Fuel Return Line Pressure?

Should the fuel line from the shutoff valve to the fuel tank remain pressurized after shutting off the fuel pump?
Also what kind of return should I expect from the fuel return line with an input pressure of 75 psi; a drip, a trickle or a gusher?
Troubleshooting the hard start, low power and no throttle response on my M813, I rigged up an LS Diesel pump from the filter housing directly into the fuel line, bypassing the shutoff valve. It's a Wrustvnabc 24 volt, 300Lph, 75psi universal in-line pump. Essentially an aftermarket version of the Bosch 12 volt sharing the same part #0580254044.
After pressurizing all I get on the return line is a trickle of fuel. The fuel line remained pressurized for at least 10 minutes after shut off.

There is zero reason to add a 75 PSI pump to a PT fuel system. These are not like a bosch system. There is almost zero pressure at idle and up to almost 1000 RPM, only flow, then the governor plunger, weights, button start ramping the fuel pressure and rate up to where it needs to be. There is low pressure in the pump body and the rail pressure is made as the fuel goes through the governor portion of the pump. 75 PSI will most likely put fuel in the oil through the seals in the front cover.

The inlet is staying pressurized because the injector plungers are not allowing fuel into the injector due to the inlet not being aligned with the metering passage and plunger. IF a plunger is aligned with the port and the injector has high leakage between the barrel and plunger, it will force fuel out between the two components and out the top of the injector, putting fuel in the oil.

The fuel needs to go through the injector, which means the plunger needs to be cycling for the fuel to get pushed out the return port. There is maybe 2 PSI of return pressure on a fully loaded, working engine, any more than that and the fuel is held in the injector and the engine gets more of a squirt, kinda like increasing rail pressure. This was something truckers did back in the day to make more power, but it can and will damage things . High return pressure will cause the idle to not come down to curb idle and, worst case, an uncontrolled, high rpm, like a run away.

Why are you doing this?

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Why are you doing this?
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Static fuel test to determine if the fuel rail is blocked.

• The pump is connected directly to the fuel rail, bypassing the PT pump and shutoff valve.
• The engine is not running (static test), so the injectors are not cycling, and fuel flow is limited to what passes through the rail and out the return line.

So what's the maximum pressure the PT pump delivers to the engine at load? I assumed it was 200 psi, I've seen figures as high as 400 psi?

Don't know the fuel code off the top of my head, but a 614 CPL 400 was 160-175 at rated RPM.

Why do you think there is a blockage? If there were a blockage between the inlet of the engine and the outlet, that would equal pinching off the return line, the idle would creep, after driving the RPM would hang at greater than idle speed.

It is possible that the inlet screens get gooped up if the fuel was old and full of algae. Also, if there was a pump failure pieces parts could have got by the in pump screen filter and into the inlet screens. The pump will work without the in pump screen too, so, if the pump messed up sometime prior to your getting the truck, and the filter wasn't installed, that could do it too.

Before you go pulling things apart, again, why do you think there is a restriction? What is the engine doing or not doing?

Hard start and no power, couldn't get her over 1000 RPM with no pedal response. After about 5 minutes it would die. Fuel filter a line was still full of fuel. Checked for air, replace the fuel filter and housing, just to be sure. Cleaned all O-ring mating surfaces and replaced the O-rings, no joy. Replaced the PT Pump screen, looks like it was clogged, maybe small particles of rust but no metal on the magnet.

Does not sound like the pump failed since no metal in the screen filter. Only way to tell if the screens are plugged is to run it with a gauge on the FP outlet and see if the rail pressure is in spec or pull the injectors and have a look see. Easier to pull things apart.

But first, make sure that the throttle shaft is turning in the pump. Should be 32 degrees of travel. Make sure it is getting full throttle. The lever should just break over.

One more stupid thing, pull the inlet fitting and check that there is no garbage in the fitting causing a giant restriction.

Try running it out of a bucket of clean fuel right into the pump and see if it is better.

A giant intake or exhaust restriction can do the same thing, usually accompanied by black smoke for the inlet and if the exhaust is plugged, no smoke gets out, will barely idle and will shut off and be very hard to re start.

In a nut shell, before you go rippin' the engine apart, check throttle shaft movement, throttle break over or full throttle, check the inlet fuel fitting, run from a bucket, disconnect inlet pipe then do the exhaust, all one at a time so if what you did fixes it, you know which thing it was.

If all the above checks out, best over the interwebs troubleshooting is pull the injectors and have a peek.

WillWagner said:

check throttle shaft movement, throttle break over or full throttle

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This is what I have from Idle to Full Throttle

That looks a bit shy of the 32 degrees. There is supposed to be a closed end lock nut about an inch or so long on the screw sticking out of the pump body at the rear of the house shaped cover on the pump. That is what sets the throttle travel, that and the part under that cover that is secured to the shaft. Those two adjustments are what aligns the holes in the throttle shaft with the passages in the pump body.

If someone has tried to "adjust" that screw, the throttle shaft will not align with it's counterparts in the pump. Low power, hard starting, no acceleration, dying. The only reason it would die from the settings on those two adjustments is if the stop inside was moved to make it so that the leakage through the passages in the pump and shaft were too small, closed off. The external adjustment will cause all but the dying.

I would put a protractor on it. There is also no nut on that screw. Should be a thin nut, lonh closed end nut and seal wired for anti tampering.

More interweb TS! I got out a protractor. The way we did it on the stand was with a magnetic angle finder in the latter days and before that one of those swinging pendulum angle finders. I got 24. Easy thing to do, without really disassembling the engine would be to do the rest of the checks above except for the injectors, and if it still runs the same, pull the pump and find a local shop that can stand check it to see if there is an issue. The cover hasn't been bent back, so IMHO, the inner hasn't been messed with, just the outside one.

That is just an obvious sign of someone being there before you. There still could be more to it, but since this is staring at you, start with this and continue on if needed.

OR, you could just adjust it yourself and see what it does. Scribe a line on the cover with the lever at full throttle, turn the screw out until you get 32 degrees or so and see what happens. After the other things are looked at, of course.

Wreckclues said:

Should the fuel line from the shutoff valve to the fuel tank remain pressurized after shutting off the fuel pump?
After pressurizing all I get on the return line is a trickle of fuel. The fuel line remained pressurized for at least 10 minutes after shut off.

Click to expand...

question #1 = No

next situation , there should be a constant trickle on the return line while engine is running..

There will be no trickle when engine is off , even if your test pump is making pressure on the inlet .

Will Wagner is the cummins expert, and i don't have enough time to read all he has said before i have to leave today , so i will toss this out there . .Have you checked the screen underneath the screwdriver-slotted cap on top of the PT pump yet?

msgjd said:

Have you checked the screen underneath the screwdriver-slotted cap on top of the PT pump yet?

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Yup, replaced it. Looked like the interior was coated with rust, however there was no metal on the magnet.
So aside from the lift pump delivering fuel to the rail, what is it in the fuel manifold that requires the engine to be spinning for fuel to flow?

Wreckclues said:

Yup, replaced it. Looked like the interior was coated with rust, however there was no metal on the magnet.
So aside from the lift pump delivering fuel to the rail, what is it in the fuel manifold that requires the engine to be spinning for fuel to flow?

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The interior of the little screen is the intake side , which is unusual but that's how those are..

if I understand these engines correctly, it's the injectors themselves that open to release the high pressure fuel and create return flow, not the PT pump... The PT only draws from the tank and creates the volume (and about 200psi?) to give a constant feed to the injectors simultaneously.

If I understand Will Wagner correctly, if the PT isn't pumping to the heads, nor the injectors moving, the injectors aren't releasing any unused fuel .. The injectors on these engines are like mini injection pumps, all by themselves.. A push rod off the cam actuates the injector rocker, which then pushes on the injector to cause the squirt .. I myself have tried to make sense of this system thinking it was like a "normal" fuel rail system , but it isn't ..

That's also a big reason why the engine will eventually speed up to a scream all by itself when the throttle lever is pulled part way for a fast idle. The speed just keeps multiplying with each faster and faster injection cycle, a domino effect. I have yet to understand why it happens with the throttle, because it doesn't seem to happen if you hold the accelerator pedal at a fast idle .. doesn't make sense

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QUOTE="WillWagner, post: 2607117, member: 2057"]
There is supposed to be a closed end lock nut about an inch or so long on the screw sticking out of the pump body at the rear of the house shaped cover on the pump.
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This?

msgjd said:

I myself have tried to make sense of this system thinking it was like a "normal" fuel rail system , but it isn't ..

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Dang! I wanted to use the electric pump to prime the PT pump and fuel rail before startup

WillWagner said:

That looks a bit shy of the 32 degrees. There is supposed to be a closed end lock nut about an inch or so long on the screw sticking out of the pump body at the rear of the house shaped cover on the pump. That is what sets the throttle travel, that and the part under that cover that is secured to the shaft. Those two adjustments are what aligns the holes in the throttle shaft with the passages in the pump body.

If someone has tried to "adjust" that screw, the throttle shaft will not align with it's counterparts in the pump. Low power, hard starting, no acceleration, dying. The only reason it would die from the settings on those two adjustments is if the stop inside was moved to make it so that the leakage through the passages in the pump and shaft were too small, closed off. The external adjustment will cause all but the dying.

I would put a protractor on it. There is also no nut on that screw. Should be a thin nut, lonh closed end nut and seal wired for anti tampering.

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Hi Will, here's perhaps some pertinent backstory of the truck to help solve the mystery.. I was the PO of this truck and it came from a town hwy dept.. I have no idea what they did to it except I discovered an open solenoid fuel shutoff bypass knob and there was a minor chip in the fuel filter casting flange .. I closed the bypass and the solenoid operated as it should.. The filter flange hasn't seemed to be a problem.. I didn't notice a missing PT pump seal but likely not seen it even if it was there.. The town used it for dust control a few times each summer, but it otherwise sat outdoors all year..

It didn't see hardly any work for me either but it ran strong whenever I had a job for it.. Lack of activity would've been different had I located an affordable Prentiss-type log picker & log bunk for it.. After a long sleep, the initial start took some extra cranking but it would catch and run strong and be throttle-responsive after a minute or two..

Within the last couple years of my ownership, the inlet screen was changed twice (rust inside) as well as the fuel filter, regardless that it hadn't run more than a few hours total all year.. The tank got drained/flushed a couple times in previous years but it always produced a rust-colored residue in the tank and on fuel filters regardless ..

During the last 8 months of ownership it mostly sat.. When I went to move it to get it ready for the pre-sale inspection, it did not want to pick up fuel, but it ran excellent with good throttle response from a new plastic fuel jug.. I checked/tightened connections, changed the filter, pressurized the tank, made sure the filter bolt gasket was sealing, changed the PT inlet screen, and added a half tank or so of fuel which got it running good enough to move around and load etc. But it was still acting like it was sucking a little air..

As Wreckclues mentioned in a post elsewhere, when he got it home he continued looking into the mystery and discovered a corroded/perforated pickup tube, replaced it, and cleaned more crud from the tank..

I think you are correct to suggest trying to run it out of a jug first in order to eliminate injector or PT pump issues... But what's the chance of gum buildup inside the lines past the PT pump because of it sitting so much ? As previously mentioned, the truck has been a strong runner until recent years , and certainly has had fuel input issues during the recent years..

Wreckclues said:

Dang! I wanted to use the electric pump to prime the PT pump and fuel rail before startup

Click to expand...

One would be really handy for filling the filter cans on these things and priming that hose to the PT pump .. A feed-thru bypass type of pump might work okay for that , if they make one with a large-enough bypass volume while deenergized ..

Another one of these M809-series I got from a different town had a commercial truck spin-on filter housing w/hand priming pump on it .. I don't know how long it had been on the truck but it was sucking air from around the hand pump stem.. Replacing the oh-oh rings and tightening the stem nut didn't fix it

Wreckclues said:

This?

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I don't think I've seen a seal deal looking like that on any of the 855 cummins' I have or had .. Now I'm curious ! ..

And ,,, has anyone mentioned checking the oh-oh ring underneath the filter can bolt? I've had trouble with that little bugger on that truck and on one of my M818's