2003 2500hd 6.0 not so random misfire

[Will03]

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

I was thinking about this last night. Would I just kill the injector and hook the gauge up and run it up to rpm? My gauge holds the highest reading so I'd have to relieve pressure from it while it was at 2000rpms and get a reading before I drop the rpms back down. That'll be a little difficult for me to do with no help and fly by wire throttle body.

[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?

[Jsdirt]

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

[Will03]

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

there was a little coming out through the crankcase, and a little through the intake. There seemed to be very little getting out the exhaust but it was harder to hear and feel for it without removing the manifold. The gauge I was using showed anything under 40% acceptable, so I didn't think anything of it at the time with every cylinder checking close to the same.

[Will03]

I have no problem spending the money on a low mileage replacement engine, I just want to be 100% sure that's where the problem actually is. With everything checking out the same through the compression and leakdown test I have a hard time wrapping my head around it being wore out and only throwing a misfire on 2 cylinders and not being able to feel it.

[Jsdirt]

Usually at 40% they barely run, if at all, in my experience. Of course there are differences from gauge to gauge, so what mine reads might be 10% different from what anyone else's reads, so there's that to factor in too.

 

I think that 20%+ level COULD be what's causing the misfires. I've never dealt with a worn out OBD-II engine (up here in MA, the vehicle falls apart LONG before the engine ever does), so I can't say for sure how the computer reacts. I do know they can only compensate so much.

 

I guess what would tell the tale would be fuel trims. If there are zero vacuum leaks and the trims are maxed out or close to it, that would say to me that the computer is at it's limit for the amount of wear the engine has.

 

EDIT: sorry my brain is smoked - just re-read your original post, and I see your trims are good. I would definitely check the valve springs out like Duelin suggested. From there, I'm out of ideas! As long as the spark plugs are new, and gapped properly.

 

Here's another thought I just had - you might just need a timing chain. Excessive slop will throw out the cam/crank correlation a little (not enough to throw a light for it), and the slop back and forth could be tricking the computer into thinking there is a misfire.

[Will03]

I had thought of the timing chain previously and suggested it to the tech who did the relearn. His thought was more of the misfire would happen when you dropped the throttle and the chain goes from having torque applied to having the torque removed and the chain goes slack on the backside then catches back up.

 

Is there a way to check valve springs without removing them that you know of? I've heard of a vacuum test for it, I'm not sure of the procedure though.

 

I have found an independent shop locally that is supposed to be very good with GM diagnostics and I'm Contemplating taking the truck to them later in the week and let them poke around for a little while with an o-scope and see what they come up with.

[Jsdirt]

Been a long time since I've used my vacuum gauge, but I think if the needle is wavering rapidly between about a 10Hg spread, that indicates a weak spring. It will be steady at idle, and get worse with RPM - pretty much where your symptoms start. Duelin may have nailed this one I think.

 

Keep us posted as to what you find, whether you, or a shop figures it out.

[Swimmer8366]

Sensors on throttle body and pedal. I'm having a misfire problem on my truck can't find it either. All your grounds are good?

 

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[Will03]

Been a long time since I've used my vacuum gauge, but I think if the needle is wavering rapidly between about a 10Hg spread, that indicates a weak spring. It will be steady at idle, and get worse with RPM - pretty much where your symptoms start. Duelin may have nailed this one I think.

 

Keep us posted as to what you find, whether you, or a shop figures it out.

This is similar to the test I did to check the cats. It would hold 25hg steady at 2200 rpms. I won't have another chance to work on it till Thursday but I'll run through this test again.

[Will03]

Sensors on throttle body and pedal. I'm having a misfire problem on my truck can't find it either. All your grounds are good?

 

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Grounds have now been checked 3 times. I'll look at the pedal sensor next time I can work on the truck.

[Will03]

Dropped the truck off at the shop last night and they messed around with it this morning. Their diagnosis is worn valve guides Letting the valves move around at higher rpms and not seal as well as worn valve springs. They wanted to pull the heads but I had them put it back together for the sake of the bill. I didn't have much time to discuss it with them at the moment to ask if they hooked an o scope up and watched the coils and injectors but I'd assume if they're taking it apart to look at the heads they have. I hope so anyway.

[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]

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]

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.

 

Well, with a junkyard engine, you're still in that same boat ... only now you have someone else's problems that you'll only discover the hard way. To counter that, lots of people have successfully installed junkyard engines and run them for years. To me, it's a gamble .. and I HATE gambling, because the only sure thing is my wallet getting lighter.

 

Definitely cheaper to keep what you've got, for sure, from a tax, insurance, title and registration perspective. Cheapest way out would be to do all the work yourself, and just rebuild what you have. Let the machine shop handle measuring, boring, honing, and resurfacing, but do everything else yourself. A short block is the best as far as speed and ease of completion goes, and a long block even better, since you need heads anyway. Plenty of options out there.

 

All my buddies think I'm nuts for spending $4,000 in parts on a 23-30 year old vehicle, but anyway you slice it, that's WAY cheaper than a payment on a new one, not even considering taxes, insurance, etc., and the vehicle will be reliable for years to come. My '07 Silverado is the newest of my fleet, and I'll be running that one right into the ground.