Apologies if this is an old subject but I’ve only just acquired a 1993 Plus8 3.9 with the Lucas Hot-wire EFI system, and I believe my problem is known as ‘idle hang up’ and it’s driving me nuts !
Symptoms :- Starts perfectly from cold, runs at about 1500 rpm for a few seconds then drops to a steady 800. Throttle response is a bit sensitive at first with careful application of throttle required to prevent the engine 'bogging' down. Having replaced a defective thermostat, the engine warms up quickly. Now having driven away, the engine idle will only drop to 1400 even when the engine is fully hot, UNLESS one stops, when the speed sensor switch kicks in to drop it to 800 rpm. The speed sensor switch always works hot or cold, but once driving, the idle is back to 1400. Eventually after as much as 20 or 30 minutes of driving, the idle will drop nicely to 1000, unless of course the cars stationary.
Interestingly, the other day I drove off very slowly not exceeding 40 mph using very low revs and the symptoms completely disappeared. Next day, driving at normal traffic speeds and rpm, the symptoms were back.
So far, I have carefully cleaned the idle by-pass air valve, (rear right of the Plenum), and fitted a new temperature sensor that feeds the ECU.
It’s really annoying and embarrassing trying to drive with such a high idle, so any solutions would be most welcome thank you.
With regards, John.
