PO300 and brutal fuel mileage

[Docky50]

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.

I love my 98 Tahoe. Still runs and purrs great

 

[CurtBrass]

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.

Thank you, that is a great place to start.

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?

I haven't put a scanner on it (I don't have one, just a code reader), but because it runs fine when cold and only idles poorly when warmed up, wouldn't it be more likely be related to a temperature sensitive item (O2 sensor or cats), rather than a spark issue?

 

[BigRed300r]

clogged cats can do that, misfires and all.

Spot on. My wife's 04 was throwing the same code. P0300 Random Misfire. Gutted cats and now runs like a new one. But now o2 sensor codes. But that's no issue to me at this time.

 

[Scottydoggs]

Spot on. My wife's 04 was throwing the same code. P0300 Random Misfire. Gutted cats and now runs like a new one. But now o2 sensor codes. But that's no issue to me at this time.

give this a shot.

 

[BigRed300r]

Thanks

 

[iamdub]

I haven't put a scanner on it (I don't have one, just a code reader), but because it runs fine when cold and only idles poorly when warmed up, wouldn't it be more likely be related to a temperature sensitive item (O2 sensor or cats), rather than a spark issue?

When cold, it's running in open loop. This means it's running off of a "base map" of sorts and ignoring inputs from various sensors. When a certain temperature is reached, it switches to closed loop and begins making fuel and spark adjustments based on sensor inputs.

You might have a faulty sensor that is ignored when in open loop. Then, once warmed up and in closed loop, the PCM starts making undesired adjustments based on that faulty sensor's output.

If the cats were clogged, this would mechanically affect the drivability since it would limit the airflow through the engine. I'm not one to condone hard throttle before an engine is fully warmed. But, if you know a point (as far as throttle input) where it acts up, it should do it in closed and open loop.

As the others have said, a scanner that shows live data would be a huge help here.

Of course, you could have both problems- faulty sensor(s) and the resulting clogged cat(s). A faulty sensor making it run rich (MAF, MAP, O2), but still within what the PCM accepts as a normal range could go unnoticed for a long time, slowly clogging the cats and/or fouling out an O2 sensor or two. The fouled O2 sensor not being able to accurately sniff the true content of fuel in the exhaust could make the PCM think the exhaust is lean and erroneously/unnecessarily add more fuel, exacerbating the problem.

 

[rockola1971]

Quick and dirty to rule out the cats is take a temperature reading of cats inlet and outlet. The temps should be fairly close to each other. If a cat is plugged up the outlet will be substantially lower in temp at the outlet when compared to the inlet temp. This is because the cat isnt flowing much of the hot exhaust to the outlet so it wont be running as hot as the inlet. This test requires a IR thermometer (Infrared) and they are cheap at harbor freight and the like.

 

[CurtBrass]

Thanks everyone,
My next day off is on Monday so I will definitely try reading the temp before and after the cats and check the O2 sensors. I'll also see if I can borrow a reader to get some more data.

 

[rockola1971]

Thanks everyone,
My next day off is on Monday so I will definitely try reading the temp before and after the cats and check the O2 sensors. I'll also see if I can borrow a reader to get some more data.

Any exhaust shop can check it for you. Its their standard test for a plugged cat. Typically a plugged cat will glow orange/red and its sticks out like a sore thumb at night so get down on the ground and have a look after engine has been running awhile.