codyguy

The fuel PSI at idle is usually high because there is no real draw on the VP under load. For ex. if you tow and are going up a grade while pulling a load, in this situation you will starve the stock LP because of the vacum created between the VP, which needs much more fuel than the stock LP can provide with the small 1/8"ths fuel lines. With a larger diameter tube between the LP fuel filter and VP, there will inevtiably still be some vacum pressure between the two under load, but with a larger diameter it is also compensated positively with a higher amount of fuel, in exchange keeping the VP cooler, and letting the truck have the proper amount of fuel for the application. This is of course just my HO, and I I have tested this hypothesis on 3 24v trucks now, and in ever same instance, I came to the same conslusion.

dkenny

check you voltage at the pump.
you might it low at ldle and higher when you rev up the engine.more voltage the faster the pump spins.

-dkenny

KATOOM

If your fuel pressure wants to jump up off of idle then I be concerned with whether or not your fuel pump is getting enough voltage or if your batteries are up to par. Especially in the summer heat when the electrical system is working the hardest against the temperature and running the A/C as well which draws a lot of juice. The fuel module screen is very obvious since it makes up the entire bottom of the unit. And no, you dont need to install a drawstraw just because you're dropping the tank. I personally wouldn't install a drawstraw given the multitude of issues guys have with them, and its not even something to bother with unless you have a 150 GPH or more fuel pump.

sluggo42

My batteries are only 2 years old, and they are high dollar jobs. I did check voltage earlier on in the process and found the voltage to be a solid 13.2dcv.

It's also a cold engine deal, no issues once she warms up... just strange...

I guess I should just get the Raptor FRRP and be done with it, but with my luck, I'll dump 3 bills out and have the same problem with the Raptor.

Offhand, does anyone know if the Raptor uses the same electrical connector as stock?

KATOOM

Since you mentioned that this only does this when the engine is cold..... Have you watched to see if the voltage gauge moves in conjunction with the fluctuating fuel pressure? If it does then whats happening is its strangely cold enough in southern CA to cycle the intake manifold grid heater for a minute after the engine first starts. The grid heater will do that as long as the ECM registers the outside temp 60* or less upon start up.

high bid

Holley blue w/a spin on fuel filter setup, $150 total(comes w/a regulator). It's lasted for 4 years and constant 16psi. JMO. I know Homestead is sold on the carter and it's probably the cheapest route.

johnh

vacum pressure...........no such thing

Junk Man

Where did you measure & under what conditions ie engine running, lights, AC? That is a little low for engine running without an electrical load. It is over charged for a battery without the alternator turning.

sluggo42

Actually ,

it was about 13.5 I think, I measured when warmed up. I saw pressure change, but voltage remained the same...

KATOOM

I think Junk Man meant when asking "where" did you take the voltage reading because where you're most interested in knowing the voltage in relation to the fuel pressure is at the connector which comes from the ECM and goes to the fuel pump. The voltage at that connector should not vary when the engine is running otherwise fuel pressure would fluctuate. And the voltage at that connector is governed by the ECM.

sluggo42

Oh I see,
I clipped some splices in about 6 inches from the pump, so I could watch it as it ran sitting still in my driveway. It would never waiver (voltage) as the pressure jumped about.

My best run came from back blowing the fuel line into the tank with some fairly low pressure to the point of hearing the tank bubble for about 10 seconds, maybe 3 times. That seemed to stop the fluctuations for a couple hundred miles. It seems to be doing it more and more again.
It will run at 14-16 mostly, then it will have brief moments where it will dip down to 8-10. Then I rev the motor, and it jumps back up.

Today it didn't jump back up with a motor rev, and it just stayed in the 8-10 range for a minute or two, then it jus blinks back to 15...

I also put a "tank cleaner additive" into this tank to see if it helps overall.
I've got it down to just a little less than half a tank now, and I'm trying to get it down real low so I can drop the tank and see if there is a problem in there. I think I'll give it another backwards blow-out again tomorrow to see if that does it again for a while...

I guess the good part is that it doesn't really go below 8 psi, but WHY it goes up and down is what's driving me crazy!

KATOOM

The back blowing helping makes me think that you have garbage in the either the fuel pumps pre-screen or that your fuel module in the tank is covered in garbage or has a ripped screen. I cant think of any reason at the moment as to why revving the engine helps.

sluggo42

I've got it down to just less than a half tank now, trying to burn as much fuel as possible prior to dropping the tank.
I'm dropping a LOT more often now into the 8-10 psi range, more often then not now. I'm going to back blow the tank again and see if that makes it good for a while again, I hope... I'll post results in a day or two. I need to get this issue resolved this week cuz I hav a fishing tourney next Sat. night and I don't want any problems while towing the boat... aaack!

Seems like it takes forever for the tank to go down when you want it to, but it's the opposite when you don't want it to...

kerry.king

Here's a stumper. Lately I've noticed my fuel psi gauge seems to be reading backwards. I'm at a constant 20psi at start, then the longer i drive, the needle climbs. At w.o.t it doesn't drop. Any ideas. Its kinda making me nervous

sluggo42

I am now convinced that there is some type of something wrong behind the lift pump. I had been getting worse and worse fuel pressure, even down to 4 at one moment, but going lower and lower by the day. I back blew the tanks a couple of weeks ago with good results for a while, and I also poured in a good dose of liquid tank cleaner additive. Not sure what that did for me... But after a week the pressure started faltering again.

So, it was to the point of not being able to go 100 feet without the alarm going off at 7 psi.

So, this morning I disconnected the line feeding the lift pump and blew compressed air back into the fuel tank. This time I let it "boil" for about 20-30 seconds, and repeated that for 3 times. I am having a rag in-between the air nozzel and using one hand to hold the nozzel against the fuel line, then slowly cracking the ball-balve open. (it's a 1/2" ball-valve, used for quick changing of the lift pump) but my point is that it's a semi low air pressure. I basically can hear the tank boiling.

ANyways, I re-connected the line to the lift pump and have now made 4 trips about town, and the pressure has not dropped below 12 yet, and only went to 12 under a hard throttle for just a moment. pressure seems to be a pretty solid 14-16 psi.

SO, what I really need now, is to see a picture of a fuel tank module, and perhaps some insite on tank dropping thoughts. It is obvious to me now that there is some kind of obstructing issue happening. I don't know enough about diesel/tanks/drawstraw issues yet to be able to pinpoint the exact problem, and I need to continue to burn off fuel to get the tank light enough to handle without 100 lbs of fuel to fight with.

I'm also wondering if there is a correct way to "clean" a fuel tank?

Anyways, I think this thread is getting close to closure, just as soon as I get the tank dropped/cleaned, and I get the fuel module/screen cleaned. So off to the search engine I go once again.... **sigh**