p1346 Which would be a cam sensor code. If the cam sensor pickup is out of range you might have random misfires.

I would have the codes read by another scanner device and report back please.

 

[freak007]

I know there is no EGT on a Mounty (especially not a GAS one)... but that is what came up...

It is the wifes truck, but I will be taking it to work so I can throw the Solus on it... I planned on running a scan, as well as KOEO/KEOR tests, but I am really not sure what I am looking for on this one

 

[shadowless127]

Sounds like a bad o2 sensor, and a bad camshaft sensor like Jon said.

Also check the coil packs.

 

[freak007]

well, finally got a "real" scan tool on it (Modis), only code found (both VIN specific and generic) was a P0420. Checked O2 sensor activity... Both upstream sensors were lazy, and both downstream sensors were switching... LTFT was +/-2%, no freeze frame data was available... I did not go into mode 6 due to time constraints...

I will be replacing the front sensors, and siming out the rears, but I doubt this is the cause of my misfire...

Any more suggestions?

 

[ranger7ltr]

Clogged/dirty fuel filter

When was the last time you changed the fuel filter? A clogged/dirty fuel filter will cause random misfires to happen since fuel volume is being restricted to the injectors...

That can explain the stalling out and the rough running...Pull the inlet side and see what the fuel looks like...If it is not clean on the inlet side, change the fuel filter and then worry why the inlet side is not clean...

Hint on that; it is not dirty or comtanminated fuel...It's coming from inside the tank....