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Im going to start from the begining again on this one and see if anyone can help me further.

Firstly i have a 1997 p reg 4.6 hse thats been lpg converted ive replaced my perfect running for example didnt miss etc but overheated engne for one from my local scrappy that was from a 96 4.6 that had also been converted for lpg.

after alot of stressful sundays i got the new engine in but it missed as i gave it to a friend to finish off.
he found the source of the miss was the complete o/s bank was down so mhe replaced all the plugs and leads with no avail.
then it was fond to be a multiplug that was slightly undone that caused the misfire.
once connected it ticked over fine for 10 min my friend reved it up and all was god he though he had it fixed.

then he gave it a rev and it backed fired stopped

now it starts fine ticks over great on all 8 cylinders but as soon as you go to rev it it goes to cut out and their is a distinc noise of air being drawn in from somewhere. we cant find if its a breather or anything we havent got a clue. its strange becaus eit ticks over great just doesnt want to rev? it sometimes when reved trys to go but misses and splutters and backfires slightly into the airbox as though its getting to much air?Also if you take the air intake pipe of from the intake it cuts out completely?

Its so thrustating as its so close yet so far? does anyone know of what the sucking noise could be that came after the pop? which is now their everytime you try and rev it? anyone know why it wont rev?

any ideas advice things to try are all gratefully appreciated
 

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It sounds like you either have an air leak somewhere, like intake manifold, missed hose connection, throttle body,etc.
If you disconnect the intake hose with the engine running you are also disconnecting the Mass Airflow Sensor. That is why the engine cuts-out.
You are going to have go back and step by step see what you missed. If you have a vacuum gauge, it would help you to know for sure if its a vacuum leak.
 

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Have you checked the electrcal connections to the MAF/throttle-postiion sensors? It sound like the ECM is not receiving inputs from them. The engine will still run/idle without one of these inputs, but will net rev correctly/have any power...
 

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Since it backfired have you ckecked for any damage to MAF or the intake pipe between the plenum and MAF?
 
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