Optimax Low pressure pump putting out 40 psi?

[Chipper1012]

Anyone ever ran across this? I downloaded an Optimax service manual for my 2004 Optimax 135 after replacing a bad low pressure pump. It says the output psi should be between 6-9. I put a t-fitting on the LP output side to test, and I'm at 40psi!

This started after my stock LP pump went out. I got on Amazon and ordered another one. Everything seemed to go fine and it was a simple swap. Solved my problem of not getting on plane. However, my engine now has a hard time idling. Runs great in the upper RPMs but stalls at idle. My idle fuel pressure is 88 on the fuel rail side; air is 78. When the new LP pump is connected, I can watch my fuel pressure rise till about 100psi then it starts to stumble and die. Air stays around 80.

I sent the LP pump back and ordered another (still from Amazon but a different vendor). Plugged it in and the same thing happens. Fuel pressure climbs to 100psi shortly after start-up and the motor dies. LP output is 40 psi on the most recent pump, as well. Did I really get unlucky enough to receive two bad pumps in a row?

[lpugh]

Serial number required
external or internal low pressure pump

[SilverFox579]

Anyone ever ran across this? I downloaded an Optimax service manual for my 2004 Optimax 135 after replacing a bad low pressure pump. It says the output psi should be between 6-9. I put a t-fitting on the LP output side to test, and I'm at 40psi!

This started after my stock LP pump went out. I got on Amazon and ordered another one. Everything seemed to go fine and it was a simple swap. Solved my problem of not getting on plane. However, my engine now has a hard time idling. Runs great in the upper RPMs but stalls at idle. My idle fuel pressure is 88 on the fuel rail side; air is 78. When the new LP pump is connected, I can watch my fuel pressure rise till about 100psi then it starts to stumble and die. Air stays around 80.

I sent the LP pump back and ordered another (still from Amazon but a different vendor). Plugged it in and the same thing happens. Fuel pressure climbs to 100psi shortly after start-up and the motor dies. LP output is 40 psi on the most recent pump, as well. Did I really get unlucky enough to receive two bad pumps in a row?

The highlighted text reveals the are you need to investigate, your air/fuel pressures are not right. In order to accurately test, you need a dual, glycerine filled gauge set to test air/fuel pressures at the same time.

Are you buying genuine Mercury fuel pumps or aftermarket? Be sure to include serial # when you reply.

Btw, any "downloaded" service manual is a bootlegged copy of a copyrighted document and may not even apply to your engine. Your best option is to order a genuine service manual based on your engine serial number.

[Chipper1012]

Serial # is OT834066.

As to the fuel/air pressures, I am testing with dual liquid gauges at the same time. Both have schrader valve screw on attachments. 78 psi (air) and 88 psi (fuel) are within spec, as the manual states 80 +/- 2psi or 90 +/- 2 psi. It runs fine at increased rpms (anything above 1500), but it stumbles at an idle.

The two low-pressure pumps I've purchased so far have been aftermarket off Amazon.

I think the fact that my LP pump is putting out 40psi, is the issue. Spec is 6-9psi. Would that raise the pressure in the fuel rails causing the motor to load up and run rich? As soon as I plug in the LP pump, my pressure in the fuel rails starts to creep towards 100psi where it eventually dies. If left unplugged, the motor will idle smoothly with consistent fuel/air pressures until eventually the high pressure pump runs out of fuel.

I guess I'm going to order a $300 mercury pump and give it a try. I just feel like theres a good chance I might be overlooking something, since the odds of receiving two new defective pumps are slim.

Thanks,
Chipper

[lpugh]

Ones that that know more about this motor have not responded yet so I will offer what I can
By the ser number I think the LP pump is external to the VST is that correct?
I think I would check the pressure connected directly to the gauge (no tee) just to be sure it is no coming from somewhere other than the LP pump. If it really is that high the volume could increase to more than the regulator could control

I may be wrong but I thought all the ones with the internal pump had Schrader valve on the VST to check the LP pressure and a tee was used for the external pumps
I would also check the return circuit for restriction. The fuel return circuit on the diagram I have shows a 40 lb check valve in the return line, don't confuse that for a restriction
Seem odd that the check valve is the same PSI as the pressure you are getting on the low pressure circuit, Kinda raises a yellow flag and make me think, is there relationship?

[JOE54]

does have external pump, he got something else going on most likely check valve stuck in return or in vst,wouldnt order that pump just yet, look at that check valve in return line make sure its not stuck

[Chipper1012]

Ipugh,

Thanks for the reply. Yes, my LP pump is external to the VST. The only schrader valves I have are on the port fuel rail. Top is for fuel, side is for air.

We are thinking on the same wavelength. I too, thought about the 40 psi check valve having a relationship to my reading. I pulled the check valve and ran various PSI's of regulated air through it to test. it works fine, opens up at 40 psi, and doesn't allow a back flow through the other side. However, I am wondering if the 40 psi it allows to return to the VST reservoir (to the HP pump reservoir section) isn't making its way back up the output line of my LP pump. Looks like they are plumbed right next to each other. To me, what de-bunks that theory is the fact that when my LP pump is disconnected from electrical and my engine is idling, there is no pressure showing on the LP pump line. It would still be pressurized from the VST return line if that was the case. Also, if I pinch or block off the LP output line below where I have my tee and gauge installed, I still get a 40+ PSI reading; even when just turning the key to prime the LP Pump.

Shouldn't the LP pump have an internal check valve to regulate its output pressure to the 6-9 psi spec calls for?

[lpugh]

Disconnect the electrical connector to the HP pump and recheck the LP pump pressure, I Think the high pressure return is plumbed into the low pressure output in the vst
Might consider removing the return line to the vst and running it into a can to isolate a restriction, be sure to cap the fitting in the vst when doing this I think

[Chipper1012]

Yes, I disconnected the HP pump when I pinched off the LP line downstream of my gauge. That way the LP pump would be the only pump responsible for pressurizing that gauge. Sure enough, when I turned the key to prime, the LP pump kicked on and the gauge went up to almost 60 PSI.

New LP pump is on order. Should be here tomorrow, I'll reply back with the results.

Thanks!

[lpugh]

Sounds like the wrong pump being supplied to you!

[EuropeanAM]

Anyone ever ran across this? I downloaded an Optimax service manual for my 2004 Optimax 135 after replacing a bad low pressure pump. It says the output psi should be between 6-9. I put a t-fitting on the LP output side to test, and I'm at 40psi!

This started after my stock LP pump went out. I got on Amazon and ordered another one. Everything seemed to go fine and it was a simple swap. Solved my problem of not getting on plane. However, my engine now has a hard time idling. Runs great in the upper RPMs but stalls at idle. My idle fuel pressure is 88 on the fuel rail side; air is 78. When the new LP pump is connected, I can watch my fuel pressure rise till about 100psi then it starts to stumble and die. Air stays around 80.

I sent the LP pump back and ordered another (still from Amazon but a different vendor). Plugged it in and the same thing happens. Fuel pressure climbs to 100psi shortly after start-up and the motor dies. LP output is 40 psi on the most recent pump, as well. Did I really get unlucky enough to receive two bad pumps in a row?

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