250hp OX66 low fuel pressure

[McKayMarine]

Hello,

I have a problem with low fuel pressure and losing 600rpm on the top end of 1998 250 ox66

I have changed low pressure pumps, VST filter, water separator, plugs, fuel lines.
When I put fuel pressure gauge on I have 35psi at idle but it drops when I throttle up.
I ran it off of a remote tank and same thing.

Any suggestions on what to check next would be much appreciated.

[boscoe99]

Fuel pressure should be increasing as you throttle up. Not going down. How much fuel pressure do you have when the problem manifests itself? In other words, how low does the fuel pressure go?

[McKayMarine]

It was a few weeks ago that i had the gauge on but i believe it would drop to 25psi

[rodbolt17]

actually, it should hang about 35-38 PSI throughout the RPM range.
try starting the engine, running a jumper wire from the BLUE pump terminal to ground.
retest it at the failure RPM and load.
if it works chase the connection problem from the pump to the ECU. don't go chasing the resistor circuit.

[McKayMarine]

Thank you, i'll try that and report back

[McKayMarine]

I had the boat out yesterday and figured a few things out but let me go back a couple weeks and try to explain a bit more.

I bought two new VST filters a few weeks ago and installed one, primed the tank but didn't try the motor for two weeks. When I put the boat in last week same problem of 600-700rpm loss.

I didn’t have time to try the boat with Rodbolts suggestion on Monday but I thought I would check the VST filter again. When I checked the filter it was clogged with what looks like a grey dust, super fine stuff that you can't really see unless you hold up to a light. Pic Attached

I installed the other new VST filter yesterday took the boat out and it worked, 5500 RPM top end. I did test the fuel pressure and it is not staying at 35-36psi.
It drops to around 29-30psi. Rodbolts post says it should stay 35-36 so I still have that problem but the boat runs great.
I tried Rodbolts suggestion with running a jumper wire to ground and it didn’t fix the problem.

The boat ran fine for the short time out maybe 15-20miles, but on the way back started stumbling again so I bet today its back to losing rpm.

The filter won't last a few hours and if it sits in the fuel for a few weeks it clogs up, I've had the tank polished twice and they use 2 micron filters and said its clean.
I also have a 2 micron racor so I don’t think it’s the boats fuel. The boat ran great all last year (I bought it in May 2013 and had the motor rebuilt) it has 91hrs)

Does anyone have any ideas what could be clogging this filter, this engine is driving me nuts I'm almost ready to get rid of it, this has ruined my summer.

[greasyshaft]

Could be brush dust from the high pressure pump, generally ends up all through the fuel rail as well.
Is any of this grey shit in the strainer before the lift pumps?

[McKayMarine]

It doesn't look like it's in the strainer. It's hard to tell its even on the vst filter until you hold it up to a light.

I've thought high pressure pump too but the dust is on the vst filter before the pump.
As soon as a new filter is on the engine runs great.

[ausnoelm]

brush dust from the pump? please explain.....

[boscoe99]

brush dust from the pump? please explain.....

The brushes in the electric motor are cooled and lubricated by fuel. Yes, fuel is actually flowing right through the motor and over and around the brushes. Some of the fuel gets routed back to the VST via the fuel pressure regulator. Some of that fuel might have brush dust/debris in it. Hopefully it gets captured by the VST filter.