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.

 

[Byoung1330]

I just bought one off eBay for 150 bucks. Comes with gm software installed. I bought it cuz I do a lot of my own work an had a new crank sensor to install but didn’t wanna install it without the crank relearn. So I got it an did the relearn. It does so much other stuff to

 

[Byoung1330]

Here it is

 

[OR VietVet]

Half the price and a little less of a Tech 2. If it does what you want, great get.

 

[silky28]

I would jump all over that scanner but its the same problem I have with the VCX Nane: to get it in Canada I will have to wait quite a while.

Question: if it is just a matter of a crank relearn is the P0300 basically reporting phantom misfires? I ask because, as others have said on this, there is absolutely no evidence of a misfire when it happens. Truck runs exactly the same!

 

[BentleyArnage]

What you learn working on a Chevy will work on just about every other Chevy. Recently, one of the fleet trucks we service had a TPS code with failsafe (power derate). It's an 8.1 in a Kodiak. We found the sensor to be bad and replaced it with an aftermarket unit. It worked for a day or two, then back to it's old game, but this time there's no moving it out of the yard once it warms up. Me thinking Closed/Open loop issue and go right to the O2 sensors. Well, they're frozen at .860. Full rich. Exhaust smells very lean. Fuel trims showing -25 LT and ST both, and on both sides. If I clear the codes, it runs well and stays running as long as I don't go to idle. Hit idle, and it's back to the previous situation.

Having issues with aftermarket parts from time to time, I disconnected the O2 sensors and the truck ran down the road fairly well. I ordered factory O2 sensors. I also ordered a factory TPS, which is the whole body and sensor for $10 more than the aftermarket sensor that didn't work. In go the new sensors and here comes the flat on it's ass after warmup situation again. But this time, the sensors are working and the exhaust is more in line with what it should be. A single code remains. TPS. In goes the new throttle body and voila! Perfection! Fuel trims near zero up and down, truck runs like a racecar and it's back on the road.

Lesson learned. We have 60 of these and expect the usual problems with the crank sensor, intake gasket, wrong plugs and gap. Buy the factory O2 sensors. Especially buy the factory TPS. And use irridium plugs, gapped to .045. Save some time and have more fun. Even save money. I hate the dealers, but there's some stuff that won't work in aftermarket.

My Tahoe had the dreaded P0300 code. It would run rich and flash the light. Replaced the left side upstream O2 sensor and problem solved. You need to learn how to read the datastream to diagnose fuel problems. Example: O2 sensor showing full rich when the engine isn't will result in the computer pulling the pulses at the injector, effectively taking a stoichiometric mixture full lean. Read the O2 sensor voltage. If it's not going high low (over .45 then under .45), it will be frozen in one place. That's a bad sensor. It's telling the computer false info. The computer is blind, it only knows what it's told. It reacts to what it's told. Even if the info is false. MAF low flow code isn't a Maf issue. It's an airflow issue. Why? Too rich will quench combustion and reduce vacuum. A plugged cat will slow exhaust leaving the system. You have to find which.

Just a few ideas to help. The brain fades.

 

[silky28]

Hey thanks for the reply. The o2s appear to be working fine. The voltage on the upstream sensors bounces around as it should...the downstream sensors bounce around a bit but not nearly as much as the upstream. I don’t think it is an o2 sensor issue.

I just drove 1000miles and the truck ran perfectly...but I still got flashing cel and p0300. I even got close to 17mpg cruising at 80mph. Anyhow, I am in the USA now so I will order a scanner that can do crank relearn and go from there.

 

[silky28]

Ok, so I finally got the vcx nano scanner up and running with the tech2. I ran the crank shaft sensor relearn procedure and no more misfires being reported and no more p0300.

 

[RED TAHOE LS]

Let's consider gaining speed/rpm effects code. My gut tells me exhaust restriction either convertor of muffler. You know the drill, drop the exhaust pipe and secure for test drive. JMHO