Engine Racing problem?

My P38 4.6 has had a replacement low mileage Gems engine 82k.
I have done around 5k since installed by an independent LR specialist and has run great till recently.

On start up it shoots to 4000 revs and will not drop, other times it idles nicely then goes up in revs then almost stalls. Fuel consumption has dropped by 7 MPG.

I have had two independent LR specialists do a diagnostic and no faults show?

I found one way of controlling the revs was by Elongating the holes on the Potentiometer (throttle position sensor) and by altering its position the engine behaved better.

To this end I tried to buy a new Potentiometer, LR say obsolete item no stock.
Ebay got me one but when fitted original revving back.

I then purchased a new plenum chamber, no difference, but is smoother with the modified Potentiometer fitted, I have also checked Fuel Air, filter, and checked for air leaks and also changed the Air Mass sensor, and had the Idle control valve checked.
My MPG is now up to 16 slightly better but not near what the old engine did.

At present the P38 is running great albeit modified, no oil usage no smoke,no misfires, but a baffling fault.

Any ideas to the cause appreciated.
Cheers Marshall.

Only a vague guess, engine ecu?

Druim said:

Only a vague guess, engine ecu?

Click to expand...

Thanks, this has also been suggested by another enthusiast, also inlet manifold leak.
Marshall.

Cruise control cable or throttle cable strands snagging? Or if it is drawing too much air check the grooves in the air hoses for splits. If it's intermittent it could be self sealing then opening up again. When it starts and races to 4000, what happens when you switch off and re-start? The same or does it settle?