96 Tahoe Random Misfire P0300

[east302]

This morning on startup both #5 and #7 were misfiring at idle but under load most of the misfires with move to #1.

Since it’s specific to one bank, that should rule out a vacuum leak in the intake or anything that would impact both banks (fuel pump, MAF, etc). Ignition could be the culprit as larryjb noted, or it could be something on that exhaust bank. Compare the graphs for the two upstream oxygen sensors to see if the sensor on that suspect bank is oscillating or if it’s flatlined and giving the computer bad information. Alternately, swap the upstream sensors to see if the misfires move with it.

P0300s are aggravating sometimes.

 

[OR VietVet]

Since it’s specific to one bank, that should rule out a vacuum leak in the intake or anything that would impact both banks (fuel pump, MAF, etc). Ignition could be the culprit as larryjb noted, or it could be something on that exhaust bank. Compare the graphs for the two upstream oxygen sensors to see if the sensor on that suspect bank is oscillating or if it’s flatlined and giving the computer bad information. Alternately, swap the upstream sensors to see if the misfires move with it.

P0300s are aggravating sometimes.

Agreed. Code for one side rules out so many things that effect both sides. Just had that happen on a 2011 Tundra that I was working on. Both sides sensors codes set at same time. Found a loose and open air intake tube that effected both sensors at same time.

 

[Hoesgottaeat2]

...Things I've tried so far.
- Replaced cap, rotor, and plugs. Wires appear OK

Do you keep the truck parked outside? If so, the condensation in the morning could be causing a short. My nearly identical symptoms wound up being directly related to the morning condensation when being parked outdoors. Problematic only at start in the morning. And then ran flawless rest of the day.
You say the wires are ok but I'd be looking at them as being my next potential culprit.

...I'm leaning towards condensation in the distributor...

My issue wound up being the distributor cap. I suggested plug wires because you indicated your distributor cap had already been replaced. Distributor caps for the 5.7 these days are too often very substandard in quality in my experience. Wound up having to go AC Delco with mine.

 

[exp500]

Sometimes the factory distributors vent screen on the bottom gets corroded and plugged up, worth a look for a cheap fix till the new distributor arrives. Because it doesn't clear up quickly after start it is suspect.

 

[laredo1307]

Just to give an update....I still haven't fixed the issue but I believe it's my fuel pressure regulator. I performed a pressure test on my pump which was good and wasn't bleeding off, but I didn't block off the return line to see if the regulator was holding. However I think it's more of an issue where it's sticking closed for some of the reasons below.

Things I tried
- Performed a smoke leak test and came out good
- Checked all my wires and they seem good. They only have about 15K miles.
- Tried to soak my ignition and all connections with water but was never able to get anything to misfire.
- Pulled my cap and rotor before leaving for work and there was no moisture inside. I did however replace my distributor with a new one this weekend. In the process of that I killed my battery which I thought was getting weak but come to find out I had a charging problem. All my cables were corroded on the insides so I replaced everything. Something I had been wanting to do for awhile. I was hoping this was the actual issue and it was just my MPFI not getting the voltage it needed.
- I also pulled the plugs on the two cylinders that are always misfiring before my drive, #5 & #7. I expected to find them soaked with gas but they were dry as a bone. So it seems my injectors are not stuck open or the regulator is draining off. I've read that the #5 is where most of the gas accumulates when that happens.
- I've also check my dipstick. Doesn't appear to be gas in my oil.


One interesting point is that my driveway has a small but decent incline and I always park with the nose of the truck pointed up. Yesterday I parked my truck the opposite so the nose was pointed down. Today was the first morning in over 2 weeks where it started and ran just fine. No misfires at all.

Going to replace my regulator here in the next couple days. I will keep you guys updated.

 

[justirv]

Hello Gentlemen,

I'm at a loss on trying to find the source of my misfire. The strange thing is it only happens when I fire it up in the mornings and will last my whole commute to work, which is about 30miles. It only throws a P0300 code and something is definitely telling it to dump too much fuel. By lunch time I can go out the truck and it will run just fine. My commute home there's no issues, runs like a top. I have about ~316K miles and the crank, heads, intake are all original. Otherwise every item you can think of including the fuel injection (updated model) has been replaced. Most of it about 50K miles ago.

When it's misfiring I've tried spraying started fluid every where but can't seem to find a vacuum leak. I'm going to borrow a smoke machine next.

Things I've tried so far.
- Replaced cap, rotor, and plugs. Wires appear OK
- Replaced the intake coolant sensor
- Checked the PCV valve, seems good
- Swapped the MAF out with my old one (I think is still good). Getting the same results

I have the Dash Commander app and both the MAF and MAP seems to be operating within their limits. This morning on the way in my fuel trims were all over the place and my O2 sensors were reading rich. Just now after sitting an hour I went outside and my truck is running normal. Fuel trims seems to be inline and the O2 sensors are reading much better. The only difference I notice is the Intake Temperature seems to be reading much higher. On the way in it was around 90F but now that the truck is running ok it's reading 118-122F. Of course it has warmed up outside and the truck is now sitting idle. I live is Houston BTW. I also have a new temp sensor on the way.

Just seems like it has to be a faulty sensor or maybe a vacuum leak sense it only happens in the mornings. Of course if it was a vacuum leak you think it would go away after the engine warms up and not after my drive and it's been sitting for awhile. If it was the fuel injection wouldn't it be happening all the time? Another thing I noticed was Sunday I didn't start my truck till lunch time and it was considerably hot outside. Truck ran just fine while I ran errands around town.

Things I'll check this evening.
- EGR valve (2yrs old)
- Fuel Pressure
- Replace Air intake sensor (2yrs old)
- Check Map sensor (2yrs old)
- Check Canister Purge Valve

Any suggestions would be great.

If you have not changed out your intake manifold gaskets, that may be it. When cold you have shrinkage, causing vacuum leak(s). I had a similar issue with my 2002 5.3 LM7 Z71 4x4. While your there, may want to replace your knock sensors.

 

[laredo1307]

Yeah, I upgraded those a few years ago. Smoke test this weekend didn't expose and leaks there.

 

[irwires]

I had the P0300 code a couple of weeks ago on my 2007 5.7 in my van. As it sits for sometimes a month I figured that I would fill it up with premium fuel. Twenty five minutes later after a short trip to HD I came out, started it and it was missing really bad. I limped it home, put the scanner on and it was P0300 with cylinders 2 and 7 missing. It appears that the computer could not compensate for the higher octane fuel. I ended up replacing 3/4 of the tank with 87 octane, replaced the plugs and wires, cleared the code and everything has been great since.

 

[Hoesgottaeat2]

I figured that I would fill it up with premium fuel....It appears that the computer could not compensate for the higher octane fuel....I ended up replacing 3/4 of the tank with 87 octane...

Look at the bright side. At least your lawnmower will be running great for a while.

 

[Larryjb]

I have never had an issue with hgh octane fuel. I suspect something different, perhaps bad gas? Water in the tank? Although regular 87 fuel should have a 5 or 10% ethanol mix which should take care of the water.

Was the high octane gas from a "Top Tier" gas station? Although I have heard that even some top tier stations serve bad gas too sometimes.