This problem started suddenly. Very rough idle with a repetitive cycle that is smooth for a couple of seconds then misses badly, occasionally stalling. Worse problem is bad hesitation when accelerating after throttle at idle position, sometimes stalling. Worst after stationary idle, but also after moving at low to medium speed with closed throttle and then accelerating. Also very hard to start because of periodic 'backfire' while cranking that brings engine to standstill.
Idle is perfectly smooth for first 1 - 2 minutes after overnight cold start, before missing at idle starts. Missing and hesitation worse the hotter it gets. Turning on air condtioning makes ilde worse, with more frequent stalls. However, during normal driving over about 2000rpm and nomral traffic speeds is perfectly smooth - no missing, no roughness no lack of power, no excess fuel consumtption.
Spent approx $2500.00 on two trips to mechanic, without improvement. Replaced complete distributor, cap, leads, plugs, coil, PCV valve. Tried alternative (used) ECU. No faults being reported by either ECU. Removed throttle body and cleaned thoroughly. None of the above made any difference.
Checked for vacuum leaks, timing, temp sensor, air flow meter, throttle position sensor. All within spec. Idle speed control removed, cleaned and checked. Mechanic performed wet and dry compression test, and while not great (car has done 350,000 km) compression was pretty good and even for a car this age.
Discovered by accident that disconnecting the TPS causes the idle to be perfectly smooth. So smooth you could balance a coin on the engine even at low idle revs of around 650 rpm. In fact drivig with the TPS disconnected is quite acceptable, except that automatic gear change down does not happen (as expected).
Have had the car for approx 13 years and has been a great, reliable family car. Reluctant to dump the car for soemthing that should be fixable. Please help!