2003 2500hd 6.0 not so random misfire

[Will03]

I know this topic has been beat to death, and I've read every thread on it and still haven't found my problem. Truck is a 2003 2500hd 6.0 with 265,000 miles. Only throws the code at 2000-2500 rpms and isn't noticeable. Misfires only happen on cylinders 2&3 (P0300) and no other codes are thrown. Fuel trims stay +/- 5% short term and stay right around 6% long term with no change when the misfires start. Both upstream 02 sensors have voltage movement as expected. I have swapped plugs, wires, coil packs, injectors (also checked resistance and all were 10.2-10.5), swapped a known good MAF from another truck, had a crank variation relearn done, compression on both cylinders is 148/150 (cylinder 1 was checked for reference and it was 150 also), replaced intake manifold gasket, and haven't found a vacuum leak. fuel pressure is 55psi running with key off and 50-51 running. i have checked fuel pressure regulator and it is working. I have checked the coil and injector wiring to the main harness and looks good up to there. Checked cats with a vacuum gauge and they are not clogged. I'm not sure where to look next. The only correlation between the 2 cylinders I can think of is that they are companion cylinders in the firing order, which would point to a crank sensor (in my mind), but I am told if a cam or crank sensor fails it would be more of a sporadic misfire and wouldn't only affect 2 cylinders. my next plan is to pull the motor and get the oil pan off to check the reluctor (would really prefer to just swap out a lower mileage motor if I'm already pulling it, but I don't want this problem to follow to another motor). Any and all input is greats appreciated.

[Jsdirt]

If it happens at that RPM at light throttle, that suggests a vacuum leak. Get a spray bottle of water and go over everything with it. If there's a vacuum leak, you should find it that way - you'll hear the water getting sucked in, and/or hear the engine RPM change. If there's an ignition problem, the water should find that too.

 

If that doesn't turn up anything, I'd do a leak down test on both those cylinders. Could be a valve spring or valve sealing issue, or even a head gasket issue.

 

You might be reading 2 misfires when there is only one in reality. This usually happens alot on Fords, but can happen with GM too if the misfire is severe enough. The crank sensor can't tell the difference between companion cylinders in some cases.

[Will03]

If it happens at that RPM at light throttle, that suggests a vacuum leak. Get a spray bottle of water and go over everything with it. If there's a vacuum leak, you should find it that way - you'll hear the water getting sucked in, and/or hear the engine RPM change. If there's an ignition problem, the water should find that too.

 

If that doesn't turn up anything, I'd do a leak down test on both those cylinders. Could be a valve spring or valve sealing issue, or even a head gasket issue.

 

You might be reading 2 misfires when there is only one in reality. This usually happens alot on Fords, but can happen with GM too if the misfire is severe enough. The crank sensor can't tell the difference between companion cylinders in some cases.

There are no misfires at idle. It doesn't log a single one until rpms reach 2000, then the misfire data goes crazy. I have tried spraying water looking for a vacuum leak and haven't found any. I will give it a shot on the ignition components. For some reason it has completely slipped my mind to do a leakdown test. I'll see if I can come up with the equipment and do that this weekend. Thank you for your advise.

[Jsdirt]

No problem. Good luck.

[Swimmer8366]

How's the wiring going to the throttle body sensor?

 

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[Swimmer8366]

What's your pressure running at idle and running at 2-2.5k?

 

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[Will03]

What's your pressure running at idle and running at 2-2.5k?

 

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Fuel pressure is between 53 and 55 at 2k and 50 at idle. throttle body wires look good.

[Will03]

Here are the instant data graphs of the misfires. Pic 1 they start exactly at 2000rpms and pic 2 they stopped immediately when I dropped the throttle.

 

 

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[Swimmer8366]

Check your volts at your sensors

 

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[Will03]

Check your volts at your sensors

 

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O2 sensors?

[Jsdirt]

I'm starting to think this may be an ignition problem, based on the misfires at higher RPM.

 

Snap the throttle a few times, and see what the counts do then. If it gets worse on the snap, then you've definitely got an ignition issue.

 

Since you did the swaptronics game already with the coils, I'd yank the plugs and see how they look. You may have some extra large plug gaps from wear.

[Will03]

I'm starting to think this may be an ignition problem, based on the misfires at higher RPM.

 

Snap the throttle a few times, and see what the counts do then. If it gets worse on the snap, then you've definitely got an ignition issue.

 

Since you did the swaptronics game already with the coils, I'd yank the plugs and see how they look. You may have some extra large plug gaps from wear.

doesnt log any misfires snapping the throttle. Kind of have to hold it for a split second to get them to start. If you vary the rpms between 2000 and 2500 it seems to take a little longer than holding steady for the misfires to start. Plugs all look new and gaps are at .040. I just did a full tuneup this past summer (roughly 1500 miles ago). I did swap plugs around also when I was moving coils and wires in case there was something going on with them not visible.

[Jsdirt]

Ahh, gotcha.

 

If all the stuff I mentioned a few posts back checks out, then I'd start looking at ignition wiring. This is an instance where a lab scope would pay for itself here - you'd be able to definitively prove either an ignition or running compression problem in minutes.

 

I'd break out the water bottle and work your way around the engine, and note any changes. Running it at 2-2,500 RPM while doing this would be the way to go I think.

[Will03]

Ahh, gotcha.

 

If all the stuff I mentioned a few posts back checks out, then I'd start looking at ignition wiring. This is an instance where a lab scope would pay for itself here - you'd be able to definitively prove either an ignition or running compression problem in minutes.

 

I'd break out the water bottle and work your way around the engine, and note any changes. Running it at 2-2,500 RPM while doing this would be the way to go I think.

I plan on spending as much time as necessary trying to get it figured out this weekend. Now I have more things to check into. Thank you

[Duelin]

I plan on spending as much time as necessary trying to get it figured out this weekend. Now I have more things to check into. Thank you

Well here is another thing you might want to check into. What is the cylinder compression at 2000 rpm? If a valve spring is a little weak the valve could start floating. Just a thought. Good Luck