Calling out the big brains. Fuel pressure should be around 39 lbs at idle (no vacumn) mine is 25. Problem appears to be intermittent. Relaced fuel pumps( two sets of hi flow Walbros), no help. Now on my second adjustable fuel regulator (CPR and Aeromotive). 25 pounds no matter the adjustment on on the regulator. 25 pounds when return line is blocked as well. Does anyone know what the low and high voltages should be to our pumps, as our pumps are two stage? Any other ideas on solution to this problem?

99L

 

Sal(PSP) should be able to help you..............

Dan

 

LIGHTNINROD,
I know this sounds to easy and is not ment to insult your intellegence. Did you replace the fuel filter at any point when changing out the pumps?

 

How about the gauge accuracy? All those changes and not any change in PSI?

 

I have same 25 psi problem. Checked filter -ok. Mods are (2) Walbro gss340 (255i) pumps, super fmu, 50lb/hr injectors. Contacted Walbro - lightning replacement pumps gss310 drew 4.5amps@40psig and the gss340&gss342 drew 7amps@40psig.
I measured 12.7 amps@40psig with the gss340. 1 pump 40psi 2 pumps@25psig. Measured draw 6.5amps@40psig each. Switching to gss310.

Some trucks get away with 255's ours wont. It basically is to much current draw before the highlow relay clicks on. If you want to see if your truck has the same problem short #30 to #87 on your high/low relay. If it fixes your problem I suggest making the same change.

Good luck!!!!

Walbro #989-872-2131

nist@dukesinc.com

 

I'm working on this issue. I spoke to the performance engineer at Walbro today, he is stumped. Fuel pressure is controlled by the regulator, so he can't see why LARGER pumps would cause a drop in pressure. The only thing he said, was that maybe the low sdie voltage is too low. Steve measured his low side voltage and only saw 5volts. Walbro said the pumps don't work well under 7 volts. However, I've sold hundreds of these 255 kits over the past few years, and not one truck ever had a problem, until recent. Five volts seems a bit low to me. I'm wondering if some trucks have higher resistance on the low side ground resistor wire than others, dropping the voltage too low to properly operate the 255s. I'll keep evereyone posted on what we find. BTW, the GSS340 is not the correct pump for us, you need the GSS342s.

 

So, did you guys just end up bypassing the resister(above the intercooler pump)? If you did do you run new wire to handle the increase in voltage? Go with external fuel pumps?
I've had no problems like that with the Walbro GSS342's I got from Sal.

 

Sal,
Any updates, I have the same issue. Just installed 2 225s.
Mark

 

Yes the issue has been solved. The problem was that the 255s are of a design of pump that doesn't like low voltage. When they have normal voltage (i.e. WOT on the Lightning), they flow to their specified capacity. But when they are at the lower stepped voltage, they actually flow less fuel than the stock pumps. Now this is perfectly fine for a stock truck, as they still supply enough fuel for the regulator to maintain base fuel pressure. The problem comes when you have larger rails. Then the pumps do not move enough fuel under stepped voltage to keep the rails full. That's where the low fuel pressure issue comes from.

The solution? Buy a second fuel pump dropping resistor, and wire it in in parallel with the original. That changes the stepped voltage from 7 volts to about 9 volts, and then the 255s move enough fuel to make it all happy again.

Hope this helps.

 

sal my l shows 24lbs at idle and 55 at wot do i need a resistor

i will be running 60lb injectors on the stock 03 rails