I have a forced air heater made by pinnacle. It is a 175,000 btu heater. It runs not too bad running on a full tank of fuel but the gauge says it's running on 13-14 psi and suppose to be at 7 psi. You have to adjust the pressure once the fuel gets down a ways. It did the same thing last year but it is a lot worse now. I have replaced fuel filter, the gauge, fuel line, air line, air filters, back plate and end plate, fan, cleaned the nozzle out. Pretty much all the parts you can replace except the pump assembly and heat chamber. The tank gets cleaned out every spring. What could be causing this? This use to be a good heater.