7.3 idi fuel supply/return issues

[Jerby]

I have a 93 7.3idi N/A that is having issues with fuel pressure delivery. I was running the Facet 40222 which is capable of 33gph. When pulling camper up a hill at 60-65mph, and when I say hill it's no longer than a 1/2 mile, the fuel pressure will drop down below zero on gauge and fuel filter light will come on. I would have to throttle back to 45-50mph so as not to starve the IP. I then upgraded to the Facet 40290 which is 50 gph and I put a regulator on it to 8 pounds not running, while idling it will carry 4.5 pounds, which oddly is the same as the 40222. New stock filter. Stock IP not turned up, still has the anti tamper pinkish red wax on all the adjustment screws. Probably not wax but what ever they use.

So I took it back on the highway and having the same issues. Now I'm thinking it's a return problem as returning too much. Heard of others putting a needle valve from return tee'd back into fuel filter??? not sure about that as it going to flow with least amount of resistance. Plenty of supply as the fuel bowl on facet is always full, well I say that but in the driveway, guess I couldn't honestly say while under full load. I did however install new tanks and blew lines out, not that it needed it as there was no junk coming out.

If closing off/pinching down the return line where's the best place to do it, or is this not a good option?

Thanks in advance for the help. If I can answer any more questions I'll answer them promptly.

 

[Thewespaul]

Jerby, I would say you have a restriction up stream of the pump that is causing the the supply issues. Try blowing low air pressure through the lines back to the tank. Make sure a previous owner hasn’t added a little filter on the frame that has gotten clogged

 

[Jerby]

Yea I blew through both directions when tanks were off. Actually traced line out while back to check same thing. Sorry I didn't mention that.

What about a restriction in the tanks switch valve? Though about running both facet pumps in parallel but that's just going to be a bandaid. I like to fix everything. Truck was pretty tore up when I got with no TLC to it. New ZF5, pulled heads for head gaskets, new clutch fan, water pump, radiator, 3 ujoints, both upper and lower control arm ball joints, motor harness wiring that I spent weeks on myself instead of buying one. And many, many more ......

 

[Thewespaul]

The selector valve commonly gets clogged with gunk from debris or crumbling plastic in the tanks, but usually you have tank selection issues if that’s the case. You can tee into the return lines and measure what pressure you have, I don’t recall the spec but it’s not much pressure

 

[IDIBRONCO]

I don’t recall the spec but it’s not much pressure

It would be more a matter of volume rather than pressure. I'm not sure that you'd want to restrict the return. You may run into issues.

 

[Jerby]

Very true on restricting the return. I did that tonight and it didn't like it. Stumbles. Placed a 1/4" valve on it and pinched it down 3/4 way. Must be something to do with injection pump not releasing the pressure for return. I have two options, run both of my facet pumps in parallel or get the carter diesel pump that will supply 100 gph. Just hate to waste money.

 

[Jerby]

Very odd of how the stock mechanical pump from specs online will only supply 30 gph.

 

[Thewespaul]

It’s got to be a supply issue, if there was a restriction on the return side it usually will keep popping off the rear return caps and making a leak. Put an inline filter before you pump and make sure it’s not getting fed debris.

Do you have a relay for the pump?

 

[The_Josh_Bear]

It’s got to be a supply issue, if there was a restriction on the return side it usually will keep popping off the rear return caps and making a leak. Put an inline filter before you pump and make sure it’s not getting fed debris.

Do you have a relay for the pump?

Those Facets have a good sized 50 micron servicable screen built into them with a clear bowl, if its clogged just service it.

Good call on the relay.
OP, you might do the 1 quart timed test and see what the GPH is. Then you'll know for sure if you have supply or other issue.

As for restricting the return from the IP, that's a thought I keep having since I noticed my pressure drop off under load. Mine goes from 9psi to 2psi, idle to WOT. I've read of people wanting to add a 5psi poppet valve after the IP to keep 5psi inside at all times. Makes a lot of sense but I haven't heard of anyone having done it with results yet.

 

[Thewespaul]

Doing so will reduce fuel output to the injectors, retard timing and add more heat to the ip, possibly causing hot start issues.

 

[Jerby]

Relay is good. I have a led light hooked up right in front of electric pump to tell me if it's has voltage or not. Thought of that too awhile back thinking relay might be bad.

Thewespaul your correct that's what it did When I tried restricting it.

I'll hook both facets up today and if it's a supply issue I'll know right away.

 

[Jerby]

Just to clarify that led light is inside cab so I can constantly monitor it while under a load

 

[Macrobb]

As for restricting the return from the IP, that's a thought I keep having since I noticed my pressure drop off under load. Mine goes from 9psi to 2psi, idle to WOT. I've read of people wanting to add a 5psi poppet valve after the IP to keep 5psi inside at all times. Makes a lot of sense but I haven't heard of anyone having done it with results yet.

...It already has one. The return fitting inside the return line(black fitting coming out of the IP "lid") has a ball and spring inside which keeps inside pressure at 3-5 PSI.

When cold, the cold advance solenoid pushes a pin against this ball, forcing it open 100% of the time. IP housing pressure drops to 0, and relative timing advances.

Pretty sure that the housing pressure doesn't really affect flow out of the IP - remember, it's an incompressable fluid, and the input side of the IP is a sliding vane positive displacement pump. So it's going to (attempt) to push X fluid through the IP no matter what. Restricting the return line will simply retard the timing, and (at extreme levels) prevent the plungers from opening up, preventing fuel from being injected.

 

[Jerby]

Raining today and didn't feel like being out at the work shop all day so maybe later hook both up

 

[Thewespaul]

...It already has one. The return fitting inside the return line(black fitting coming out of the IP "lid") has a ball and spring inside which keeps inside pressure at 3-5 PSI.

When cold, the cold advance solenoid pushes a pin against this ball, forcing it open 100% of the time. IP housing pressure drops to 0, and relative timing advances.

Pretty sure that the housing pressure doesn't really affect flow out of the IP - remember, it's an incompressable fluid, and the input side of the IP is a sliding vane positive displacement pump. So it's going to (attempt) to push X fluid through the IP no matter what. Restricting the return line will simply retard the timing, and (at extreme levels) prevent the plungers from opening up, preventing fuel from being injected.

It does affect it, I have found that when setting up the governor for idle fuel rate on the flow bench (with the top cover off) that it flows about 5-10cc less with the cover back on. I knocked out the ball in fitting of the return port and gained some flow. By bumping up my inlet fuel pressure my fuel temperature also went down which increased flow as well. I’m working on a write up for just stock fuel mods to get as much as you can out of a stock pump.