PO300 and brutal fuel mileage

[CurtBrass]

I've got a 2002 Yukon Denali, 6.0 Vortec. It recently started throwing a PO300 code and my fuel mileage has gone from about 14 mpg to about 9 mpg. Idles fine when cold but rough once it has warmed up. I've replaced spark plugs and spark plug wires, but that hasn't fixed anything. Wondering if anyone has any brilliant ideas about how to troubleshoot the problem.

 

[Scottydoggs]

clogged cats can do that, misfires and all.

 

[Fless]

Put a scanner on it and see what your fuel trims look like, STFT & LTFT, both at idle and at around 1500-2000 rpm. Post 'em up and give us a look.

 

[tomloans]

... Watching

 

[Rocket Man]

Following. Mine threw that code and more later, then I discovered my cats were clogged.

 

[rockola1971]

OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected

Random misfires caused likely by spark plugs, wires or Bad Ignition Coil(s). Likely the wires unless they are under 100k miles old. Easiest way to troubleshoot a bad wire or coil is wait until its dark outside. Start engine and let idle. Pop the hood. Keep your hands in your pocket. Have a look around for a lightning show. If the wires are cracked or an ignition coil is cracked on its high tension side then you will see sparks jumping to chassis ground (anything metal under the hood). You will see the light flashes. You stick your hands down around it and your neighbors will hear you scream like a little girl and your significant other will notice that you have peed yourself.

 

[Doubeleive]

bad cat's will cause a drop in fuel mileage, you can check the temps with a scanner, laser thermometer, or pull a 02 sensor and stick a camera in, or have a backpressure test done.

 

[CurtBrass]

OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected

Random misfires caused likely by spark plugs, wires or Bad Ignition Coil(s). Likely the wires unless they are under 100k miles old. Easiest way to troubleshoot a bad wire or coil is wait until its dark outside. Start engine and let idle. Pop the hood. Keep your hands in your pocket. Have a look around for a lightning show. If the wires are cracked or an ignition coil is cracked on its high tension side then you will see sparks jumping to chassis ground (anything metal under the hood). You will see the light flashes. You stick your hands down around it and your neighbors will hear you scream like a little girl and your significant other will notice that you have peed yourself.

I installed new NGK wires and plugs, but I haven't tried running it in the dark. Been waiting on coil packs because I don't want to spend $400 needlessly, but I might have to.

 

[rockola1971]

I installed new NGK wires and plugs, but I haven't tried running it in the dark. Been waiting on coil packs because I don't want to spend $400 needlessly, but I might have to.

You need to put engine on a scanner and look at real time misfire data. Is it limited to 1 or 2 cylinders or all of them? If its one cylinder then move that cylinder's coil to another spot and see if the misfire moves to the second cylinder. If it does then the coil that you moved initially is the problem child. Mark all your coils with a marker with their original cylinder position then you can keep track which is which during the moving.

Did all this misfire problem start before or after you replaced plugs? Sure you have the correct plugs? Correct spark plug gap?

 

[Ronald Lechleiter]

I have a 04 Tahoe with the 5.3, before this one I drove a 98 Tahoe with the 6.0, both of them did the exact thing yours is doing. I just got my 04 Tahoe back Saturday 3/26. Both had the same problem, intake manifold gaskets leak somehow it damaged the upstream 02 oxygen sensor. A scan will save you from putting a bunch of parts on that do not need replaced.