Chasing a P0300

[silky28]

I have a similar problem on my 03’ Hoe.

at highway speeds I will get a flashing cel that goes and comes back. Often times no there is not DTC code...when there is I get a P0300 and that is it. I do not have a scanner that can detect which cylinder is misfiring unfortunately.

I’ve noticed an issue with my fuel trims are off. STFT seems ok fluctuation between -2 and +3 on both bank...but LTFTs on both banks sit around +10.

I assume since both banks are positive fuel trim that this is a sign that there is a vacuum leak somewhere. Where should I start looking?

 

[exp500]

silky28- Recommend you post your own question, as you can see, sometimes they go quite long even if we don't go off topic much.
And yes, vac leak, even fuel pump.

 

[Cornelius]

WASSSUUUUPP!!!! reviving this thread for an update: I still have never seen any codes other than P0300, but last week I had a random (never happened before) crank/no start event, and I happened to be looking at the dash and noticed zero rpm. Someone a few pages ago had suggested crank sensor, which at the time I didn't give much attention to since they tend to be pretty reliable and throw codes when bad. But since I'm just about out of parts to throw at it I figured what the hell and replaced it. Started right up, ran smooooooth, and on test drive NO MISFIRES!!!!! Could this be the fix? Well, no not exactly. While it didn't misfire or light a cel where it usually did (highway speeds, on decel mostly), it did light a BLINKING cel intermittently at WOT, then shut off on its own. misfires on 3 and 8 this time. The MAF sensor I had replaced turned out to be defective so I returned it, but when looking at maf live data on this test drive, I saw very low readings, 2-4 lbs at 2000 rpms, and little change at idle. I think those numbers should be quite a bit higher at higher rpms right? Against better judgement I grabbed a used OEM one from the junkyard today and I hope its good. I should probably spring for a new one but I enjoy the extra work, frustration and confusion.

 

[Fless]

WASSSUUUUPP!!!! reviving this thread for an update: I still have never seen any codes other than P0300, but last week I had a random (never happened before) crank/no start event, and I happened to be looking at the dash and noticed zero rpm. Someone a few pages ago had suggested crank sensor, which at the time I didn't give much attention to since they tend to be pretty reliable and throw codes when bad. But since I'm just about out of parts to throw at it I figured what the hell and replaced it. Started right up, ran smooooooth, and on test drive NO MISFIRES!!!!! Could this be the fix? Well, no not exactly. While it didn't misfire or light a cel where it usually did (highway speeds, on decel mostly), it did light a BLINKING cel intermittently at WOT, then shut off on its own. misfires on 3 and 8 this time. The MAF sensor I had replaced turned out to be defective so I returned it, but when looking at maf live data on this test drive, I saw very low readings, 2-4 lbs at 2000 rpms, and little change at idle. I think those numbers should be quite a bit higher at higher rpms right? Against better judgement I grabbed a used OEM one from the junkyard today and I hope its good. I should probably spring for a new one but I enjoy the extra work, frustration and confusion.

After you change that crank sensor you should do a crankshaft position variation relearn. Short procedure with a bidirectional scanner.

 

[Cornelius]

About that....Does the relearn NEED to be done? If it runs after the changeout and no codes, how crucial is this step? Plus, I do not have a bidirectional scanner and will resist for as long as possible paying what the thieves around here charge for simple s*** like that

 

[Fless]

Highly recommended, but if it's running okay with no misfires, you can probably slide by. Anyone nearby with a Tech 2 can do it. Other advanced tools can do it, too.

 

[Cornelius]

I can’t say it’s NOT misfiring but compared to before being 4 cylinders in the dozens and hundreds, it’s down to #3 being the consistent one.

 

[Cornelius]

So, after all that, I finally ran out of shit to replace, went back to the fundamentals of swapping wires and plugs coils, discovered that the #3 wire was bad. this is the set I had bought and installed to rule out plug wires. I had saved the ones I removed, and I heard angels singing as, for the first time in a couple years, DIDN'T HAVE A FLASHING CEL!!! So I looked at misfire data again, 0 on #3, but other cylinders had a few, so I went through and just put the old set back on. Drives like buttah. Ohmed the "new" set to find higher resistance in the four that showed misfires. Yes they were AC Delco, but perhaps counterfeits, perhaps returns, who knows. She runs like a champ now. I am officially a chump for throwing all those parts at it, but they would need to be replaced at some point I figure. I'm sure I had other issues because it did run better after a few of the parts I replaced, and the misifires moved around, but eventually #3 became consistent, which led me back to basic troubleshooting and finally solving it.