Truck had a # 8 cyl. misfire code , replaced plugs to copper, did not fix. Anyone had this happen before? 119k on truck

 

I'd post this in the 3rd gen section you'll get more replies there

 

I will move it over for him. No need to double post.

2nd Gen = 1994-2001 model years
3rd Gen = 2002-2008 model years

Welcome to DF!

 

I believe the first thing I'd check is the coils. Try moving #8 to another cylinder and visa verca. Clear the code and see if it comes back on 8 or if it moves to 6. Obviously then if it moves, it's the coil. Replace that and you're golden. Plug wires are another easy one to replace.

If not, you might have some trouble in the head or block. Maybe a valve problem. Maybe a spring or push rod problem. Maybe something more simple like a valve cover gasket leak (or #8s spark plug gasket specifically). Valve cover and spark plugs gasket set is a $30 packet at pep boys, and isn't much more involved than changing spark plugs.

Might also be the fuel injector.

 

Welcome to the site.

 

Thanks guys, I am a little new to the forum deal so please forgive me for posting in the wrong generation initially. I have an 04 thunderroad edition laramie with the 5.7. Out of the blue I had a problem with the truck surging as soon as cranked it. Took a code read with # 8 cyl missfire. I have replaced the plugs( orig copper ), wires autolite, and the # 8 coil.....Nothing. It cranked up fine after install, and ran smooth until I shut it off and recranked it. It started surging as if I hadn't changed anything. The only thing I think it could be would be the # 8 injector may be leaking, and possibly flooding that particular cyl out. I do not have a pressure tester to do this. I don't know if I am heading in the right direction, but money is tight and there is no warrenty. Dealer is not an option at this time, and I am down one vehicle. Thank you for any info on this!

 

It's a little tricky with misfires, because there are so many reasons they can happen. The "throw parts at it" method is all I can suggest unless there are other symptoms. Injector isn't a very common failure (as in there's not a large pattern of injector's causing problems on the 3rd gen trucks).

Did you already change the plug wires, and verify that all of the plug wires are in the proper firing order? Did you do any other maintenance shortly before this started happening?

Another thing you might try that's free is to simply reset the PCM and see if it's the adaptive memory that's causing the problems. If you changed plugs, you might as well so it can relearn from scratch with the new plugs. On my '04 the PCM is fuse #23. Pull that out, turn the key to the "start" position and hold it there for about a minute (or until you hear it chime). Reinstall the fuse and take it easy around the block for awhile pausing to let it idle for several seconds every time you stop. If it goes away, great. If not, I don't know what to tell you. You'd need some diagnostic tests to figure out the cause without just throwing parts at it. Could be a sensor. Could be vacuum related. Could be wiring. Could be compression.