I bought a 2003 2500 Suburban pretty cheap that was thrashed hard. When I bought it the water pump was leaking, alignment was bad and it had no muffler and tail pipe. It was odd all the way aroud. But I wanted a 6.0L/4L80E burban. The guy only had it a few months. He bought it from a used car lot and after a bit the muffler started rattling so he cut it off behind the cats. It was leaking oil from the oil pan and valve covers like the gaskets werent there..it only had 150k miles on it. The interior was trashed. I couldn't fatham a dealer selling something so rough nor see someone trashing it so fast. Well I drove it and it seemed to run strong and had a solid shift so I low balled him and he accepted. I brought it home and pulled the water pump and was about to replace it when I noticed the block had 4.8/5.3 behind the alternator. Immediately crawled under it to verify it had a 4L80E...it did. I pulled a plug and tossed a scope in to see if it had flat tops or dished pistons...dished; 5.3l... sigh of relief. I scratched my head for a while and ultimately decided to continue with the repair. After I put the water pump in I replaced the valve cover gaskets. To my surprise the motor was pristine...definitely a low mileage motor but even hard to believe it was anywhere close to being 10+ years old. I got a tailpipe and muffler at a junkyard...put that on. I fired it up for the first time with exhaust and the motor sounds tight...no typical 5.3 ticks or knocks. The tranny was rebuilt and it has a brand new torque converter in it. I spoke to the seller and he said he has no clue. He was clearly not very mechanically inclined and the carfax matched his story. The used car lot he bought it from mainly sells 3/4 ton trucks.... My theory is it was bought by the dealer at auction with a blown motor/tranny and they dropped a rebuilt 5.3 in with the rebuilt 4L80E and sold it with the rough interior. The oil leak and water pump leaks are because he over heated the crap out of it when heater hose QDs started leaking. So now Im trying to figure out whats what. The 6.0L tailpipe bolted up to the burbans exhaust so Im guessing the exhaust is unchanged but there are two hangers on the exhaust at the front right of the tranny where the two pipes begin to run parallel to the rear... but nothing to hang them on. The rear 02 harnesses arent routed in a factory path. The furthest rear one is zip tied to the body above the tailshaft of the tranny...perhaps a 5.3 harness? Everything under the hood looks appropriate though. The tranny mount was completely gone due to the motor oil eating at it. I replaced it but the tranny wants to push to the rhs...it wasnt easy getting the new mount in there. Its definitely on a bind. 5.3Ls dont directly bolt up to 4L80Es without a flexplate adapter or a 6.0L flexplate. I have 3 major concerns 1. Why is the tranny biased hard to the rhs? Are the 6.0 and 5.3 motor mounts different enough to cause this? 2. Is it possible the 5.3 is running off the 6.0 computer? Would a 5.3 harness even work with a 4L80E? 3. It has a pretty decent shift...a tad too hard. It also feels as though it has an rv/towing stall converter. - is it possible they didn't use the 6.0 flexplate or adapter and the trq converter isn't fully seated? Would that cause the hard shifts? Wouldn't the front pumpnseal start leaking if it wasnt fully seated? - is it possible the 5.3 isnt matching up to the 6.0 mapping (assuming its got a 6.0 ecm) throwing the shifts out of wack? It doesn't throw codes. My OBD-II is happy thinking its a 6.0L. My next coarse of action is to pull the trq converter dust cover off and see 1. If i can see an adapter. No idea if I will be able to. 2. See if the trq converter clearance to bell housing looks normal. Then look at the ECM for a PN. Would the pn even differentiate between a 5.3 and 6.0?