Low Fuel Pressure

[Syty84]

I am getting about 20 psi fuel pressure at the fuel rail during idle. What do most people get on average. I swapped the fuel pump with two brand new pumps, replaced the hotwire pump relay, new sportmachines fuel pressure regulator, brand new 80lb injectors. Not really sure what's going on here but I'm running out of ideas.

 

[Slyclone]

Re: Low Fuel Pressure

did you happen to turn the adjuster in or down?

back off nut, turn down threaded rod with Allen key.. to adjust fuel pressure.

With hose disconnected I believe it should be at 40-43 psi at idle.

any recent change in fuel filter? could be restricted you never know.

 

[Syty84]

Re: Low Fuel Pressure

Brand new fuel filter installed.

I adjusted the fuel pressure regulator and it made no difference on the fuel pressure gauge.

 

[Eric Sy GN]

Re: Low Fuel Pressure

Fuel pressure gauge working correctly?

 

[anryo]

Re: Low Fuel Pressure

Is there any how-to's for changing FPR on our trucks? Mine also needs replacement, but not sure where to start.

 

[Don W.]

Re: Low Fuel Pressure

Is there any how-to's for changing FPR on our trucks? Mine also needs replacement, but not sure where to start.

First, not that it's a bad idea but how do you know it needs replacement?

Anyway. First get Sportmachines AFPR. Tools: 3/16 allen, 5/8 open-end and 3/4 open-end. Parts: one inj o-ring. (Sportmachines AFPR looks like it comes with an 0-ring.) and one fuel line o-ring. You may have to buy kits, not sure individual o-rings are available.

With engine cold because you’ll probably spill some fuel remove the manifold pressure sense line from the FPR. Use the ¾ wrench to support the FPR and the 5/8 to remove the fuel drain line from the bottom of the FPR. Use the allen to remove the FPR. Lightly oil the new fuel line 0-ring and install it on the fuel return line. Lightly oil the new FPR o-ring and install the new FPR on the fuel rail. Reinstall the fuel return line. Reinstall the sense line.

To do this correctly you’ll need a way to measure fuel pressure ga to set the new FPR. I don’t believe they come preset.

I imagine this all comes on an instruction sheet with the new AFPR.

Someone please chime in if I forgot something…

 

[anryo]

Re: Low Fuel Pressure

Thanks for reply. I'm suspecting FPR fault, as my fuel pressure jumped rapidly around 2.2-3.0 bar (32-44psi). And what is strange, that some weeks ago this stopped and pressure was nice and steady. And now few days ago it started again.
I have aftermarket ECU, so I can see teh "live" fuel pressure. But I have no ideas, what could cause this....

 

[Slyclone]

Re: Low Fuel Pressure

^ could be a bad fuel pump. or weak. do you know the history or condition of your current fuel pump? if it's original. if not a brand? could be aftermarket :screwy: ...

 

[Syty84]

Re: Low Fuel Pressure

Last pump was a walbro 255 which worked fine for a few years, i then upgraded to the racetronix 340 pump which is when I experienced the drop in fuel pressure. I then swapped that pump for a 450 pump that my tuner friend uses in all his vehicles that make over 500hp. Amazon.com: Walbro F90000267 450 LPH E85 fuel pump: Automotive

What's odd is that the fuel pressure is the same on the last 2 pumps, I'm going to switch out the fuel filter with another today, maybe it's a bad brand new filter?

 

[anryo]

Re: Low Fuel Pressure

Thanks for reply. I'm suspecting FPR fault, as my fuel pressure jumped rapidly around 2.2-3.0 bar (32-44psi). And what is strange, that some weeks ago this stopped and pressure was nice and steady. And now few days ago it started again.
I have aftermarket ECU, so I can see teh "live" fuel pressure. But I have no ideas, what could cause this....

My truck also has a Walbro fuel pump, but it does not have "hot-wire" kit. I've been also thinking, that hot-wire or bad filter could be the issue, but why it sometimes gets better and sometimes worse?
Under boost it seems to be always more stable than on idle or cruiisng speeds.

 

[Syty84]

Re: Low Fuel Pressure

Swapped in my old FPR and instantly had 40lbs of pressure. Thanks everyone.

 

[anryo]

How fast the pressure should drop after turning ignition on/off? Mine is dropping to almost 0 in ~30sek. Will it indicate bad FRP?

 

[Don W.]

How fast the pressure should drop after turning ignition on/off? Mine is dropping to almost 0 in ~30sek. Will it indicate bad FRP?

It should hold for for a long time. Minutes. If yours drops that fast you have a leak somewhere. FPR being only one possible location. the others are: bad fuel injector(s), bad check valve or other issue in the fuel pump, an actual, fitting or hose, leak somewhere.

 

[liquidswords88]

You can test by pinching off the fuel line at the rubber section at the top center area of the transmission. Prime the pump then pinch off the return line if it holds the fpr is bad. Prime it up and pinch off the pressure line, if it holds the check valve in the pump is bad. Prime it up and pinch off both pressure and return, if it drops than the injectors are bad.

 

[anryo]

You can test by pinching off the fuel line at the rubber section at the top center area of the transmission. Prime the pump then pinch off the return line if it holds the fpr is bad. Prime it up and pinch off the pressure line, if it holds the check valve in the pump is bad. Prime it up and pinch off both pressure and return, if it drops than the injectors are bad.

Perfect answer! Thank you!

 

[Don W.]

That's what the manual says actually but I'd be cautious. How do you know for sure that you've fully pinched off the line? Just a thought.

 

[liquidswords88]

It's very simple. There is actually a tool made just for pinching off rubber lines. The method works and is very simple and quick.

 

[Don W.]

Yup, agreed. Done it many times in the past on a different application. I've seen how difficult it is to -fully- close off a line. So just sayin' to be aware.