Anybody? I could really use some help with this, I'm pretty stuck.

 

Wish I could help. I'm currently chasing down a cylinder 2 misfire myself.

 

I would say with the milage. AC delco plugs and wires would be my next step.

 

Wish I could help. I'm currently chasing down a cylinder 2 misfire myself.

Good luck. It's a pain. Yours will hopefully be easier to track since you've only got one cylinder to trace down.

 

I would say with the milage. AC delco plugs and wires would be my next step.

Thank you. That would definitely be the most typical cause. I forgot to mention, the P/O (previous owner) allegedly changed all of the spark plugs, spark plug wires, and coil packs last summer. I can't confirm that he did, but they do look new. However, even if all of that had gone to ****, that still doesn't explain the throttle position reading wrong.

Does anybody think it is possible that my replacement throttle position sensor (which says on it that it was made in China) could be reading wrong? I know problems can occur by not buying OEM. I'm considering replacing the throttle body (which has the sensor on it) with the OEM part, but I can't afford to keeping throwing parts at it without having something to go on.

Thanks for the input so far.

 

I had a 6.0 with a broken valve spring which gave an erratic misfire. The dealer said it needed an injector which of course didn't help. I finally ran a compression test and on one cylinder it would sometimes have compression and the next time not. after removing the valve cover the broken spring was visible

 

Did the truck run rough? Why I ask is my truck runs like a clock smooth no shaking nothing. I've tried everything except removing the valve cover gasket. Might just have to do that.

 

Just had a LQ4 with p0300 , p0101 noisy valve train has broken valve spring , drivers valve with power tools is like 15 minutes to get off. looking at it was only way to tell.

 

IM having the same problems but with a brand new motor. 12 year old injectors, new throttle body. When i tap the pedal in park it hesitates and climbs then drops to low. I cant figure it out, and cylinder 2 and 6 are miss-firing the most.

 

Did this ever get resolved? I am currently trying to research why my truck has a random misfire, but only causes a torque converter unlock and/or flashing CEL once every blue moon.

 

IM having the same problems but with a brand new motor. 12 year old injectors, new throttle body. When i tap the pedal in park it hesitates and climbs then drops to low. I cant figure it out, and cylinder 2 and 6 are miss-firing the most.

You might want to check fuel pressure, both at idle and when the motor is at say 2500 rpm. The FP should be fairly constant and around 55-60 psi.

 

I just did plugs and wires yesterday. The #7 is the hardest to reach but having a stool to stand on makes the job 10 times easier. My truck has about 73000 miles on it and 3000 hours. I don't remember the OEM plug number, but I replaced them with Ac Delco 41110 iridium plugs. The looked like the Ac Delco plugs that came out but the OEM plug number was different. The truck runs noticeably better now. More smooth at idle, better throttle response and better acceleration.
Those were the stock plugs that came out, so they have been there since 2004. Stock gap is .040" and those plugs were worn to about .050"-.060". Previous owner stated it had a random misfire since new. If its now gone, I suspect a faulty plug from the factory but time will tell I guess. Either way, its running noticeable better.

 

I'm chasing a misfire in cylinder 7 and can't find what's wrong .changed plug did improve also one of the cats was bad fixed that and new o2 sensor ls improved .I've done spark test fuel tes injector test all pass only code I have is p0300 . I've seen that there are 2 ground wires on driver side firewall next to cylinder 7, one is missing can that cause My misfire

 

Hello, first post here at SilveradoSierra.com.

If you're wanting to read this quick, skip most of the first paragraph below.

Within the last month or two I bought a 2003 GMC Sierra 2500HD 4x4 extended cab short bed with the 6.0 Vortec motor. It has 125,000 miles on it. I bought it with a bit of a misfire, which I figured wouldn't be too hard to fix. I was wrong. I can't for the life of me track down this issue. So far I've put a new Accelerator Pedal Position Sensor and Throttle Position Sensor (on the throttle body), because AutoZone scanned the truck and pulled up some throttle position sensor correlation errors. That didn't change anything. Then I discovered that I thought my fuel pressure was a little low (about 50-51psi, I've only tested it with one gauge). I was also VERY rapidly losing pressure after shut off. That turned out to be my fuel pump check valve, but long story short I put a new OEM Fuel Pump, OEM Fuel Pressure Regulator, and a new fuel filter. I still haven't tested it with another gauge but I'm not losing pressure (beyond specification). Pressure still reads 51. I don't think that's my problem honestly. I had my original pump as well as two others in there and it was the same. I cleaned the ground back there at the pump, tested for proper voltage, and load tested the ground, and it all checked out good.

After a lot of online research, I decided I should try doing a crank relearn. My buddy with EFI Live came over to read information off of my truck and do a crank relearn. First, the misfire really is random. At idle it is mostly at cylinder 4 and also 3, and at higher rpms its mostly 5 and 8, but almost every cylinder misfires at some point. And here's what's interesting. When he went to do a crank relearn, it says there is an error. The engine has to be at a certain temperature, which is was, and it can't be moving, which it wasn't. However, it also says the throttle position must read 0. No matter what we do, the throttle position will not go below approx. 3.7%. I've cleaned and reconnected the ground coming from the throttle position sensor (located on the rear of the passenger cylinder head), and somewhat searched for any broken wires, especially near the sensor. I replaced the first 8" or so of wiring to the sensor plug.

I'm trying to think if I have more information. Please ask for any more information needed. I'm really lost, I have no idea what to try next. I would really appreciate some help. This misfire is really annoying, and it also causes my torque converter to be unlocked most of the time when I'm driving which bothers me even more. I really don't want to take it to the dealer (AKA Stealership), I can't afford it lol.

Thanks guys.

Same issue … ive replaced everything…a friend had the same
Issue with his truck.. took it to a shop. They said he had a loose ignition wire in the harness. Fixed it for $150.00 taking mine to the same shop next week.. will keep you posted

 

Same issue … ive replaced everything…a friend had the same Issue with his truck.. took it to a shop. They said he had a loose ignition wire in the harness. Fixed it for $150.00 taking mine to the same shop next week.. will keep you posted

Did fix your truck? Anything new that you found out?

 

I know this is an old thread. But I've been fighting a random missed code p0400. Have a pretty decent computer where I can watch the misfires. Majority of them come from cylinder 6. Then you will have a few on 4,6 and 8. Change the throttle body first. Then plugs and wires. Doesn't do it when I first started up. Minute I drive it for a while stop at a traffic light at an idle anywhere. The misfire comes back mainly on 6. Here's a strange thing, if I turn the engine off and back on again. It starts running good again. Went through all the grounds cleaning them. Seems like it still would be more electrical the fact that I can turn the engine off turn it back on and it runs good for a while. Haven't pulled the valve cover yet just don't seem to make any sense.....

 

Hi Sir

I came across this thread after a month of searching. I had a customer's vehicle for almost 2 months and tried everything. Tried almost 4 ecu's and only the relearn worked and the misfires disappeared.

Thanks a lot for all your help. I just registered on this forum to thank you.
😎🙌

 

I see lots of details on changing parts, as we all do before the more simple things seem to help the most... 6.0 1500 HD is my vice and I've had this problem from early ownership until now. One thing I noticed was it seemed to run better with a can of octane booster, $8 each... so I thought I'll just buy mid grade 89 octane gasoline instead of 86... now, only $4 more at fill up... seems to get rid of my running around town misfires when the truck is running it's hottest? no more random misfires... good luck, may not work for you, but it does for us...

 

Since this necro thread has already been revived, I’ll throw some troubleshooting tips in here.

I had a similar issue with lean condition and pinging. Plastic intakes can warp and the intake gaskets weren’t the greatest for a number of years. FelPro has a steel backed replacement gasket that corrects this.

Check the long and short fuel trims while it’s at idle and in park at 2500-3k rpm. If there is a significant difference then it’s probably an unmetered air leak. My gaskets were leaking and an evap fitting was loose on the back of the intake. Smoke machines help with diagnoses but hard to see under the intake and it may not leak enough until it’s at full operating temp. Something else to check before throwing parts at it… like I did lol

 

...just fyi, Electromat just made that single post 2 years ago, so I wouldn't expect a response from that user.