Optimax Low Pressure Pump making 40 PSI!

[Chipper1012]

1


Having trouble with the fuel pressure on my 2004 135 Optimax. Ideal pressures are 90 for the fuel, 80 for the air, thus maintaining a 10 psi difference. Checking my motor with dual, liquid filled gauges, my fuel is 88 and my air is 78 (close enough). After idling for a few seconds after start-up the motor starts to run rough and I notice that my fuel pressure creeps up to 100psi where it eventually dies. Air pressure stays the same (78 ish)

This only happens when my low pressure fuel pump is plugged in. If I unplug the LP pump, it idles smoothly at the correct pressures. As soon as I plug in the LP pump's blue [COLOR=#67B045 !important]connector[/COLOR], the motor starts to stumble as the fuel pressure [COLOR=#67B045 !important]climbs[/COLOR] on the gauge. I've gone through two LP pumps, because I thought the first one I ordered was bad out of the box, but the newest one does the same thing.

If I advance the throttle to 1500 rpm the motor will stay running with the LP pump plugged in, but the fuel pressure is still around 100 PSI and the air is 80ish PSI. Again, If I unplug the LP pump at this "high idle" the motor continues to run but my fuel pressure drops back to 90 PSI (where it needs to be) and the air stays at 80 PSI.

So, I checked my LP pump to see if it was putting out the correct pressure (6-9psi). I couldn't believe it, but it's putting out 40 psi. I checked it against the first pump I ordered and it too had the same results. Anything else that would cause this other than the fact I may have got two bad pumps?

#2
Chipper1012 , Today 02:55 PM

[Motv18]

Iirc the electric low pump is 40, the low pressure diafram fills the vst then the low electric feeds the hp electric.

Sounds more like a regulator problem. It should open at 90 psi if air is 80psi. And air should climb up with fuel psi sice it will block air from leaving. It might be stuck in open air closed fuel.

If you had stabil or got a few gallons of butyl ether it can swell or melt dissolve the rubber making it stick.

I would expect it to be more surge than rough. More or less calibration is off due to the pressure making it rich.

I’ll check the service manual or you can look up the spec for the low side pump.