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.

[Will03]

Have you done a Crank Variation Relearn?

I haven't. There is a shop next to where I work that did it for me for $10 previously so I'll probably get it done one day next week.

[Jsdirt]

Ahh, that one slipped my mind! I didn't even think of that.

 

Could be a good excuse to slap some performance heads on there. Pricey though ... and then there's the 200k+ transmission, lower end, and differentials to worry about.

 

With the age, it might be wise to do a long block, but you could get away with just a head job for several years I would think. The lower ends on these engines are pretty stout, I'll give them that. I recently saw a Youtube vid of a guy that took a junkyard 5.3 lower end, and built the pee out of it with forced induction and nitrous - had 1k RWHP! They beat the hell out of this thing relentlessly, racing it, and doing dyno runs. Guy said they did about 30 dyno runs before the wrist pins decided they had had enough, and exited the block rather violently, with a crowd of people watching. The lower end had over 100k on it to start with too.

[Will03]

Did a leakdown test yesterday. All cylinders were 22%-25%. I found the ground on the driver side lower block wasn't connected (g102 or g103). Hooked it up and made no difference. Also checked the rest of the grounds again. Sprayed water after dark on the electrical components for all cylinders as well as the wiring harness and didn't See any arcing. Checked compression on the rest of the cylinders and it was in line with the 3 I had checked previously. How noticeable would a weak valve spring be? I've had valve float on 350's before and it was definitely noticeable. Would it be possible for a valve spring to be weak and cause a misfire that can't be felt?

[Swimmer8366]

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

 

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[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]

That could explain why you didn't feel the misfire. This also highlights the importance of THOROUGH testing.

 

It's possible, but not 100% convinced that could cause that problem, since the cam/crank sensors have to detect a slowing of the crankshaft when there should be a speeding up, due to a particular cylinder's firing event. Timing chain slop could certainly trick the computer into thinking the engine slowed. I would think that would throw a correlation code, but you never know with these vehicles today. What happens 20x in a row could be totally different on the next one that comes in the shop. Nothing is set in stone anymore.

 

Since you already have it apart, I'd send the heads out to be magnafluxed and rebuilt - that will eliminate any possibility of leaks of any kind. The machine shop can measure the guides and either confirm or disprove the worn guide theory. Timing chain and oil pump would be next, along with new head gaskets of course, knock sensors, intake gaskets (upper and lower), and I'd want to inspect the cam and lifters as well. If it were me I'd just do it all. Don't forget a new oil pan gasket and rear main seal. Might as well.

 

If the engine was quiet, you probably have several years left on the lower end. Since it's apart, you could throw a set of rings in there easily, and LIGHTLY hone the cylinders, and then check the lower end bearings while you're there. Kind of a tough call to say when enough is enough, but since you have it apart, I'd do ALL the work NOW while it's out.

 

It sucks removing an engine the second time - been there, done that! Any time I have tried to save a buck on a job like this, it ALWAYS bites me in the ass later. Do it all, do it right, and do it ONCE. But at the end of the day, you can only do what you have a budget for. Your call there.

I already have a replacement engine with 105k sitting in the shop ready to drop in. for the money and time involved a replacement is a better solution for me right now. I am keeping my core and I'll be going through it as I have the time.

 

What made me think the motor mount could cause the misfire, I have read several cases (more on the 5.3, but still the same thing pretty much) of people getting a p0300 from a slightly stretched belt, or slightly worn tensioner causing vibrations through the engine.

[Will03]

New motor is in and the truck runs great. I haven't had a crank relearn done yet but there is no more CEL flashing. Very happy I decided to take the motor swap route. I didn't realize how tired the old engine was until now.

 

 

Just a side note for anyone swapping an LQ4 to an LQ9, a computer from an LQ4 vehicle will work fine on an LQ9 (assuming you're using the trans the computer is for). 90% of everything I read said I would need a tune or to reflash it.

[Will03]

I've been under the valve covers already checking springs. I was actually surprised how clean it was for the mileage.

 

 

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[Swimmer8366]

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

 

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[Will03]

Ahh, that one slipped my mind! I didn't even think of that.

 

Could be a good excuse to slap some performance heads on there. Pricey though ... and then there's the 200k+ transmission, lower end, and differentials to worry about.

 

With the age, it might be wise to do a long block, but you could get away with just a head job for several years I would think. The lower ends on these engines are pretty stout, I'll give them that. I recently saw a Youtube vid of a guy that took a junkyard 5.3 lower end, and built the pee out of it with forced induction and nitrous - had 1k RWHP! They beat the hell out of this thing relentlessly, racing it, and doing dyno runs. Guy said they did about 30 dyno runs before the wrist pins decided they had had enough, and exited the block rather violently, with a crowd of people watching. The lower end had over 100k on it to start with too.

I have absolutely no problem spending the cash on a motor. With the mileage on it, it has definitely served its purpose. I have less than $3000 in the truck as it sits now and I would spend at least $6000 to replace it with another high mileage truck. I've been checking around and I can pick up a 120k-150k replacement for around $1200 and I'll have an extra set of manifolds, intake, injectors, coils, etc. I know as soon as I put a good set of heads and cam in this motor something else will let go. That's how my luck goes.

[Jsdirt]

Anything over 10% is getting worn. Over 20%, IMO, is VERY worn. Where was the leakage going? Rings, intake, exhaust, or ???

[Jsdirt]

Pulled my driver side cover today to find the springs are fine not broke or anything but I did find this

 

...

 

So yay me time to pull heads and get them redone and do gaskets timing chain oil pump would really just like to do a build than drop but money is not there for that can do all the gaskets timing chain and oil pump and water pump for 200-250$ and clearly a couple days of work. I have never seen a head gasket cause a misfire just at idle though??

 

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Idle is high vacuum, since the throttle plate is shut. Cruise and idle will pull the highest vacuum - those 2 instances will be when coolant or oil is drawn into the combustion chamber. The leak is probably small enough to only create an issue at idle - anything above that and it just burns the coolant. If it's a compression-related misfire, higher RPM will overcome that too. More speed = less time for leakage.

[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]

That could explain why you didn't feel the misfire. This also highlights the importance of THOROUGH testing.

 

It's possible, but not 100% convinced that could cause that problem, since the cam/crank sensors have to detect a slowing of the crankshaft when there should be a speeding up, due to a particular cylinder's firing event. Timing chain slop could certainly trick the computer into thinking the engine slowed. I would think that would throw a correlation code, but you never know with these vehicles today. What happens 20x in a row could be totally different on the next one that comes in the shop. Nothing is set in stone anymore.

 

Since you already have it apart, I'd send the heads out to be magnafluxed and rebuilt - that will eliminate any possibility of leaks of any kind. The machine shop can measure the guides and either confirm or disprove the worn guide theory. Timing chain and oil pump would be next, along with new head gaskets of course, knock sensors, intake gaskets (upper and lower), and I'd want to inspect the cam and lifters as well. If it were me I'd just do it all. Don't forget a new oil pan gasket and rear main seal. Might as well.

 

If the engine was quiet, you probably have several years left on the lower end. Since it's apart, you could throw a set of rings in there easily, and LIGHTLY hone the cylinders, and then check the lower end bearings while you're there. Kind of a tough call to say when enough is enough, but since you have it apart, I'd do ALL the work NOW while it's out.

 

It sucks removing an engine the second time - been there, done that! Any time I have tried to save a buck on a job like this, it ALWAYS bites me in the ass later. Do it all, do it right, and do it ONCE. But at the end of the day, you can only do what you have a budget for. Your call there.