96 Sierra K1500, Misfires until full throttle applied

JJPittsley · October 4, 2017

'96 GMC Sierra K1500 5.7L V8 manual trans

Having trouble pinpointing the problem with my truck. It has a habitual problem of running rough and misfiring (gets worse during acceleration) until throttle is full open for a sec or two then it stops misfiring but after reducing the throttle even a little it will go back to misfiring. It is not as noticeable in 1st and 2nd but when it gets the higher gears the misfiring is bad enough that it will slow down and I have to cycle flooring the gas peddle on and off to keep it going at speed. Also 75% of the time, after driving 5-10 miles it will start running smooth, no misses, and act normal for the rest of the drive. But that doesn't always happen and it seems that engine temp does not effect it one way or the other.

My father is a mechanic (foreign motors specialist) and I have access to a garage full of tools, OBD-II scanner, multi-meter, ETC; but we are still having trouble figuring this out. A month ago decided to upgrade to a direct fuel injection system and found that at some point before I owned it there was a fire inside the intake manifold that burned up the CMFI fuel lines and melted the end of the idle air valve. But even after fixing all that it still has the same problem. We have replaced the MAF sensor, upgraded to direct fuel injection spider, replaced idle air valve, tested the TPS (good), tested fuel pressure & changed fuel filter. Oh and the previous owner took off the catalytic converter. We have gotten misfire codes and the code that the cat is bad. I know it could be a computer issue but I want to rule out everything else before considering replacing the ECM.

Any help with what to check next would be greatly appreciated, need to get fixed before the snow hits.