'92 GMC has fast idle, erratic shifting after warm up

[bwdearman]

I'm new to the forum so let me take the opportunity to say Thanks in advance.

I have a '92 GMC with a (not so recently) rebuilt 5.7 and an auto tranny. The truck runs like a scalded dog. In order to get it to pass inspection last year, I replaced the EGR, TPS, IAC, coil, plugs/wires/cap and rotor, ignition module, rebuilt the TBI, even replaced the catalytic converter. It finally passed and with flying colors.

Recently, it started acting really weird. In the morning, it starts fine and the idle starts a little high, but slows down to around 700 in a couple of minutes. I take off driving and it runs and shifts normally. After it warms up, things start to get weird. The idle runs up to around 1500 and pretty much stays there. The tranny starts shifting weird… in and out of OD for no apparent reason. It goes into OD at as low as 35 mph, but doesn't want to stay in OD at 65 mph. By the way, there's been no sign of a SES light.

I've checked for vacuum leaks from top to bottom and have found nothing, although I must confess I haven't actually measured the amount of vacuum with a gauge. I replaced the IAC again on the off chance the new one from last year failed. I reset the ECM and reset the new IAC as per normal procedure, but the problem remains.

The debate in my shop has come down to either a bad MAP (which ironically wasn't replaced last year with everything else), a bad TPS (which actually was replaced last year), or possibly the inside or outside speed sensors on the tray. The plan at this point is to measure the vacuum with the gauge and check the timing. My son has a scanner that we're thinking about hooking up but being that it's a '92, it probably won't tell us much. Worst case, I replace the MAP, then the TPS if necessary.

Sorry about the dissertation, but I wanted to be thorough. If anyone out there has anything to offer on this situation, please shoot me an email as soon as possible.