I have an 05 4.8L Sierra with 293k miles. The last few weeks my truck has had a bit of a rough idle but runs fine other than that. Then my engine light flashed a few times which was P0308. I don't have a scanner that shows missfire count but i can say that in the last two weeks of driving back and forth to work 8 miles both ways, the CEL has only flashed once for P0308.

I have swapped coils, wires, plugs, and injectors and it made no difference. I've sprayed carb cleaner around the intake with no change in idle/sound. After fighting with a compression gauge with damaged threads and dropping a plug on the concrete, ground strap down, i finally got to test compression today but the engine was cold and i only tested one side.

Dry tests:

Cyl 8: 140
Cyl 6: 195
Cyl 4: 190
Cyl 2: 215

Cylinder 8 wet test came out at 215.

Fuel pressure with key on was 50, running at idle: 58.

With all that said, this truck burns around half a quart of oil between oil changes (5k intervals)

The compression test seems pretty conclusive that the rings are bad in cylinder 8. Should I give up and accept the fact that this engine is just about done or could there be something else causing the missfire?

At almost 300K miles, I would say its due for a rebuild.

Cylinder 8 could either be bad rings, or a valve issue. Won't know until you pop that head off.

You could pop that valve cover off and check the valves on that side. Just to verify that they are opening/closing all the way.


Either way, your looking at some labor and down time.

those engines are pretty cheap, everybody wants the 5.3 or 6.0, i would think its not worth rebuilding find one under a hundred thousand miles for a few hundred bucks and swap it out

Chilton manual says fuel pressure should be 55-62 with key on, engine off. This weekend i'm going to change the y-pipe out. I've been having a replacement off a wrecked lower mileage donor for months now. Just haven't had the time. I'll change the air filter as well as clean the MAF and throttle body.

I purchased a compression gauge so i'll do complete compression test correctly this time with the engine warm after the changes. I'll post any findings.

Anyone know which gaskets I need when i swap the y-pipe? Fel-pro only shows two, 61323 & 61089 which look like the two from the manifold. What about the one at the muffler joint?

Gm flange gaskets are left side part no 12555806 and the right side is part no 88891747.

Ok. Exactly which ones do I need? There'* 3 joints to seal. Two that goes to each exhaust manifold and the collector that goes into the muffler.

Appreciate the help.

In your case, both the part numbers he listed are items 4 and 5 in the diagram here...

Awesome. Thanks, guys. My exhaust is a bit different than the schematic. The collector, on the muffler side, is a bolted flange onto the muffler with a single output. The flange seems to look similar to the ones on the header side. Not sure which gasket would be needed for it. Any idea?

What is your complete model of truck? 1500 short/long/2wd 4wd etc etc.