Resolved Fuel pressure high at 52 new FPR same thing

afjLX · Jan 2, 2025

Noobz347 said:
It not actually the first time I've seen this happen but it's been rare.

The trouble is that if the return side is just [barely] able to handle the volume then the pressure rise accumulates fast (hydromechanics are a beeoch).

The good news is that you're not burning much fuel at idle.

it takes a little bit to get one's head wrapped around the difference between pressure and volume in a closed hydraulic system. I could probably take your return system, put it on my car, and it would be just fine.

So I drove it around and ran auto tune with the MegaSquirt for about an hour and I noticed the AF is not bouncing all over the place randomly anymore. It’s much more stable. I’m just now remembering I did not put that little duck bill restrictor thingy back on at the fuel pump assembly return. I do have a tube on it extended about the length of the pump to prevent aeration. I did that when I was trying to test flow with the 255. With the Walbro 190, I was able to set fuel pressure to 38psi idle with vacuum on and it doesn’t drop when I rev it up. Hope that little duckbill backflow thingy doesn’t come back to bite me in the ass down the road.

afjLX · Jan 2, 2025

AeroCoupe said:
All I have to say is glad you got it figured out and this is the first time I have ever seen or heard of this but I have never thought to research a 255 being too large based on my experience and other cars I have been around with them. Kinda crazy how you can run an intank 255 plus an inline 255 and not over pressure the stock lines with a supercharger or turbo car at idle. IDK but still really glad you got yours figured out.

Maybe my 255 is defective?

AeroCoupe · Jan 2, 2025

Dunno man but chalk it up to a win. Ford put the nozzle on the return line to spray the fuel pump when the gas got below it to help cool the pump off. I run a Pro-M hanger and that line is now down to the bottom of the tank and directed away from the pump to prevent cavitation at low levels. The new pumps do not run as hot as the old ones so I would not give it another thought.

Noobz347 · Jan 2, 2025

Wut he sed

afjLX · Jan 2, 2025

AeroCoupe said:
Dunno man but chalk it up to a win. Ford put the nozzle on the return line to spray the fuel pump when the gas got below it to help cool the pump off. I run a Pro-M hanger and that line is now down to the bottom of the tank and directed away from the pump to prevent cavitation at low levels. The new pumps do not run as hot as the old ones so I would not give it another thought.

Thanks for the advice! I will chalk it up as a win!

Up next, getting the heater and blower working.

Noobz347 · Jan 2, 2025

afjLX said:
Thanks for the advice! I will chalk it up as a win!

Up next, getting the heater and blower working.

Uhh... blower motor! No, ... switch!

Blown88GT · Jan 4, 2025

afjLX said:
Walbro 255

So I’m just now finding out there is a walbro 255 low pressure and a walbro 255 high pressure. The LP is rated at 50psi while the HP is rated at 80psi. I’m wondering if I somehow was sent a HP pump by mistake?

Sorry, late to the party here. Went through all this over 10 years ago. The only FPR that will work with the Walbro 255 high pressure (GSS 340, 341, 342) is the Kirban. Even with that one, the return orifice inside the FPR can be drilled out slightly (the center hole). Still won't go lower than 40psi with the vacuum line off. Also, it's the only FPR with a return line check valve (also called anti-drain back) inside the FPR. They used to put them inside the pump, but no more. You can see the check valve in the center of the diaphragm, there's a little spring behind it. With the Megasquit, you can run 50psi, but you have to change settings to account for it. Result is less duty cycle on the injectors at idle.
https://walbrofuelpumps.com/walbro-gss-fuel-pumps

Yes, you are correct. Ford did put the nozzle (duckbill) on the return line to spray the fuel pump when the gas got below it to help cool the pump off.

BTW, the Kirban was originally designed for the Buick Grand National with the turbo 3.8 V6.

afjLX · Jan 4, 2025

Blown88GT said:
Sorry, late to the party here. Went through all this over 10 years ago. The only FPR that will work with the Walbro 255 high pressure (GSS 340, 341, 342) is the Kirban. Even with that one, the return orifice inside the FPR can be drilled out slightly (the center hole). Still won't go lower than 40psi with the vacuum line off. Also, it's the only FPR with a return line check valve (also called anti-drain back) inside the FPR. They used to put them inside the pump, but no more. You can see the check valve in the center of the diaphragm, there's a little spring behind it. With the Megasquit, you can run 50psi, but you have to change settings to account for it. Result is less duty cycle on the injectors at idle.
https://walbrofuelpumps.com/walbro-gss-fuel-pumps

Yes, you are correct. Ford did put the nozzle (duckbill) on the return line to spray the fuel pump when the gas got below it to help cool the pump off.

BTW, the Kirban was originally designed for the Buick Grand National with the turbo 3.8 V6.

Thanks for the reply! I did find posts scattered throughout google about the 255 overrunning the fuel system in other cars. But so many people in the Fox world saying they had no issues. This was the reason I bought the 255 in the first place. It is a head scratcher for sure. I’ll definitely get a Kirban if I ever go back to the 255.

AeroCoupe · Jan 4, 2025

Man that is really good to know. Evidently I do not have the high pressure model as my Aeromotive FPR has a wide range on fuel pressure.

Noobz347 · Jan 4, 2025

Blown88GT said:
Sorry, late to the party here. Went through all this over 10 years ago. The only FPR that will work with the Walbro 255 high pressure (GSS 340, 341, 342) is the Kirban.

I have not found this to be the case [at all] and in the OP's instance, it's the everything [after] the FPR that was causing his issue.

I installed my 255L HP when I ordered my KB kit for my original engine. It ran with the 255L HP and no other mods with no issues or codes.
Later, the SC and FMU
Later, 42s and Tune
Later, DART Combo and Flowzilla and retune

I [still] run an OEM FPR because it's the only one that fits under the KB intake manifold.

AeroCoupe · Jan 4, 2025

Well now I’m going to go dig up my receipt for the pump and see what I have. Hopefully I still have the paperwork.

afjLX · Jan 4, 2025

Noobz347 said:
I have not found this to be the case [at all] and in the OP's instance, it's the everything [after] the FPR that was causing his issue.

I installed my 255L HP when I ordered my KB kit for my original engine. It ran with the 255L HP and no other mods with no issues or codes.
Later, the SC and FMU
Later, 42s and Tune
Later, DART Combo and Flowzilla and retune

I [still] run an OEM FPR because it's the only one that fits under the KB intake manifold.

You ran a 255 on a stock engine? Verified fuel pressure? My only question with my test where I dumped into a bucket that was right where the metal fuel rail ended. About a gallon of fuel came out in seconds. That’s a crap ton of fuel. I could see pressure buildup trying to flow through that quarter inch line all the way back to the tank with the 90 deg bend in the stock line. That was why I questioned the validity of the bucket dump test. But again, many Fox Mustang owners have stated they ran the 255 on stock lines with no issues. Maybe I have a pump that is super efficient pushing 99% of rated flow where most pumps only get 65-70%? Or I actually do have a restriction somewhere?

Also, one thought popped up in my brain. I am running a CVF racing high current alternator. Could that be causing the 255 to flow more than expected? More power, more efficient, more consistent voltage over 14v, higher output at any DC motor? Maybe most running a 255 on a stock motor had the low power Ford alternator?

Noobz347 · Jan 4, 2025

afjLX said:
You ran a 255 on a stock engine? Verified fuel pressure?

Yes

You really need to download and read that article I posted earlier in this thread. It will help make sense of it.

Wait until you see what a simple change in voltage does to the pump.

afjLX · Jan 4, 2025

Noobz347 said:
Yes

You really need to download and read that article I posted earlier in this thread. It will help make sense of it.

Wait until you see what a simple change in voltage does to the pump.

I’m wondering if my CVF Racing high output alternator was causing it to push more than rated flow.

Noobz347 · Jan 4, 2025

afjLX said:
I’m wondering if my CVF Racing high output alternator was causing it to push more than rated flow.

That is possible.

I have an in-line voltage regulator installed with my 255L HP. So, no matter what going on in my electrical system, the voltage to the pump stays constant.

This smoothed out my Wideband quite a bit and made things easier to tune.

afjLX · Jan 4, 2025

Noobz347 said:
That is possible.

I have an in-line voltage regulator installed with my 255L HP. So, no matter what going on in my electrical system, the voltage to the pump stays constant.

This smoothed out my Wideband quite a bit and made things easier to tune.

Verifying voltage stability is on my laundry list of things to do on this damn car. It’s consuming too much of my time already. Wife hates the fox already.

Noobz347 · Jan 4, 2025

Boost-A-Pump® Plug & Play - Kenne Bell Superchargers

Boost-A-Pump® Plug & Play. The BOOST-A-PUMP® is clearly the most efficient and cost effective method of increasing pump flow and the HP potential of any modified vehicle (supercharged, t

kennebell.net

Noobz347 · Jan 4, 2025

afjLX said:
Wife hates the fox already.

All that means is that the world is still spinning in the correct direction.

afjLX · Jan 4, 2025

Noobz347 said:
All that means is that the world is till spinning in the correct direction.

LOL

afjLX · Jan 4, 2025

Noobz347 said:
Boost-A-Pump® Plug & Play - Kenne Bell Superchargers

Boost-A-Pump® Plug & Play. The BOOST-A-PUMP® is clearly the most efficient and cost effective method of increasing pump flow and the HP potential of any modified vehicle (supercharged, t

kennebell.net

Well, there you go. Increases voltage output boosting flow. I’m wondering if that’s what is happening now with my CVf racing alternator.

Resolved Fuel pressure high at 52 new FPR same thing

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