Fuel Pressure drops under boost.....NEED HELP!

[LowLife]

1993 Ty w/ATR Pitbull chip (purchased truck w/this chip)

Baseline fuel pressure set at 42psi with vac line disconnected....fuel pressure is consistent up to around 8psi of boost and then the fuel pressure drops down similar to 0psi boost.

Things I've already tried:

1. New Walbro 255 w/Hotwire Kit (all from Ractronix)
2. New Fuel Filter (OEM)
3. Accufab AFPR (this was already on the Ty....checked for gas smell in vac line, small hint of gas, but nothing overwhelming)

Ty will only boost to 8psi, it will not build more boost (possibly bc it's starving for fuel?). At 8psi, pinging/detonation sound. Local shop w/AWD dyno said it's probably the fuel pump going bad.....that's why I purchased the new Racetronix kit. However, that still didn't fix the issue.

Instead of dropping another $100 on dyno session.....does anyone have any ideas? What would cause fuel pressure to drop under boost? Do I need to rebuild the Accfab (even though there was only a hint of gas smell)? Could the ATR Pitbull be messing something up with fuel pressure?

Any help, feedback, suggestions is appreciated. Thanks!

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

Any thoughts? I'm out of ideas. Thanks!

 

[denny]

Re: Fuel Pressure drops under boost.....NEED HELP!

Check the vacuum line to FPR, maybe bad FPR.
Ecu/chip have nothing to do wiht fuel pressure, two seperate things.

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

Thanks!

I forgot to mention that all the vac lines are brand new....less than a month old.

I ordered a rebuild kit from Accufab for the AFPR.......hopefully it should be here by this coming weekend.

 

[jusssi]

Re: Fuel Pressure drops under boost.....NEED HELP!

How about check valves in vac line? If starting leak pressure when boost...?

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

New check valves replaced with new vac lines.....and this problem occurred with old lines and old check valves too....so hard to imagine, they'd be bad on both old and new vac lines.

 

[93 TY 2185]

Re: Fuel Pressure drops under boost.....NEED HELP!

It has to be your AFPR. This may be a long shot but check your return line.
Sometimes they get a kink in them right at the AFPR.

 

[RealFastV6]

Re: Fuel Pressure drops under boost.....NEED HELP!

Since you replaced the obvious one (fuel filter), I'd start checking for a kinked supply line.

Also when you replaced the pump, did you replace the lines inside the tank?

 

[RealFastV6]

Re: Fuel Pressure drops under boost.....NEED HELP!

It has to be your AFPR. This may be a long shot but check your return line.
Sometimes they get a kink in them right at the AFPR.

If there's a restriction in the fuel line, the pump is going to be working harder than your fuel pressure would indicate.

When the FPR asks it to work even harder (under boost), you will "run out of pump", even though you're scratching your head saying "But the pump should be able to put out 60psi." What's really happening though is it's failing to put out the (let's say) 90psi that it's having to put out just to see 60 at the rail.

Now there's a number of possibilities (lines in the tank leaking, kinked fuel line, "shit" in the fuel lines) but bottom line is likely restricted fuel flow between the pump and the FPR.

:2cents:

 

[93 TY 2185]

Re: Fuel Pressure drops under boost.....NEED HELP!

If there's a restriction in the fuel line, the pump is going to be working harder than your fuel pressure would indicate.

When the FPR asks it to work even harder (under boost), you will "run out of pump", even though you're scratching your head saying "But the pump should be able to put out 60psi." What's really happening though is it's failing to put out the (let's say) 90psi that it's having to put out just to see 60 at the rail.

Now there's a number of possibilities (lines in the tank leaking, kinked fuel line, "shit" in the fuel lines) but bottom line is likely restricted fuel flow between the pump and the FPR.

:2cents:

I agree with you. I was just throwing out some other things that I have run across.

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

Thanks everyone!

When I replaced the fuel pump, I replaced the fuel sending unit with a new Spectra brand. Seems like it got good reviews from SyTy, so I did it too.

I'll check the fuel lines from back to front next....look for kinks. If I can't find anything visible....and after I rebuild the FPR if I'm still having the same issues......then, I'll assume the fuel lines are blocked or restricted. I guess the only option would be to replace the lines at that point in time.

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

Rebuilt the FPR, with new diaphragm.....still having fuel pressure drop in boost.

I'm thinking the ONLY thing it can be is fuel lines at this point.......correct? Just want to get some reassurance before I spend the money on braided lines.

 

[DaveP]

Re: Fuel Pressure drops under boost.....NEED HELP!

Borrow a radiator pressure tester. Or buy a cheap one at Harbor Freight. With the engine idling, and your fuel pressure gauge connected, connect the radiator tester to the AFPR. Pump it up, and watch the fuel pressure. You should get appx 1 pound of increase in fuel pressure for each psi of diaphragm pressure.You'll know in less than 5 seconds if it's the fuel pump, or AFPR. I suspect it's not fuel pressure related at all..

The 8psi threshold is "base boost". If the ECM doesn't PWM the boost solenoid, the waste-gate receives full pressure, and regulates boost to 8 psi. I suspect you have a boost control problem, not a fuel delivery problem limiting you to 8psi.

 

[DaveP]

Re: Fuel Pressure drops under boost.....NEED HELP!

If there's a restriction in the fuel line, the pump is going to be working harder than your fuel pressure would indicate.

^^Not really true^^.

It depends on volume required at the FPR pressure. If there is a restriction, the pump may produce up to appx 60-70psi. There will be pressure drop across the restriction. As engine usage increases, when the pressure drop matches or exceeds the FPR setting, then the pressure at the FPR will also fall.

The fuel pump will work "just as hard" with no restriction as it will with the restriction. It takes the same amount of energy to move 10 gallons of fuel at 10 psi as it does to move 1 gallon of fuel at 100psi.(More or less. There's some efficiency loss). But high pressure, or restricted output, causes the pump to run faster. This wears out the brushes and commutators. Not the high pressure itself.

 

[Don W.]

Re: Fuel Pressure drops under boost.....NEED HELP!

SInce 8# seems to be a consistent number I tend to agree with Dave. Disable the controller and carefully run the boost up and see what happens. If it was fuel delivery I'd suspect the 8# to vary. The fuel pump can be checked by squeezing the return line and watching FP. That only checks pressure output, not pressure AND volume which is what the motor requires. Could it be the little "sock" on the bottom of the fuel pump restricting flow?

 

[RealFastV6]

Re: Fuel Pressure drops under boost.....NEED HELP!

It depends on volume required at the FPR pressure. If there is a restriction, the pump may produce up to appx 60-70psi. There will be pressure drop across the restriction. As engine usage increases, when the pressure drop matches or exceeds the FPR setting, then the pressure at the FPR will also fall.

We're on the same page - I just think you explained it better. :tup:

When I said "working hard" I really mean increasing pressure at the pump.

 

[Osmosis]

Re: Fuel Pressure drops under boost.....NEED HELP!

This may seem like a simple question and I"m sure you've done this already......What is the voltage to your fuel pump? It could be an electrical problem. I had a similar problem and it turned out to be my fuel pump. Your situation sure sounds like a bad fuel pump....I wonder if your new pump is bad? Stranger things have happened. I had two walbro 255lph pumps that went bad so I went back to the stock unit and have had no problems since.

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

SInce 8# seems to be a consistent number I tend to agree with Dave. Disable the controller and carefully run the boost up and see what happens. If it was fuel delivery I'd suspect the 8# to vary. The fuel pump can be checked by squeezing the return line and watching FP. That only checks pressure output, not pressure AND volume which is what the motor requires. Could it be the little "sock" on the bottom of the fuel pump restricting flow?

What's the easiest way to disable the controller? I can check that pretty easily.

 

[Don W.]

Re: Fuel Pressure drops under boost.....NEED HELP!

Just remove the line to the actuator. Just be real careful here. I would suggest as a first step squeezing off the fuel return line while watching FP and see where it goes. 100% safe way to test max FP from the pump.

 

[LowLife]

Re: Fuel Pressure drops under boost.....NEED HELP!

Disconnected vac line to WG actuator......tested it very slowly and it hit a max of 15psi on the Autometer. Seems like something is still holding it back......I doubt it would max out at 15psi on it's own?