Random Misfire

[carlos2012]

Hello all! I'm having a misfile at idle problem on my 98 C1500 with the Vortec 5.0 engine. It has the following codes:

P0147 - 02 Heater circuit - pending
P0452 - Evap emission pressure sensor / low input
P0172 - System too rich bank 1
P0175 - system too rich bank 2
P0300 - random misfire

When viewing the misfires per cylinder via a Snap-On scanner, cylinder 5 has a bad misfire, along with cylinder 7. When pressing on the throttle in park, the misfire goes away, though sometimes cylinder 3 reports a brief misfire. The truck drives like normal, it just idles really rough and has a very strong exhaust smell. Cylinders 5 and 7 have new spark plugs, and the cap and rotor have been changed. Looking at other posts online, this seems to be common issue that can be tricky to isolate the cause of.

 

[kenh]

Could you be getting some cross firing and be read as a misfire? Cross fire as in inductance between plug wires.

 

[Erik the Awful]

You have an 02 sensor code and a "system too rich" code for each bank of cylinders. I'd start by troubleshooting the O2 sensor circuit. Check your voltage swings on your front O2 sensors.

 

[stutaeng]

Need to look at live data while running to fix this...specifically, fuel trims and O2 sensor response.

the evap code is most likely unrelated to the others.

From my limited experience...I'd guess injectors stuck open (seemingly on both banks). That's the 2 rich codes and that's the source of the misfire and O2 codes. faulty injectors are probably just dumping a lot more fuel into cylinder(s)...again just a guess based on the pretty common injector issues. Time for a injector balance test.

 

[Schurkey]

Misfires go hand-in-hand with LEAN codes. To have misfires, and RICH codes makes me very suspicious of the O2 sensors.

Fix them first.

The Evap code could be as simple as the gas cap not sealing properly--perhaps just not tightened enough. Verify with the service manual.

 

[carlos2012]

Could you be getting some cross firing and be read as a misfire? Cross fire as in inductance between plug wires.

So far I have swapped wires between banks, along with plugs, and the cylinder 5 misfire remains.

 

[Schurkey]

What is the cranking compression pressure of #5 compared to a cylinder that runs right?

 

[1997]

are you running the stock spider assembly or has it been upgraded?
my p0300 code was caused by a bad poppet in the spider assembly.

 

[carlos2012]

What is the cranking compression pressure of #5 compared to a cylinder that runs right?

I'll need the check on a good cylinder, but it does appear that cylinders 5, and 7 which are both misfiring reflect about 150 PSI of compression. I have swapped wires and plugs from cylinders with no problems, but I end up getting the same result. Before I take the plunge and buy an injector assembly, is there a recommended scan tool that can be used to do the injector balance test? I thought the Snapon MT2500 scan tool would do it, but I'm not seeing the option in any of the engine menus.

 

[carlos2012]

are you running the stock spider assembly or has it been upgraded?
my p0300 code was caused by a bad poppet in the spider assembly.

I bought the truck used in 2019 from a car lot. I'm not sure if the injector assembly is original, though judging by various parts installed, it appears that the previous owner put higher quality parts so it's possible that the assembly may have been changed. I suppose I'll have to dismantle the plenum to see what's currently installed.