So the problem is now resolved and the motor home runs fantastic. As some sugested here and elsewhere it was indeed the fuel pump. Fuel pressure was fine when first starting when cold but after it ran for 10 of so mins the pressure would drop to low 20s. New fuel pump ordered and installed. (Bought the Delphi) and drove it today 100 miles and didn't skip a beat. Kept a steady 36psi all the way.

Thank you all.

 

Was it your high-pressure pump or your in-tank pump?

Steve

 

There is only one pump as far as I'm aware. The in-tank one.

 

In my 1990, there is an in-tank fuel pump, and an external high-pressure pump.

Steve

 

An F53 never had a external high-pressure pump from the factory.

 

I think that is a whole different set up. The 1994 only has the in-tank high pressure pump.
as best as I can tell. Anyone that knows better can maybe clarify.

 

Your 1990 was an E-series RV and not an F53. That is why it had the external high-pressure pump.
The 1991 was the last E-series with the external high-pressure pump.

 

Thanks for the info.

Steve