Jump to content
  • 0

2003 2500hd 6.0 not so random misfire


Will03

Question

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.

Link to comment
Share on other sites

  • Answers 45
  • Created
  • Last Reply

Recommended Posts

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.

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

I hear you there. Gotta do what you gotta do.

 

Since you had a little leakage past the rings and intake valves during the leak down test, that's probably the best thing to do.

Link to comment
Share on other sites

Good deal William - glad you've got it fixed! :thumbs::cheers:

 

Always a huge change comparing worn to good engines back to back. I'll never forget the seat-of-the-pants increase in power I got in my '89 S10 Blazer - I yanked the original engine (odometer said 77k, but I think they rewound it from 277k, as I had odometer issues later on) that burned a quart of 20w-50 every 50 miles in the dead of winter. Had a spark plug defouler installed on #2 it was so bad - would foul that plug every 3 days (about 170 miles). I grabbed an engine out of my '00 Jimmy that was having non-stop failures. Ripped all the EFI crap off and installed an Edelbrock 1404 & a HEI ignition system, & dropped it in the Blazer. Felt like a 200HP gain compared back to back - night and day difference!

 

When I tore the old 4.3 down, I found each cylinder had at least a 1/8" ridge at the top of the cylinders - that's some serious wear! It's a wonder the engine ever ran!

 

Here's a pic I love showing of just how bad that engine was - this was from the exhaust after warming the engine up after a blizzard hit:

 

Blazeroilyexhaust.jpg

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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?

 

Sent from my Z981 using Tapatalk

Once you find problem please post back.

 

Sent from my Z981 using Tapatalk

Link to comment
Share on other sites

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

 

84ccce1da8e5483f2c52bef0078dc0ee.jpg

 

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??

 

Sent from my Z981 using Tapatalk

Link to comment
Share on other sites

I got the motor pulled this morning and the motor mount on the passenger side is wore out. Think it's possible that was causing the "misfire"? I've pulled the head and removed heads and springs and the valve guides are tight and the seats are in good shape kind of disproving the valve movement and not sealing so I'm kind of back to square one, but with the engine removed.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.




×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.