96 Tahoe Random Misfire P0300

[laredo1307]

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.

 

[YukonGTmaster]

Do you have any leaking coolant? My lower intake gaskets were completely disintegrated, and I had the same symptoms you are describing. Were the lower intake gasket replaced with the upgraded part 50k ago? I used the Felpro gasket and bolts.

 

[OR VietVet]

More Information for DELPHI CV10011

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A whole list of things come to mind. Idle Air Control Valve works differently depending on load and temperature.
Plug wires "appear ok"? Age and should be done at same time as cap, rotor and plugs.
Temp sensor is like choke for injection system.
Recent other thread had a random misfire on an older rig, like yours, and was the EGR valve.

I say "older rig" but I assure you, I had a 1996 Tahoe at one time and wish I had it back.

 

[Hoesgottaeat2]

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.

 

[laredo1307]

Do you have any leaking coolant? My lower intake gaskets were completely disintegrated, and I had the same symptoms you are describing. Were the lower intake gasket replaced with the upgraded part 50k ago? I used the Felpro gasket and bolts.

No coolant leak I'm aware of. I also replaced the intake gaskets with FEL Pro a few years back.

 

[Eman85]

Cheap test. Start the truck and use a spray bottle of water. Have someone lightly powerbrake it while you lightly mist water around each plug wire and the cap. If it starts missing you need plug wires. You can add just a drop or 2 of dish soap to make the water cling to the wires a little better.

 

[laredo1307]

More Information for DELPHI CV10011

More Information for DELPHI EG10026

More Information for ACDELCO 9718Q

More Information for GM GENUINE 213928

A whole list of things come to mind. Idle Air Control Valve works differently depending on load and temperature.
Plug wires "appear ok"? Age and should be done at same time as cap, rotor and plugs.
Temp sensor is like choke for injection system.
Recent other thread had a random misfire on an older rig, like yours, and was the EGR valve. air idle control valve is also fairly new,

I say "older rig" but I assure you, I had a 1996 Tahoe at one time and wish I had it back.

I just replace the coolant temp sensor yesterday.

The EGR valve is fairly new but I'm goin going to pull it off and check it.

The air idle control valve is fairly new as well.

I'll probably do the wires next but they have about 40K miles.

I always buy ACDelco parts too

 

[laredo1307]

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.

Yeah, I do. this could be it. I'll take a closer look at the wires tonight.

 

[Larryjb]

Yeah, I do. this could be it. I'll take a closer look at the wires tonight.

When you do, put a good coating of silicone dielectric grease on the boots for the plugs and coil.

Do you have any lean codes? Can you put a scan tool to see whether the left or right bank is primarily affected? For me this was one clue that the air intake gasket had failed. As things warm up it tended to seal better and I wouldn't have any more misses.

I tried the starter fluid around the intake and couldn't get any results from that either. I just had to take the gamble to replace the intake manifold gaskets. It's not a very difficult job to do, and this might be my second suggestion after plugs and wires.

 

[laredo1307]

When you do, put a good coating of silicone dielectric grease on the boots for the plugs and coil.

Do you have any lean codes? Can you put a scan tool to see whether the left or right bank is primarily affected? For me this was one clue that the air intake gasket had failed. As things warm up it tended to seal better and I wouldn't have any more misses.

I tried the starter fluid around the intake and couldn't get any results from that either. I just had to take the gamble to replace the intake manifold gaskets. It's not a very difficult job to do, and this might be my second suggestion after plugs and wires.

Right now there's no lean codes. I only get the P0300 random misfire. I replaced the intake gaskets 2yrs ago when I replace the spider injection. I'm going to borrow a smoke machine this weekend so maybe that will detect a leak.

I'm going to try spraying the wire connections like the other user suggested when I get home.