Random misfire on a 4.8L swap is driving me crazy...

[inline]

First off, I want to say that I appreciate everyone's knowledge on this forum. It has helped me out greatly on previous swaps.

I have a '56 Pontiac wagon that I am in the process of swapping a 4.8L/4L60E into. This swap was done with a bunch of left over parts from other swaps that I have done for others. The engine is what I believe is a stock 4.8L but I do not know the year, I am assuming it is a 2001. It was given to me and it had 3 burnt valves. I ended up swapping a set of the same heads onto it when I decided to use it for this project. The intake is an LS1 with 2000 corvette 26# injectors. The ECU is from a 2000 5.3L Silverado that I copied a stock 2000 4.8L tune from the repository. I did this by comparing files and changing everything to the 4.8L tune except for the electric fans and injector size. The harness is a factory DBC harness that I modified. The biggest change to the harness is that I wired each coil individually to eliminate the large white coil plugs. I also had to change the injector plugs for the LS1 injectors.

The problem that I am having is that I am getting a P0300 random misfire code. The things that I have done are as follows:

-new O2 sensors
- new plugs
- new wires
- swap injectors locations
- swap coils with another used set. I also used a spark tester to verify that they were firing...which they were.
- I also swapped injectors with another set of 30# LS6 injectors

No matter what I do, I am getting a misfire primarily on 5 and 6 with an occasional misfire on 4. The misfire is there regardless of rpm. The attached log file shows the misfire logged near the end. It appears that the engine has to run for a bit before the ecu begins logging misfires. I am assuming that this is by design?

The engine visually appears that it does in fact have a misfire and it is not running smoothly. I turn each injector on and off via the scanner and it sounds like they each have an effect. I also have unplugged each coil with each of them having an effect as well.

I am in the process of rechecking my wiring to make sure that I didn't cross something up but haven't found anything yet.

I haven't check the compression, but I would think that if I had a blow head gasket, it wouldn't show up on opposite sides of the engine.

Any help/advice would be appreciate. I have spend a great deal of time searching on here for possible causes and I couldn't find anything relating to #5 and #6 constantly misfiring no matter what is done. Attached is my log file and tune.

misfire 1-11-24.hplStep 5 4.8L.hpt

[stroh]

Did you do the crankshaft position variation relearn?

[inline]

Did you do the crankshaft position variation relearn?

I did, several times actually.

[inline]

So, I finally checked compression and #5 had 95psi and #6 had 20 psi. Guess I found the problem. Going to swap it out with a known good 5.3l. Thanks to all that had some suggestions.