Another P0300 Thread

[drdave81]

It comes on a few trips after clearing it. I replaced plugs and wires. I've tried both .060 and now .040 gap plugs. NGK TR5 currently. Wires are Taylor. Fuel pressure is low 50s at key on, engine off, holds pressure. Fuel pressure running is high 40s.
Reading the misfire graph in Tech2, all cylinders are misfiring, but some are worse than others, and it varies. Swapping plugs, or wires, or coils does not make the misfire move. STFT and LTFT at idle, 1500 and 2500 rpms are always below 10% and within 2% of each bank. O2 voltages are toggling as expected. I think I covered everything so far. Any thoughts on what to check or where to go next with this? It feels like it idles and runs smooth, the misfire isn't detectable by driving, only because the light pops on.
The screenshots were taken at different times/days. I don't recall which one, but I believe one may have been at cold start, the others were at operating temp.

 

[OR VietVet]

Age of MAF and MAP and did you check for vacuum leaks?

 

[drdave81]

Unknown age of those. I picked this up last Oct, haven't changed either of them since everything seems to be reading within specs on both sensors. I sprayed around the intake and vacuum lines with brake cleaner, no fluctuation of RPMs at all.

 

[OR VietVet]

Compression test when plugs were out? Light comes on when idling, regular accel, hard accel.....etc.

Care to share the vehicle info? engine size? Year?....etc,

 

[OR VietVet]

FUEL PRESSURE SPECIFICATION: The fuel pressure specification for the 1999-2006 4.8L, 5.3L, 6.0L Chevrolet Silverado (Suburban, Tahoe) and GMC Sierra (Yukon, Yukon XL) is 55 to 62 PSI. Flex fuel engines: 48-54 PSI. Connect your fuel pressure tester to the Schrader valve.


With the pressures you stated and what is spec, I would guess the light comes on with any accel.

 

[Fless]

FUEL PRESSURE SPECIFICATION: The fuel pressure specification for the 1999-2006 4.8L, 5.3L, 6.0L Chevrolet Silverado (Suburban, Tahoe) and GMC Sierra (Yukon, Yukon XL) is 55 to 62 PSI. Flex fuel engines: 48-54 PSI. Connect your fuel pressure tester to the Schrader valve.


With the pressures you stated and what is spec, I would guess the light comes on with any accel.

These specs are KOEO (engine not running).

 

[OR VietVet]

I got lazy. Did a search and saw that the running fuel pressure in in the 40's and his high 40's should be good. Thanks for the catch @Fless.

Still want to know if the light comes on when accel, idle.....etc.

 

[Tonyrodz]

Compression test when plugs were out? Light comes on when idling, regular accel, hard accel.....etc.

Care to share the vehicle info? engine size? Year?....etc,

Is it best to do a compression test on a cold motor/hot or warm? Will the readings given change?

 

[OR VietVet]

I have never done a compression test on a hot/warm engine. Don't know why other than I did not want to burn my delicate fingers/hands. I don't know if the temp can reveal different readings. I do know that if in a hurry and need to check for a blown head gasket, I have just pulled the cylinders plugs that I believed to be suspect. If I am doing a complete compression test, all plugs are removed and then I do a dry and then a wet test if needed.

 

[Scottydoggs]

if it feels gut less, id guess bad cats.