250 ox66 WOT RPM loss - Cant maintain fuel pressure

[littletunny]

S250 TURX 1999

3 new LP Pumps
New VST filter.

When running WOT at 5500 RPM, RPMs slowly drop to 5000 RPMS.

Hooked up gauge to schrader valve. 35.8 psi all day long inlcluding 5500 RPM until it after 45 seconds and it falls to 30 psi. Tested between the LP pumps and the VST. Get 4.5 PSI at WOT then it falls to 4 psi which makes sense sense the LP pumps are based on crankcase pressure, the faster the motor spins, the more LP pump output. I guess I need to look for a fuel restriction before the LP pumps? The VST is not refilling fast enough at WOT ( its thirsty needing 26 GPH).

Can the O2 sensor control the High pressure pump out put? Can the TPS control fuel pressure? I am pretty sure its 35.8 psi at all ranges and never changes pressures. My understanding is that either the HP pump works or doesnt, really is no in between.

Thanks

 

[Inducted]

Re: 250 ox66 WOT RPM loss

Re: 250 ox66 WOT RPM loss

It is not clear, when does your fuel pressure drop to 30psi? Neither o2 sensor or tps DIRECTLY control fuel pressure. Pump may have a high and low speed but pressure is controlled by pump and regulator. Like you said they work or they don't. Look for a clogged filter or maybe bad lp pump. There are filters on your injectors. Good luck!!!

 

[littletunny]

Re: 250 ox66 WOT RPM loss

Re: 250 ox66 WOT RPM loss

It drops to 30 psi after 45 seconds of WOT which leads me to believe that the VST is not replenishing quick enough. I get WOT no problem for 45 seconds, then the rpms and the fuel presure backs off from 35.8 psi to 30 psi. I even cleaned the fuel pressure regulator.

 

[Inducted]

Re: 250 ox66 WOT RPM loss

Re: 250 ox66 WOT RPM loss

Usually rpm drop and fuel pressure drop means a restriction. Most times the vst filter is dirty. If you use a vacuum gauge and a clear tube with the same ID as your fuel line you can keep moving the gauge until you find the restriction. Does your engine have a check valve between the lp pumps and vst? If you had water in gas problem at least drain the vst and double check. Was your vst filter replaced with a new one or just cleaned? I think you are correct about the vst starving for fuel. Good luck!!!

 

[littletunny]

Re: 250 ox66 WOT RPM loss

Re: 250 ox66 WOT RPM loss

Problem solved!!!!

So started working on the fuel supply BEFORE the engine mounted filter.

I suspected a clogged pickup tube so I took a portable air tank and blew air in the fuel line from the fuel tank to the water seperator in the bilge. I blew some air in that line and heard bubbles in the fuel tank. Cool. Unhook the compressed and fuel starts coming out of the line I just shot air into!!! WHAT? This thing isnt pressurized. I rush over and hear air slowing leaking out of the gas cap, I remove said gas cap and the rest of the air flows out. AH HA!

Unhooked the fuel vent hose and the fuel didnt spit back out of the supply line like previously. Let the vent hose off and ran WOT no problemo! So it was basically a clogged fuel vent! Thanks everyone!