Ok.. 06 mark lt, started with misfire number num3, resolved, now truck runs like crap. Replaced just about everything on truck I could think of. Plugs coils Mas ,fuel pump driver, fuel filter .cleaned throttle body, replaced filter, smoke test, evap purge. 02 sensors injector on 7. New pcm. Outvof ideas.. runs , drives, no codes. Just rough. And loss of power.. 200 000 miles. Any ideas would be appreciated.. compression is good 185

 

Welcome to the forum.

 

Welcome. Mike check behind the r/s interior kick panel for any rust. That is where there are grounds & they may be compromised if that area is rusty. These gen trucks are known for rust in that area. I do not know if this could cause your problem, just something to check out.

 

Cats ?

 

Ditto
Posts #3 and #4

 

thanks.. already did.. no rust all good.. good thought tho..

 

How rough we talking? Making a YouTube and uploading here would be nice.

I have a hard time believing a rough running 200K mile 5.4 without unknown (to us) history is in perfect health despite the compression check. Your claim of power loss says as much.

 

All started after I had duel exhaust replaced , changed to stock single.. first i thought was a back pressure issue.. but everything seems to be fine on that end.. im guessing it could be cats .. never looked inside, but I don't see excessive heat coming from them while running.. would got red. It coukd also be phaser in timing, but without tearing engine apart no way to really know.. and it came out of nowhere.. this has me stumped. Ill check cats tomorrow gonna have to cut off.. but could be the problem.. al least hope so. Don't wanna rip frontbof motor apart to check phases or chain. Cats would be simpler

 

do you smell rotten eggs at all ?

 

You need to have a diagnostic run on your truck. There are many, many things besides cats that may be your issue.
The free computer program Forscan is your friend, check Mode 6 data. Lots and lots of info on Forscan on this forum.

BTW old cats are worth money at recyclers (not scrap yard) probably at least $200 each.