Thought it might be the fuel filter....if it exists!

P38 4.6 (LPG) 2000 runs a bit rough when on petrol but ok when I switch to LPG. Thought it might be a fuel filter problem but found out from this forum that it does not exist! So why does ebay have sellers selling fuel filters? Any other reason why she is running rough on petrol only. Changed plugs yesterday but no difference.

94-99 have fuel filters(gems) 99-on(thor) dont

Fuel Trims - The ECU logs fuel settings, and monitors the engine repsonses and trims to ensure smooth running and efficent combustion.

If it has been run on LPG for an extensive period, the ECU has learnt how the engine behaves on LPG and not petrol (as the two burn in a completly different manner) thus when you switch to petrol it is still running on LPG trims....

Thanks lads. With ref to Saint V8, is this something I shoudn't worry about too much as I run her mainly on gas - only use petrol on start up when she can run rough.

i changed my coolant temp sensor and it now runs fine on lpg or petrol hot or cold

As Saint said. But to expand a little, your gas may need a bit of a tune. It's not unknown for the vaporizer to get tired therefore reducing delivered pressure which will throw the trims out. its worth getting it checked by a gas specialist who can set it up whilst your driving it.