I have an 89 EA GL with the single-point 3.9 and a dual petrol / LPG conversion.
When I bought it, a faulty relay meant the petrol system had been unused for a couple of years. The relay cost $25 to replace, but the fuel had badly corroded the system. A new tank and fuel pump, new fuel lines and a rebuild of the injection mechanism were necessitated, and the car was then run on petrol a-ok for about 6 weeks.
About that point, I tried to run it on gas again: fine, but the engine stalled when the gas/petrol switch was made. A $200 fade-out system fixed that (kept small quantities of the just-turned-off fuel flowing while the other one was starting to be delivered). Again, another 6 weeks of fine running on both fuels.
At this point, the car began to idle roughly, and that progressed within a week to surging and stalling at idle or moving away from idle. The longer it idled, the worse it was. I took it back to the mechanic who did the LPG work, who checked and decided it was neither the brand new injectors, nor the new relays, causing the problems. He suggested a plug change - which didn't help - and pressure-tested to rule out any head problems.
The problem persisted. Thinking electric, I took it to an auto electrician: nothing wrong with anything. They fitted a new set of leads: no change.
At this point I gave up and took it to Ford, who suggested all of the things I'd tried, replaced the engine control chip, and fitted a $200 set of Bosch race-standard leads: no fix. Finally they decided (after 3 days with the car) the problem was irrepairable and recommended a replacement engine.
I'm interested to know what could possibly be causing the problem. The car involved was recently written off in an accident, but I bought the wreck from my insurer and am contemplating repairing it (in which case, I need to decide what to do about the damn engine).
Can anyone give me any ideas?