Working on a customer vehicle. 2004 LLY, 4x4 bone stock, used to have predator 70hp. Trying to accelerate it seems like it's running out of fuel, has surging like gas motor with plugged fuel filter. Installed new fuel filter, same problem. Checked desired fuel pressure with scanner and actual always meets desired. Checked all temp sensors and reading correctly. No codes set all DTC tests pass. Any suggestions?

Fuel pressure regulator will cause a surge when at idle, not sure about under acceleration , a worn out fuel pressure relief valve will make it seem like its running out of fuel,also possible worn fuel hose sucking air might do it. you can perform the bottle test to check the fprv for damage.

Or lack of boost pressure will cause loss of power. What's the boost at? Maybe the rub through issue on the boots to the intercooler.

Fuel pressure regulator will cause a surge when at idle, not sure about under acceleration , a worn out fuel pressure relief valve will make it seem like its running out of fuel,also possible worn fuel hose sucking air might do it. you can perform the bottle test to check the fprv for damage.

Bottle test? Please explain. D-maxes aren't that common here. If fprv damaged, shouldn't my actual pressure be different than that of desired?

Or lack of boost pressure will cause loss of power. What's the boost at? Maybe the rub through issue on the boots to the intercooler.

No problem on boost. Checked the boots, all good. Ran it this morning again, tons of power. Now it just turned intermittent. Thinking sticky valve or something now.

Bottle test information:

http://www.dieselplace.com/forum/showthread.php?t=342633

At what RPM does it seem to run out of fuel?

Fuel Pressure reaches 160MPa (~23,000psi)?

Is Elevated Idle on? (I know....WTF does that have to do with power? I'll explain if my hunch is correct.)

If fuel pressure hits 160MPa, bottle test is not needed.

Bottle test is a suitable alternative for when you don't have proper diagnostic equipment.

In my past experience I have had a few things exhibit symptoms such as your describing (besides the fuel filter): a MAP sensor (aka boost sensor) short out, rail pressure relief valve crack, FCA (others have called this the fuel pres regulator) sticking due to biodiesel usage, worn CP3 not keeping up with railpressure demands (vehicle had 90k miles), and vane position solenoid hanging up. Most of these are accompanied by an MIL. But stranger things have happened. Additionally, I have had a TS Performance MP8 go bad internally that caused very similar type of symptoms as a bad MAP sesor. On this vehicle, the map signal gave 5 volts @ idle. This caused VERY slow acceleration then it would just spool up after a few seconds to full boost. These are just a few that I can remember fairly accurately. Hope this helps.

At what RPM does it seem to run out of fuel?

Fuel Pressure reaches 160MPa (~23,000psi)?

Is Elevated Idle on? (I know....WTF does that have to do with power? I'll explain if my hunch is correct.)

Seems like almost any accel is the issue. I did see the fuel pressure spike to 23000 a few times, but then immediately drop down again because desired commanded less fuel.

Elevated idle is ON

The schrader valve on the front of the engine could be sucking air, (Brass Valve with black cap). Is there diesel coming out of the schrader valve? Did you check your inlet pressure to the CP3? Wonder if maybe you have a problem with the plastic pickup tube in the tank?

OK, did a bit more testing, disconnected FPRV entirely and even then pressure will only climb to 14000psi. Push on accel pedal and fuel pressure drops to 10,000psi and rpm limited to approx 2300rpm (even at WOT). Thinking either control valve stock part open or relief valve. Opinions?

BTW, turned elevated idle off, and problem still exsists.

Pretty sure I have some air entering the system before the filter housing too. Used clear line between FICM and filter housing and saw bubbles.

When you said you disconnected the FPRV (drivers side on rear of fuel rail), was that the return line (rubber tubing) or actually removed it? If you removed it you would never build any pressure and the truck wouldn't run.

With air you will never reach the 23,000 psi, and the truck will run like S*$T.

Once more, went on road test, no performance issues at all, fuel pressure up to 23000 no problem. Now just extended crank at startup and very hard starting when cold. -20*C here and plugged in.

My bet is on the air leak, cracked filter housing, leaking line, or pickup tube in tank could be cracked. Do you have the ability to pressure up the system from the tank (ie lift pump), I would pressure it up and see if you have leaks.

The typical inlet pressure to the CP3 is so low that any leak would allow air to penetrate the lines and you wouldn't even notice the leak.

When you said you disconnected the FPRV (drivers side on rear of fuel rail), was that the return line (rubber tubing) or actually removed it? If you removed it you would never build any pressure and the truck wouldn't run.

With air you will never reach the 23,000 psi, and the truck will run like S*$T.

OK, I think I said that wrong. I just took the connector off of the pressure regulator, not a relief valve. Sorry.

Again, took a testdrive and everything seems great now except for starting issue. I'm starting to pull my hair out.

When you disconnect the FPR, the CP3 will default to max output, 23,000psi. If you can't maintain this pressure with the FPR unplugged, you have a leak in the CP3 HP side, air is getting in on the LP Side of the CP3 or bad CP3. I lean towards the air getting in to the LP side of the CP3, more common problem.

When you disconnect the FPR, the CP3 will default to max output, 23,000psi. If you can't maintain this pressure with the FPR unplugged, you have a leak in the CP3 HP side, air is getting in on the LP Side of the CP3 or bad CP3. I lean towards the air getting in to the LP side of the CP3, more common problem.

After yesterday, I'm thinking air on LP side too. Goig to hook up clear hose at various connections towards the tank and see if I can't isolate. o you happen to know if there is ANY air allowed in the system? I know the Fords in recent years have allowed bubbles up to 3/8" every 6" of line. Is there any spec for the d-max?

found it, air after fuel filter housing, but not before! Parts on order.

Glad to hear, figured you would eventually find it. And the answer is no air should be allowed.

Thank you, and thanks for your help Rebel!!

And now after all this diagnosis, the customer is contemplating not fixing it and taking it to the city to trade it off!! Sometimes these people frustrate me.