In hindsight, my IAC suggestion is apparantly significantly flawed. I'll keep thinking about a test of this sort, but meanwhile, I recall that I've already seen a simple test here that will work. I'm not a big fan of pinching hoses, but that's probably the simplest approach:

- with the car idling, pinch off the inlet hose to the IAC. The idle should drop noticably, and the engine might stall.

- now push the throttle slightly off the endstop, and then pinch off the inlet hose to the IAC. Now the idle should not drop significantly.

Re: the TPS. I just looked at my Mitchell CD, and found 2 things I didn't realize:

1) the TPS ground doesn't go back to the ECU; it must ground near the switch
2) the TPS signal also drives the EZK ignition ECU. I guess it affects advance at idle? Woudln't think this is the likely culprit, but something to keep in mind.

Anyway, the TPS signal is on the orange wire at the ECU - pin 2.
1) disconnect the ECU connector and measure resistance between this pin and ground while repeatedly exercising the TPS switch. The low resistance should be consistent
2) disconnect the TPS and ensure that its harness pin 2 sees ground.

You may instead want to do the simpler test
1) disconnect the harness to the TPS, and measure resistance between its 2 pins while repeatedly exercising the switch. The low resistance should be consistent
2) ensure that its harness pin 2 sees ground

The only downside of the simpler test is that it doesn't ensure the TPS signal is getting to the ECU. That's less likely to be the problem though; if there is an issue, the odds are the simpler test will find it without having to dig into the ECU.



--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto