I am suffering from a very erratic idle on a 2.5ltr sve lpg converted car, currently 58,000 miles. The car is an automatic, and seems to function fine in all other manners, but, when put into neutral the revs jump up and down from 1500 to 2200rpm making the kind of noise you would get from deliberately revving the engine. The car functions in all other ways ok, when in drive, with the brake on, there is no rev problem, but then the engine is engaged... There is a consequence of this, at speeds of low revs, the car gets a slight jerk forward as it pushes up from the 1500 to 2200.
This happens whether or not the car is running on lpg or petrol, but doesnt happen when the car is first started up, ie it runs for a short while before the problem happens.
A general mechanic who has done work for me in the past, said his fault finding equipment couldnt pick anything up and suggests i go to dealer and see if their system can locate the fault.
I dont know much when it comes to cars, but have read about lpg messing up some valves, and i do know that it has some 'valve saver' fluid bottle fitted.
Any help would be greatful.