Hey everyone - have a head scratcher here.
Just rebuilt a GM 454 (7.4L L29) with some aftermarket heads and the stock EFI system. I got some very gracious help with the tune here that has really improved drive-ability but I have one specific problem that I'm stuck on.
The truck Idles ok and shows what I assume are normal kpa readings for the idle. Once in closed loop and warm the kpa will get excessive with ~10% throttle input at 1200 rpm and the ecu will pull timing like crazy. Truck basically creeps along due to retarded timing because the ECU assumes I'm pulling a stump out of the ground at full throttle at 1200 RPM.
I assume the MAP gets priority over the TPS
Replaced the MAP and the problem still occurs - unplugged the MAP and the problem goes away.
I have not smoked the intake to look for leaks yet because the LTFTs look fine. Its a fresh build, and I build quite a few engines. All Fel-Pro gaskets, everything aligned very well, and all torque specs were done in sequence with my snap on digital wrench. I will say this though: the upper manifold bolt torque spec is 11 ft lbs, which seemed really light. Lower is 35ish I believe.
Here is what is so confusing: Why is 10% throttle getting me such a huge dip in vacuum, and do I need to adjust my timing table that drastically for low throttle input? Cam has a 114 degree LSA.
Is the 22 year old ECU getting goofy?