Chasing a P0300

[Cornelius]

Did you ever get any codes O2 related?

at one time yes, then replaced them

If a vacuum leak, (manifold) then fuel trims should go hi- over 10 up to 30+. Maf wiring also connects to TB, so a bad ground/flexing wires on Maf plug. I'm sticking with MAP, thats where the cover bolts on so check wires there too. Pull it clean it oil gasket reinstall. and add a sensor in your scanner for it and monitor.
Even a bad pcv could be enough to miss. It's the 4 unconnected cylinders thats the stumper, not side by side.

My fuel trims were that high at one point now it’s normal after vac line repair. I will look at MAP sensor like you suggested. I always thought that connection was a little soft. I should note that I did change TB gasket to eliminate one more possible point of entry unmetered air and replaced the MAF because the reading would never be more than 5 no matter what the load was. Now it reads correctly and my mpg has improved but flashing cel and misfires on 1 5 6 8 persist.

 

[Tonyrodz]

at one time yes, then replaced them


My fuel trims were that high at one point now it’s normal after vac line repair. I will look at MAP sensor like you suggested. I always thought that connection was a little soft. I should note that I did change TB gasket to eliminate one more possible point of entry unmetered air and replaced the MAF because the reading would never be more than 5 no matter what the load was. Now it reads correctly and my mpg has improved but flashing cel and misfires on 1 5 6 8 persist.

Which vacuum line did you replace? Think I missed that.

 

[exp500]

Cornelius- I'm not sure this is it, but has affected a few others.Plus it's a cheap fix if you are not afraid of it.
Disconnect battery, both terminals, touch cables a few seconds. Unplug computer with one hand grounding yourself to truck, then to computer case. Inspect pins and sockets for water and corrosion, clean both and apply dilectric grease to seal and re-install. If it is green/dirty/wet you found it.

 

[ks03]

...but flashing cel and misfires on 1 5 6 8 persist.

Flashing cel, and it fells like it’s running well while it’s flashing? Typically you’re going to feel a misfire that makes the light flash.


so, afaik misfire detection works as a drop in crank speed. if it was a bad miss I was going to say the reduction in crank speed is enough that sometimes the next cylinder in the firing order shows misses. With the firing order being 1,8,7,2,6,5,4,3 a miss on 1 can show up as misses as 1 and 8. What I’m wondering though, is that if it runs great, if there isn’t a problem with the crank reluctor wheel. I haven’t ever had a reason to even look at one, but this was a thing on 3.8s where the wheel was on the back of the crank pulley. I don’t know if it’s even possible but I’d want to try to take out the crank sensor and watch it while turning over the engine by hand... alternatively you could try changing the crank sensor, but I have no idea how much they cost, and they typically don’t present like this

 

[Cornelius]

I know you say you replaced the fuel pressure regulator, but I’m suspect of it still.

one possibility for misfires on 1 and 8 is liquids in the intake, weather it’s caused by gas, oil or something crazy.

5 and 6 are near where the vacuum line for the regulator connects.

occasionally pulling lots of fuel, then evening out, especially after a hot soak, would indicate a regulator.

im not saying replace it, but just check it a couple more times. Just pull the vacuum hose off, checking for fuel. If it’s dry, twist the regulator to point the port down and make sure everything is dry.

FPR is dry, and seems to be working. Fuel pressure holds now with the new pump

Which vacuum line did you replace? Think I missed that.

Replaced the 90 elbow of the FPR, replaced the grommet where the PCV goes, insoected and trimmed the end of the pass side breather hose from valve cover to TB

Cornelius- I'm not sure this is it, but has affected a few others.Plus it's a cheap fix if you are not afraid of it.
Disconnect battery, both terminals, touch cables a few seconds. Unplug computer with one hand grounding yourself to truck, then to computer case. Inspect pins and sockets for water and corrosion, clean both and apply dilectric grease to seal and re-install. If it is green/dirty/wet you found it.

LOL it almost sounds like I should be looking for a hidden camera for this one! What is happening with this procedure?

 

[exp500]

Glad your sense of humor is still working. Just remember everything is connected to the net! Have you found the "Candid Camera " post yet?

 

[Cornelius]

So touch pos and neg to discharge the capacitors in the ecu and inspect the plug for corrosion. Do a hokey pokey and turn myself around. I don’t quite understand how this will stop misfires. Unless there aren’t actually any and the ecu has gone rogue and is reporting fake news? I’ve never heard of that but then again I don’t know what I don’t know

 

[exp500]

What did the radio used to say? " This is a Test and Only a Test".

 

[livingez_123]

I will pop the random misfire if I let mine sit and idle for a long period of time or lots of stop and go traffic...but it's not missing. I believe it's my cam that making it think it's missing. It runs smooth, plenty of power it's just a little chuggity chuggity at an idle. I hope to get it tuned very soon to smooth the idle out. If I bring up the rpm to 700rpm it doesn't pop the code.

 

[livingez_123]

Do not disconnect battery to clear codes, use scanner. Every time you disconnect battery it goes into Re-learn mode. Needs a 50 mile drive to adjust fuel spark and trans. Don't do anything except clear codes and monitor for a few days.

I just learned something new^^^^^^^^^^^

 

[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.