Just a quick question. Could a faulty idle control valve be a cause for really poor fuel consumption on a 3.9 efi engine? like 9mpg poor? Would a faulty valve also effect the power output of the engine as well?

Cheers

I have now changed the idle control valve. It idles nicely whilst running on petrol, no problems. Running on LPG is a different story. Whilst stationary, it idles nicely, but when the car is moving, the car will idle at below 500 rpm, occassionally lower causing the car to stall. This is annoying as it is an auto so it idles a lot whilst moving.

Any ideas?

In your first post the fuel consumption might be due to the ecu being in limp mode.

As to your other query I wouldn't have thought the LPG would use any of the EFI control unless it is plumbed in to just stop the fuel pump then use the ECU to regulate idle but then there would have to be a way for the original ECU to talk to the LPG to regulate fuel flow in to the engine if you see what I mean.

Maybe worth checking the ignition timing, IIRC it has to be advanced to run LPG but do a search to check that.