2003 2500 Suburban puzzle

Discussion in 'Engine & Performance' started by JRW261, May 11, 2019.

  1. JRW261

    JRW261 New Member

    May 11, 2019
    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?
  2. Peter T

    Peter T New Member

    Dec 4, 2019
    I see it's been a while. What did you figure out?

    I heard that a 4L80e would bolt up with no issues, but I guess that's incorrect? I did see where a couple pins had to be swapped around for a solenoid difference between the 4L60 and 4L80 so that's something to look for.

    Was there a spacer between the engine and trans? Are the engine mounts gone due to oil leakage causing the bias of the trans? How did the trans fluid look/smell?
  3. lazz

    lazz New Member

    Nov 13, 2019
    Its is possible there has been a swap. But am wondering if it really was a Sub 2500 to start with and not rebadged like some people do. The ECU has the PCM built into it so if it had a 6.0 and a 5.3 was swapped providing it had the correct reluctor on the crankshaft it would run but not well without it being reprogrammed. Some ECU's will brick if you try it. The transmission could care less what motor is powering it. Scan and see if the VIN matches the ECU. if it matches then chances are its the original ECU. The 4.8/5.3 and 6.0 have a few differences in wiring harnesses and most can be interchanged with little or no modification except for the trans. there are a few wires that have to be swapped to different pins. If the trans is is shifting the way you say it is then there a trans. issue. "Service Engine Soon" does not equal to "Service Transmission Soon" so transmission codes do not turn the light on. I suspect the TCC selenoid is causing a line pressure issue which in turn causes hard shifts. Also if someone installed a aftermarket Sonnex kit in it and didnt do it correct it causes that as well. here is the test needed to figure out what is happening.

    12v verified at pin E on PCM to trans connector.

    1-2 solenoid 20ohm, A, clicks with 12v

    2-3 solenoid 21ohm, B, clicks with 12v

    PCS, pressure control solenoid 4ohm, Force Motor

    TCC solenoid 10ohm

    Temp sensor 4k ohm.

    If any of these fail then check the harness to make sure its ok and replace as nessesary.