Wait a minute, no check in supply. How does LP work w/o a check valve at input and output?? As to a tight system, sure it's tight until it's not and then drainback so a stop at T prevents it. Don't plan to run both. Epump only large enough to prime and limp to safe location, almost any pump suitable. Spare LP onboard if really needed. A larger epump ok if PWM controller works out so no bypass regulator needed and pressure can be constant over load range. Probably over the top but could even slather suction side fittings with vacuum grease, holds against ~30" in lab but would needed testing on orings. That stops air leaks when IP sucks harder than epump can supply. Did I leave anything out.
Sucking air/Losing prime
- Thread starter chris142
- Start date
An just in case this seems derailed, I plan to test a better way to prime a stock setup by running my 7.3 dry at 1/4 tank (showerheads not fixed yet). Might answer debate as to whether to open injector lines or not.
Just a thought. How will epump cause flow thru LP output check valve while LP is also pumping? Unless we are worried about when(if) LP fails. Even then it will pumping unless supply shut off, which has not been mentioned yet. Really trying to get handle on failure modes since she died 5 miles from home on first test run after taking 3 days to get started (burned out WTS, flaky GP solenoid, broken inj return, etc.). Seems unreliable but drove from SC to CT without incident.
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If the e pump is putting out more fuel than the mech pump it will push volume through it at it cycles, which is why you need a check in front of it. It won’t push back to tank with the check before it, but it will put pressure on the internal diaphragm and rupture it just like if the e pump was before the mech pump.
I guess that's possible. I have 2 filters inline one being a large water trap and then the stock 6.9 filter. Both filters have less than 3k on them. It runs fine once running. I didn't drive it today. I will see if it starts tomorrow.
I replaced mine when I had the problem at 10k feet but of course it didn't help. But it was probably due for a new filter anyway, it had unknown miles on it.
Check the short piece of clamped on hose at the lift pump too. It takes a lot of abuse and if it gets leaky, it definitely lets in air.
Oh and there is a short piece of return line hose hiding way down behind the engine by the bell housing. On my RV, that piece is easy to see and reach due to the engine cover. I would guess trucks have it too but it's very hidden.
The pic below shows where it goes. The hose was removed in the pic, so I drew it in. It connects from a tee where the rubber/braided hose from the injector return caps connect to, and it goes to that steel line for returning towards the tank.
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Wes I have not opened one up but fairly confident LP has check valves on inlet and outlet. If that's so, how can epump force flow thru LP outlet?? If epump ahead of LP, why does flow not pass thru LP, which would increase pressure on diaphragm? Have you seen this with items when installed? I don't want to be obstinate but wont design replacement system until I get a handle on existing functions.
Wes I have not taken one apart but fairly sure OEM LP has both inlet and outlet check valves or would not be much of a pump. Given that, how does parallel epump push flow back into LP? Epump also has outlet check so unless we think they are leaky, why do we need check valve before or after LP, a stop valve before stops tank draining thru diaphram (which seems to not be a high probability event). Personally I will go with manual three way selecting epump or LP and blocking the other inlet. After I get mine starting/running well, will test better prime method suitable for back country or side of freeway.
Sorry, double post since it did not show until after this. Will wait longer in future.
Sorry, double post since it did not show until after this. Will wait longer in future.
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Ok, I will drop it after this. For last time, why does LP outlet and inlet check valves, assuming it has them, not prevent drainback to tank, or crankcase, in event of air leak downstream?? Adding check valve before LP, mainly because it is easier?, does not prevent tank draining thru LP diaphragm like a stop valve would. Has anyone had this happen?? Ok, I stopped typing and thought about. Pieces of showerhead work thru FSV and stick LP inlet and outlet valves open (but not the check valve we installed !!??). Not likely. Will change my system as planned.
The mech pump does not have check valves, when it’s new and the tolerances are tight, this style design pump has check valve like properties if it’s not on the pumping lobe of the cam shaft when shutoff, but if the pump gets some wear, or is shut off in the discharge position it’s free flowing.
If you are fired up to attack this common problem, why not install some small shut-off valves temporarily in the LP inlet line, and the return line going down the back of the engine. Those are really the only two places fuel can leave the engine correct? Even cheaper and more temporary would be to just squeeze the fuel lines shut with locking pliers. You could then perform some experiments, like taking a rubber return line off one of the injectors and see if it caused a starting problem with various configurations of the two lines shut off or open.
steve phillips
Registered User
I am not wanting to stir this pot, only to be constructive. with that said, this discussion centers around something I have been curious about for years,ie the line between fuel filter and return line cap. I know there is a check valve in filter head, my guess is most are missing or bad, my truck is now 25 years old. As I see it , assuming no other air leaks in filter head this is the point of air intrusion. This line is the connection between fuel supply and return . I know this is supposed to be a quick bleed for fuel filter. If you want to maintain that feature a simple valve setup could take care of that. opening valve when bleeding, closing during normal operation. If my memory is correct, the was a ford bulletin about putting a loop in this line to prevent air or siphoning of fuel. I have been searching for it for 3 hours can t find it. It s been along time since I worked on a 6.2 or 5.7 Gm diesel but I don t think they return fuel this way. I think vent on pump only goes to return lines. It is my sincere hope this adds something useful to the discussion.
Thanks Wes, only Ford would make a pump with no check valves. Will change design accordingly. When I started this reply posts by franklin2 and steve phillips above not visible. At last some discussion. Yes, I am fired up after stalling 5 miles from home today and needing AAA flatbed to get home. I plan on blocking hoses, blowing down hoses, and no stranger to vise grips and line clamps etc. to find the problems. Vent line from filter to injector cap removed due to breaking off injector return nipple while removing hose. Emergency fix of epoxy in return cap hole working fine so will leave it that way. Cranking flow seems low from LP so will measure it tomorrow. Starts on first cylinder again and fan rotates 18 sec after shutoff so no heavy load runs yet. If LP is bad will probably ad epump like Carter P4600 with PWM controller and manual 3 way to select replacement mech LP or Carter. Thanks all for any guidance regarding priming, leaks, epumps, etc. Later
Many have referenced a tech sheet that ford put out to eliminate that return hose off the filter, I haven’t seen it myself, but I always ditch that line.
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