ster1

Hi folks,
yea I have the P0300 misfire. 5.3. It's missing on 1,3 and 7 only. At first I thought cam, but the engine is silent in the valve train. No engine noise / valve ticks at all. So I did a compression test and all were in spec. 3 was even one of the highest in the range, so I'm not thinking cam. Fuel pressure is where it's supposed to be, so I thought maybe I got crap in that rail and I had some bad injectors. I swapped em across (1 for 2, 3 for 4 etc) no change. When I did the compressions I put new plugs in too. Swapped around coil packs and leads, and nothing changes. The only wierdness I can see in my scan tool (other than 1,3,7 misfires always going up, is my CKP Active Counter is all over the place, erratic. Going from 2 to 150 to 10 to what have you. I thought that number was supposed to steadily rise, and at some point (like when the counter reached 255) reset down to 1. So I replaced the CKP sensor (pita behind the starter), did the relearn 4 times, and no change. So I started looking around more. My O2 waveforms for bank 1 were all over the place and the system was in open loop. I had originally assumed that was because of the misfires (all being in bank1), and ignored it originally. I was wondering if maybe I was wrong about that, so I replaced all 4 O2 sensors. No, I wasn't wrong about that, because no change.

misfires in 1,3 and 7 only, no matter what I do. Compressions are good, and I have no valve noise, so I'm reluctant to believe it's a cam. But I've tried everything else I can think of.

Does anybody have any ideas? I'm kinda stuck. Thanks!

mountainmanjoe

it's all on one bank . Check for vacuum leaks.

ster1

I understand that intake gasket can be problematic, so today while the engine was running, I had my scan tool open watching rpm and misfires, etc. I sprayed brake cleaner all around cyl 1 intake, watching for a change and didn't see one. I don't see how I'd use a similar approach for checking the others, maybe a real long straw? I would also think leaks elsewhere in the vacuum system would produce more systemic symptoms, as in not just limited to those 3 cylinders. Do you think my assumptions are correct?