The machanic reckons everything with my dads car is fine despite the 20mpg problem. He things the ECU needs changing.
As far as I can tell the ECU just plugs in like a cartridge and is really easy to fix, so if I replaced it what problems may I have?
I am worried by replacing it the car will simply refuse to start etc.
Immobiliser? It's quite an old car, so you may not have any problems - but with newer cars, you may have problems like key transponder match, mileage not matching between the ECU and clocks, and even more problems with 2005 or later CANBUS systems where near everything in the car can be linked to the ECU.
AT - if an ECU needs replacing, you should generally see other faults with your car; not just the 20mpg problem. Other faults could relate to timing, exhaust etc.
In regards to the ECU replacement - I don't know what car you have (rover??) - anyways - under the bonnet, near the footwell, near the door (anywhere) is where it could be. Some ECU's just have a wire either side for connection, but some have like a parallel port style connection which simply slots in.
If you do replace it, make sure you reset it first. In fact, thats what I'd be doing now; check for loose connection, reset it and then try again.
Is the mechanic suggesting that just because he's run out of other ideas? I don't think ECUs tend to go wrong much. It might be worth resetting the ECU - disconnecting the battery will do that - if it really is an ECU problem.
How old is the car ?
If it's new then most things will be linked in to it.
Does it hunt at all? On my 306 it was drinking a lot of fuel, hunting, smelling of petrol and running very rich. Turned out to be the MAP sensor hose had become disconnected, reconnected and it ran sweet as a nut! Another possibility could be the carbon canister purge valve - this was something my uncle suggested to me when I mentioned the petrol smell. It absorbs petrol fumes evaporating when the car is standing then releases them back when the engine's running, but apparently problems can be caused when the valve to purge it is blocked or malfunctioning.
If there's no other symptoms like hunting for revs, or difficulty in hot or cold starting, then I doubt an ECU fault...it could be a hundred things though. If you're a member of a Ford forum, see if there's anyone on it that has an Escort and is willing to let you swap the ECU with their known good one so you can make sure it is definitely an ECU fault. If there's any Ford specialists nearby then they may have an old ECU lying about you can try? If worse comes to worse any you do need a new ECU, look about for a reconditioned one as they're often much cheaper.