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.

 

[Tonyrodz]

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.

I need to get this done myself.

 

[drdave81]

It's an 01 Yukon Denali. No power loss or noticeable issues when driving.

The light comes on at different times. It's come on at first start up and also in the middle of a drive, not any hard accels. The misfire monitors start counting as soon as it's started, warm or cold.

I haven't done a compression test, mainly because the misfire moves, it's not specific to any cylinder(s). This is leading me to believe it's a fuel related issue, which is why I was checking pressure and watching the fuel trims, which all seem to be within spec from what I've found.

 

[ks03]

I haven’t touched a tech2 for years now, but I was thinking that you can go to freeze frame for p0300 or maybe it was just in data. Anyway I think there should be somewhere that lists the number of misfires on each cylinder. Not the sweet graphic, just the number… my theory is that when you’re watching, it’s not doing it, and sometimes it acts up and one cylinder is setting the code.

 

[drdave81]

the screen shots from the graph is live, I can monitor them as they happen. Different cylinders count more misfires each drive cycle, as you can see. Each screen shot is a completely different drive cycle.

 

[Fless]

Any other codes? Maybe try a crank sensor relearn?

 

[drdave81]

Any other codes? Maybe try a crank sensor relearn?

No other codes, other than the first time I checked codes, there was a stored P0440. That hasn't come back since though, which has been a few weeks now. I'll try a relearn tomorrow, see if that helps.

 

[drdave81]

I ran the cam/crank relearn. I noticed the misfire counter doesn't count up as fast anymore, but still counting them. At almost 200k miles, I'd almost assume some may be normal. So far no light on, but I haven't been driving a whole lot. I'll report back if the code does come back.

 

[drdave81]

Just wanted to update, in case anyone else finds this in a search someday. About 2 weeks and several hundred miles later, the light still hasn't come back on. Cam/Crank Relearn did fix it. Though it lets me know I have a timing chain replacement coming up in my future at some point.

 

[Dron]

This is old thread.
I had the same problem on 2002 Avalanche. I replaced everything (plugs, wires, fuel filter, intake gaskets), except fuel pump and injectors. I replaced injector connectors (old pigtails wasn't stays on). I replaced O2 sensors, because cats was gutted and secondary O2 sensors was an emulators. I install Y pipe with old cats from donor 03 Tahoe.
I get cheep scanner (Iden400GM $150), relearn cam sensor. No more P0300.

 

[drdave81]

The code still hasn't come back since I did the relearn. That was definitely the issue for me.