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.

 

[strutaeng]

Could be lot's of things like already pointed out.

Intake gaskets since you mentioned it happens cold. How long ago did you replace and how many miles on them? They seem to leak cold but once warmed up they swell and seal fine. The replacement was the version with the metal reinforcement plate, right?

Is the system going into closed loop? Fuel trims look okay? Is it running rich or lean when it's misfiring? O2 sensors data looks okay? Waveform looks good? You can check them by spraying some carb cleaner to induce a rich condition and temporarily unplugging a vacuum hose to induce a lean condition and watching the live data O2 sensors.

Distributers are also wear items. Same thing: how many miles on it since it was replaced? I had issues with my old 4.3 Vortec starting on damp/humid weather (actually, the darn thing didn't start, just cranked and cranked), but once it started it ran fine.

Worth doing a fuel pressure gauge test and see if it's dropping pressure. Also do an injector balance test.

 

[laredo1307]

Could be lot's of things like already pointed out.

Intake gaskets since you mentioned it happens cold. How long ago did you replace and how many miles on them? They seem to leak cold but once warmed up they swell and seal fine. The replacement was the version with the metal reinforcement plate, right?

Is the system going into closed loop? Fuel trims look okay? Is it running rich or lean when it's misfiring? O2 sensors data looks okay? Waveform looks good? You can check them by spraying some carb cleaner to induce a rich condition and temporarily unplugging a vacuum hose to induce a lean condition and watching the live data O2 sensors.

Distributers are also wear items. Same thing: how many miles on it since it was replaced? I had issues with my old 4.3 Vortec starting on damp/humid weather (actually, the darn thing didn't start, just cranked and cranked), but once it started it ran fine.

Worth doing a fuel pressure gauge test and see if it's dropping pressure. Also do an injector balance test.

Yeah, could be anything.

I replaced those gaskets about 4yrs ago with the metal reinforced ones, probably 60K miles. The 02 sensor data looks good when the truck is running normal but when it's misfiring they showing rich.

Distributor is about 2yrs old.

I'm going to check fuel pressure tonight and wires. This weekend I'll get a smoke machine.

 

[east302]

Can you view the misfire counts by cylinder to see if it’s specific to certain ones or to just one bank?

 

[laredo1307]

Can you view the misfire counts by cylinder to see if it’s specific to certain ones or to just one bank?

Yeah, I figured that out on Dash Commander yesterday.


On the way home I had no issues, no misfires as usual.


This morning on startup both #5 and #7 were misfiring at idle but under load most of the misfires with move to #1. One thing I notice though was my Cam retard yesterday was around +1.8deg when running smooth but this morning it was closer to +2.4deg. Also looked like my spark advance was much higher. Looks like I may just have a timing issue. I replaced my timing chain and crank sensor about 10K miles ago. Maybe the chain is starting to stretch a little bit. Going to fine tune that this evening.

 

[OR VietVet]

Yeah, I figured that out on Dash Commander yesterday.


On the way home I had no issues, no misfires as usual.


This morning on startup both #5 and #7 were misfiring at idle but under load most of the misfires with move to #1. One thing I notice though was my Cam retard yesterday was around +1.8deg when running smooth but this morning it was closer to +2.4deg. Also looked like my spark advance was much higher. Looks like I may just have a timing issue. I replaced my timing chain and crank sensor about 10K miles ago. Maybe the chain is starting to stretch a little bit. Going to fine tune that this evening.

Gonna adjust timing by light or vacuum?

 

[laredo1307]

Gonna adjust timing by light or vacuum?

You can't actually adjust the timing on these. It's all done by the computer.

The only adjustment is to fine tune the cam vs crank sensor correlation. You have to loosen the bolt that holds the distributor, make sure the RPMs are at least 1250prm and then slightly turn the distributor til you get within +/- 2degs. You need a scanner that has this function to show you the CMP / CKP correlation in real time.

 

[OR VietVet]

Sorry, had older 1990's rigs in my head. I apologize. Whatever you do, do not get old.

 

[Larryjb]

Yeah, I figured that out on Dash Commander yesterday.


On the way home I had no issues, no misfires as usual.


This morning on startup both #5 and #7 were misfiring at idle but under load most of the misfires with move to #1. One thing I notice though was my Cam retard yesterday was around +1.8deg when running smooth but this morning it was closer to +2.4deg. Also looked like my spark advance was much higher. Looks like I may just have a timing issue. I replaced my timing chain and crank sensor about 10K miles ago. Maybe the chain is starting to stretch a little bit. Going to fine tune that this evening.

Look for the easy fixes first, don't get distracted by what your Dash Commander is saying. Yes, it has some valuable information, look for simple fixes first. The computer is likely adjusting timing in response to the misfiring. Misfiring could simply be due to condensation inside the spark plug boots or distributor. Spraying externally around the plugs may not get enough moisture inside the boots to cause any misfiring, especially if the engine is hot.

Because the misfires seem to be on just two or three cylinders, I would focus my attention on the spark plug wires for now. I would remove and inspect all spark plug boots and use dielectric grease on all boots, including those on the distributor. (I had forgotten that yours is a 96 which has the distributor, so I'd also look there too.) I would remove and inspect/clean the rotor. Then I would use dielectric grease on the base of the rotor that sits on the distributor:

What is Dielectric Grease & Why (or Where) Should You Use It?

Learn about the benefits of dielectric grease--what it is and how to use dielectric grease on spark plugs.

www.onallcylinders.com

 

[laredo1307]

Look for the easy fixes first, don't get distracted by what your Dash Commander is saying. Yes, it has some valuable information, look for simple fixes first. The computer is likely adjusting timing in response to the misfiring. Misfiring could simply be due to condensation inside the spark plug boots or distributor. Spraying externally around the plugs may not get enough moisture inside the boots to cause any misfiring, especially if the engine is hot.

Because the misfires seem to be on just two or three cylinders, I would focus my attention on the spark plug wires for now. I would remove and inspect all spark plug boots and use dielectric grease on all boots, including those on the distributor. (I had forgotten that yours is a 96 which has the distributor, so I'd also look there too.) I would remove and inspect/clean the rotor. Then I would use dielectric grease on the base of the rotor that sits on the distributor:

What is Dielectric Grease & Why (or Where) Should You Use It?

Learn about the benefits of dielectric grease--what it is and how to use dielectric grease on spark plugs.

www.onallcylinders.com

Yeah, you're probably right. I'm leaning towards condensation in the distributor. Was going pull the cap off tomorrow morning before I drive in. But I still think I need to dial in my cam retard a little better now.

 

[strutaeng]

You may find this video useful:

If you suspect moisture in the distributer, put a hair drier in there for about 45 minutes and see if it starts.

Another thing that I discovered right before I retired my 4.3 was letting it crank for like a full minute, non-stop. Only then was I able to make fire in damp weather in the mornings. I wasn't driving that truck daily anymore, so only did it a maybe 3-4 times. I felt like I was going to melt the starter, but it worked...

Edit: oops, I thought this was the other thread on a crank-no-fire condition... I'll leave my post for anyone having that condition and the video linked is still relevant.