96 Tahoe Random Misfire P0300

[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.