P0302 and +24 STFT at idle

[dgdsm]

I revved up the engine a couple of days ago in neutral to 5K+ after an extended idle for just a few secs, and since then I've been having a rough idle problem. It eventually throws P0302 (cylinder #2 misfire) at idle and the STFT goes up to +24. Before this the fuel trim was a little high (LTFT+STFT = +12) at idle, but at this point STFT+LTFT is +36 . It goes down to normal levels when driving in gear, or if I rev up the RPMs in neutral. The numbers from the O2 sensors also go haywire at idle, like the cat isn't doing anything, but look normal while moving in gear.

Is vacuum leak around cylinder #2 the only way to interpret this? I don't see how revving the engine up momentarily to 5K+ can rupture an intake manifold gasket. Would an exhaust manifold leak explain the high idle STFTs? Or is this +24 number just the effect of the ECU dumping fuel into cylinder #2 because it's not firing? The compression numbers are 160-135-170-160, so low compression in #2 could be a part of it but it doesn't seem to explain everything.

 

[93'colttsi]

Yes, if the compression is low enough in cylinder 2, then it can cause missfire, which will dump o2 into the exhuast. This o2 is picked up by the o2 sensor, and it will give a false reading of a "lean" condition and cause the ecu to add fuel (high positive fuel trims) like your seeing.

You have identified that cylinder number 2 is the problem cylinder with a compression test. Next thing to do would be a leak down test (puts compressed air into the cylinder at TDC) and find out where exactly you are loosing the compression (exhaust valve, intake valve, piston/rings or headgasket.

Good luck!

 

[dgdsm]

OK Thanks, and yes #2 is the problem cylinder. But the reason I think the low compression doesn't explain everything is that that cylinder has had the lowest compression since I put everything back together in November, and the car hasn't thrown engine code P0302 in over 2 months. 135 is the lowest I've seen though since I put it back together. STFT at +24 at idle is also something new, it's usually 0 with LTFT at +12. I also know that the exhaust manifold gasket isn't sealed up too well. If it were only about cylinder compression, wouldn't you see the fuel trim or misfire problems all the time instead of only idle?

 

[dgdsm]

Something seems to be improving on its own, so there might be something intermittent on this. The engine ran today for 20 mins without throwing P0302, and the STFTs at idle went up to +24, but then they can come down. I wouldn't be surprised if there were a leaky or clogged fuel injector. Also while running the compression test I noticed oil leaking into spark plug wells #1 and #2 from the spark plug tube seals. Attached is a Scanmaster graph showing what the fuel trims and O2 sensors are doing at the end of 20 mins (2 mins at idle). The upstream O2 sensor seems to be working normally, I'm not sure about the downstream one. If you still think the cause is an O2 sensor giving a false "lean" condition reading let me know. Again there were no misfire codes in this 20 mins run.