sons 240 wagon. No vacuum leaks, plugs / rotor OK, New Exhaust system with new O2sensor, replaced fuel pressure regulator, cleaned throttle body, checked AMM - all OK. Tightened intake manifold, cleaned flame trap, new air filter, no holes in hose from airbox to throttle body. I am stumped...

Starts fine, idles smooth for ~30-45 seconds, then idle surges from 900 - 1500rpm, almost stalling. Any ideas?

Thank you

When it stalls at idle, does it throw a Check Engine Light and 2-3-1 code at the diagnostics box? If it does, the computer is not getting the closed signal from the throttle position switch. Check that the Throttle Position Switch is adjusted correctly. Then, to test it, you can use the diagnostic box by putting the diode in hole 6, and pressing the button twice for 2 seconds each. The led will start the blink, if you open the throttle slightly (so that it clicks) the blinking should stop, and it should blink a code. This means that the closed signal is there. If the blinking continues, then the throttle switch needs to be replaced. With the throttle body off the car, use a multimeter to test continuity on the switch (the outer most terminals with the center one, one will have continuity with the throttle closed, the other with the throttle more than half way open). The switch is easy to replace, just follow the directions in the 700/900 FAQ. Takes less than 30 minutes, and don't forget a new throttle body gasket.

All this of course after you rule out the Idle Air Control Valve as Dan described below.

Good luck,

Justin

Clean the idle air control valve and make sure it operates when you have it out. Dan