Dreaded P0300 possibly solved

[silky28]

Hey all,

so my 03 z71 hoebag has been acting up a bit lately. Other than a slightly rough idle there have been no drive ability issues. However, at certain, seemingly random times while on the highway the CEL will sometimes flash for a bit and sometimes I will get the P0300 code. I noticed that when the freeze frame data from the code shows the LTFT at +14 on both banks when this happens. I also found that in normal operation at idle I was seeing +14 and +10 on the ltft.

so needless to say I assumed it was caused by a moderate vacuum leak but I could not find it. I could hear something so I sprayed A poop ton of Quick Start around the engine but never got any spikes in RPM. Finally I think I found the culprit - the pcv valve. When I looked closely at it I found that the pcv was not even in its hole. It had somehow pulled out. The valve was in the rubber hose but out of its whole in the rocker. I put my finger over the end of the valve and I could feel the vacuum and hear the engine rpm change.

so I reinserted the pcv valve and since then my ltft has changed from about 10/14 to -2/-4. So I guess the vacuum leak is fixed. I took it out ok the highway and did not get the flashing cel or the P0300 but that does not happen all the time anyways.

so my questions are these:

1) why would the PCV valve have come out of its hole? Could internal engine pressure do it or is it likely that a mechanic removed it and forgot about it?

2) could a pcv leak cause the flashing cel and P0300?

3) is -2 and -4 acceptable for LTFT or should I now be looking for a reason for a rich condition?

thanks!

 

[ks03]

1. I’m guessing someone left it out. When I bought mine the pcv valve was out of the grommet on the valve cover, laying on top of it at an angle. I didn’t do anything special to secure it, just put it back in its hole. Hasn’t jumped out on its own, been a couple years

2. pcv attached but out of the valve cover, I wouldn’t expect a p0300, and less likely flashing cel. If the vacuum hose to the valve was off, then yes.

3. + / - 10% is my threshold for looking for trouble without other symptoms.

 

[OR VietVet]

Recently did my PCV and found as I traced the hose back to below the throttle body, that I had a bad hose but never did get a CEL:

 

[silky28]

thanks for replies. I’m gonna be taking the old girl on a 2000 mile journey in a few weeks...will see if cel still pops on.

normally I would be worried about flashing cel and code but there are absolutely no drive ability problems when it happens, and o2s and trims are not horrible.

 

[silky28]

So, not fixed. Today I got the P0300 again.

So, reattaching the PCV brought the LTFT from +10 into the negative, about -4 on average. But, when accelerating, the trims go to about +7 and when cruising at 75mph the LTFT sits about +11 on both banks. Also, when I hit about 2500rpm in any gear the cel flashes for the P0300 and I even got it once at about 2200 rpm, 75mph when descending a slight hill.

Any ideas for what is going on here? I assume that it is not a vacuum leak since it runs slightly rich on average and only lean while accelerating/cruising. Since it seems to be affecting both banks evenly it has to be something that affects all cylinders. I doubt, therefore that it is a fuel injector issue or a coil/spark plug/wire issue. I’m leaning towards MAS, fuel filter or fuel pump.

first though, I will try to get a crankshaft sensor relearn done.

any tips?

ps there are really no driveability issues. It does seem to be burning a bit more fuel than it used to but not significantly more.

 

[ks03]

when I hit about 2500rpm in any gear the cel flashes for the P0300 and I even got it once at about 2200 rpm, 75mph when descending a slight hill.

Misfire at light load and raised rpm means look for worn/faulty ignition bits for me.

know how long plugs and wires have been on? Know what replacement parts were used last time they were replaced?

 

[silky28]

New plug and wires about 6000 miles ago. Plugs are Bosch 8104 double platinum and the wires are the premium NGK set. I inspected the plugs a while after I installed them and they all looked fine with good gap. So basically, only ignition components left would be t

 

[silky28]

So I took her out, warmed her up and unplugged the MAF. Feel wise the truck ran exactly the same, fuel trim wise it did not.

fuel trims at idle were normal but now, when accelerating, the LTFT stayed around -5 and the stft jumped to -10 and even -17. With the MAF connected the LTFT would go +7 to +10. Is this a sign that it is a MAF problem?

also, I was unable to get the p0300+ flashing cel to reappear with the MAF disconnected, even if I tried to do it by keeping rpms around 2500...as soon as I plugged in the MAF I was able to get P0300 and flashing CEL to come on.

 

[ks03]

With having a flashing cel on any sort of repeatable basis, and having given it a good once over, I think it time to get a scanner connected that can show which cylinder is misfiring. Even if you’re not feeling a drivability issue, the only reason the cel flashes is for a catalyst damaging misfire.

 

[silky28]

I am trying to get my hands on a vcx nano so I can do crank relearn and misfire count. Unfortunately I’m in Canada and things just are not as easy to get here. Will take a couple of weeks from when I order to get it. Unless you know of a scanner that can do those things which isn’t ridiculously expensive I have to wait.