What would cause a fuel pressure drop?

["Turbo-T"]

Today I replaced the intank fuel pump because when I would go into boost, the fuel pressure could not get past 42-44 PSI before it would quickly start dropping off. The Scanmaster confirming knock and the wideband showing 12-15 in full boost confirmed this.

I asked this in another post and the answer I got was bad fuel pump.

The pump I pulled out was one of the F2000169 Walbros that I purchased in Feb '08 before they became known as the premature failing pump.

The new pump I installed is a Racetronix GS340M.

Today on the test drive I was only able to get up to 50-52 PSI of fuel pressure, and if I tried to push any harder, the pressure would quickly start dropping off.

So I have a new pump, I tightened all the hose clamps on the hose that connects the pump to the hanger, the hose I used was a new piece that came in the kit....before I drop the tank and pull the pump, where else could I check at that might affect the fuel pressure dropping off?

 

[SCOOBY DOO]

Hotwire kit? How's your charging system?

 

["Turbo-T"]

Yes it is hot wired, charging system runs between 13.8-14.4

 

[bmackinnon]

did you replaced the fuel filter? Maybe it's clogged and causing a restriction? Do you have the vac line to the regulator ziptied tight at both the source and the regulator, such that you are sure reference boost is not leaking out of the hose??

 

["Turbo-T"]

I replaced the fuel sock with a new one, filter was replaced about a year ago. I could try that.

Yes the line going to the regulator is zip tied as tight as can be.

 

[azbob]

When the pump fails and disintegrates the debris usually ends up in the filter.

 

["Turbo-T"]

Thanks Bob.

If I may ask, how exactly does a clogged filter contribute to a sudden fuel pressure drop off at a certain psi?

Also of interest I did notice my shroeder valve at the end of the fuel rail will leak fuel if the cap is removed; if the cap is on no fuel will leak. I don't know if that's known to cause fuel psi drop off but I did replace the valve for good measure which didn't do any good.

 

[azbob]

The efficiency of the filter is reduced at at some point it will not flow.

 

["Turbo-T"]

Thanks I will try the fuel filter. Are there any particular types of filters I should get/stay away from?

 

[forcefed3.8's]

What is your fuel level. I have noticed with a Spectra replacement tank and a double pumper, I need to keep it almost full. Around 8 gallons left, the pumps start sucking air, and the wideband goes all over.

 

["Turbo-T"]

The fuel level according to the gas gauge on the dash was at 1/3 to 1/2.

Before I dropped the tank, I pumped about 4 gallons (guesstimating) into a 5 gallon jug before the fuel coming out of the hose attached to the fuel rail started just spitting fuel. The amount of fuel I was able to actually remove from the tank was over half of the gas jug but didn't completely fill it up.

After the tank was back up in place with the new pump in it, I dumped the entire gas jug into the tank, then put in a full 5 gallon jug into the Buick's tank.

 

[Rodney87GN]

So I have a new pump, I tightened all the hose clamps on the hose that connects the pump to the hanger, the hose I used was a new piece that came in the kit....before I drop the tank and pull the pump, where else could I check at that might affect the fuel pressure dropping off?

How about the rubber hoses above the tank that connect the hard lines to the sending unit? Those have been known to split giving you low fuel psi.

 

["Turbo-T"]

While I had the tank down I did not see any splits in the rubber hoses. I did make sure to have the clamps on as tight as they'd go.

 

[Roger]

I've seen rubber hoses collapse. The vent hose from the charcoal cannister or the same line has a short hose at the tank.
I would bet on the fuel filter & maybe the pump.

Another possibility could be a pinched fuel hard line.

Please post your fix.

 

["Turbo-T"]

Would a crack in the sealant where the hard return line goes past the fuel pump hanger top affect pressure? I did find this during the pump R&R and used some RTV to seal it up.

Also on my fuel pump hanger the seat for the pump is missing so I had to use two hose clamps to secure the pump to the return line. It's been this way for awhile.

Today I changed the fuel filter, but have yet to ops check it. I'll keep you posted.

 

["Turbo-T"]

Update!

Looks like you were right...took the car out for a spin after changing the filter and I got my pressure back. I did it slowly to make sure the pressure was rising and not falling off after 50 psi. So a big thanks to you for that.

I did however encounter some knock. At first I saw some figures of .02 then .09 when at WOT, glanced at the wideband and it was in the 12's.

Did a little bit more driving and punched it again....no knock this time and wideband hit 10.70's.

Gas is 93 octane with a little bit of 110 left in it from the last time I raced at the track. The chip is at stock 23/21 timing, boost at 25 psi. Battery voltage is 13.6-14.4 according to the Scanmaster.

I did notice now at a 30 mph roll when I punch the pedal the tires squeal. It didn't used to do this unless I floored it from a dead stop. Interesting. That's going from a Walbro F20000169 pump over to a Walbro GS340M by Racetronix. Must be the slightly bigger wire Racetronix supplies....?

Anyways this weekend I'm going to do some more testing and report back.

 

[Razor]

Looks like you were right...took the car out for a spin after changing the filter and I got my pressure back. I did it slowly to make sure the pressure was rising and not falling off after 50 psi. So a big thanks to you for that.

I did however encounter some knock. At first I saw some figures of .02 then .09 when at WOT, glanced at the wideband and it was in the 12's.

Did a little bit more driving and punched it again....no knock this time and wideband hit 10.70's.

Gas is 93 octane with a little bit of 110 left in it from the last time I raced at the track. The chip is at stock 23/21 timing, boost at 25 psi. Battery voltage is 13.6-14.4 according to the Scanmaster.

I did notice now at a 30 mph roll when I punch the pedal the tires squeal. It didn't used to do this unless I floored it from a dead stop. Interesting. That's going from a Walbro F20000169 pump over to a Walbro GS340M by Racetronix. Must be the slightly bigger wire Racetronix supplies....?

Anyways this weekend I'm going to do some more testing and report back.

Clogged fuel filter cost you performance.

LEAN afr contributed to knock. Did you purge the alky kit before going WOT?

 

["Turbo-T"]

I'm pretty sure I purged it, but can't remember. :redface:

But tonight I filled up the tank with 93 and nailed it....wideband was reading 10.2/10.3 after coming back from the station. Zero knock on the Scanmaster.

Now I gotta fix the darn fuel hanger...filled the tank up and it started pissing out of the top. Probably since the return line has a cracked seal at the hanger top assembly.