Engine and Transmission Tech

I've been trying for 10 years now to get the idle under control for my Rover Hotwire 4.2L FI (MGBV8 Conversion)

The idle hunts up/down, almost stalls/stalls, requires a foot on the gas pedal to keep the RPM's under control or it will idle too high (often NOT coming back down).. I've installed several IAC valves (with differing results)... I've tried different ECU's (one "Tornado" chipped" and a couple stock), I've torn the top end apart looking for any sources of vacuum leaks, replaced the ignition SYSTEM, including the distributor a couple times and it's always the same. Poor Idle control

I've checked for vacuum leaks, replaced (several times) all the vacuum hoses and replaced every sensor (Fuel temp, coolant temp, Hot Wire/MAF, Throttle Pot) etc.. New fuel pumps, filters, FPR's (adjustable and static), new injectors, You name it, I've replaced it and or tested it.

What's maddening no matter what I replace, the idle problem eventually comes back. It might seem fixed, however it'll start acting crazy again.

I can start it and let it idle and it'll be perfect and steady (about 750 RPM), and I'll re-test that every day and it's perfect. The next time I try, it's idling at 2,000 RPM! The last drive, it was up as high as 3,000 RPM, then the IAC would step it down to 1,000, then ,... (then it would stall) etc..

There has to be a logical solution to this decade old problem... Basically I can't drive the car anywhere. I can't sit in traffic with my idle all over the place and then stalling.

I can't be the only person that's run into this... Advice???

I have no experience with that particular problem, but I'd suggest that you might start looking at other potential sources of the problem such as wiring and connectors. Is the engine properly grounded to the battery? The chassis grounded properly? Are the contacts in the connectors good. (Fox Mustangs with EEC-IV control system have a connector at the firewall that connects the engine wiring to the ECM. When the contacts in the connector get corroded, it is a notorious source of hard-to-find problems.) Is there an association between weather and the occurrence of the problem, such as wet weather? (I found a problem with a drifting load cell for a scale that occurred after the hopper got hosed down; water getting into a wiring splice.)

Trace the wiring from the IAC (Idle Air Control) and check to see that all the connectors are good and that there isn't a bare spot in the wiring. Disconnect and reconnect the connectors. If you have an Ohm-meter, measure the resistance of the sensor wires as well as the IAC wires.

Good luck.