Sucking air/Losing prime

[PROFG]

couple sentences lost here but probably not instructive and forgot anyhow

Tire pump to filter vent, massive leak from bottom oring @ 7-8psi (same as 15"vacuum). Removed bottom of fuel filter to inspect, no dirt or water, oring fine, reinstalled. Tested LP flow into water bottle 10sec front tank then 10 sec rear, less than 1/2cup. NG. Tried air pressure in tank prime (w return blocked), no joy. Tried vacuum at filter using large 4 wheel vacuum pump (Ram 1500) and vacuum brake bleeder , return still blocked, worked fine, solid fuel, no start. Tried vacuum IP return port, some bubbles very little fuel, no start (apparently can't pull fuel thru IP unless cranking). While recharging will find strap wrench and get one piece fuel filter and reprime. Was starting on first/second cyl this AM before I started 97595709 with it but now I know something that works and something that does not. Later

 

[franklin2]

We already know the simple formula that the factory used, totally sealed system on top of the engine, and the fuel will hang there and not drain back. But I don't think I have ever heard or seen anyone have a fool proof solution to where if you had leaks, what would cover that problem up and let the engine still start.

Some people have used electric pumps to try and pre-fill the system after it leaks down. This from what I gather helps, but there is still some cranking involved to get all the air out so it will start.

My idea which I have never tried, was to put a electric solenoid shut-off valve in the return line leaving the back of the engine. That along with one of those add-on checks on the input to the pump should block all pathways for the fuel to leave the engine. Theoretically you could pull a line off the return system and leave it open, and it would still start the next day wouldn't it?

But I am lazy and every few years I might have a minor leak that needs repairing, so I do that and that fixes the problem.

 

[steve phillips]

that jogs my memory. many years ago I saw an idi with an owner installed check valve at the connection where the return lines and the line back to tank meet. owner said it made truck start easier. I didn t understand why at the time, but I know now. PROFG use of vacuum pump at filter head is something I have used before. I can t remember who showed me their mod to fuel system , or I would see if they still had truck or reasoning for using it. from my thinking that would hold fuel in return lines, period and stop introduction of air. return lines will go into vacuum condition of some sort when truck is shut down. there is still atmospheric pressure acting on fuel in tank. If return line is below fuel.

 

[steve phillips]

http://www.nickpisca.com/diesel/fue...rd-van-fuel-tank-sender-replacement-tutorial/ red device on line is it check valve for return lines

 

[chris142]

Update: truck sat 36 hours. Started right up this morning and ran fine. Ran @ lunch and when I went home. All I can figure is that it sucked air and it took a few days to get out? Odd because I have ran out of fuel,switched tanks and it was fine. Never has it acted this weird after sucking some air.

 

[steve phillips]

glad you got it fixed

 

[PROFG]

I removed it when I broke the injector cap nipple it went to. Two part epoxy and wait an hour and no leak.

Posted right after page update and post @ "ditching filter return line"
Thought @ why pull Schrader to test LP volume and remembered maybe orifice in fitting. Just pulled it and tiny little hole so will retest LP flow. Maybe ok.

 

[PROFG]

Replaced Schrader with nylon 1/8 NPT to 1/4 barb, disconnect IP plug, crank 10 sec into water bottle. Meets line measured 1/4 cup + 1/3 cup water (9 fl oz~1/3Pint). This was 2X what I got AFTER the filter so need to change filter, not pump Later

 

[Thewespaul]

Post filter should put out 1 pint per 30 seconds cranking to meet the minimum spec.

 

[PROFG]

I was looking for 1/3 pint in 10 seconds to reduce cranking and it just meets minimum BEFORE filter so pump marginal (ok for now).

edit: Solenoid at return T prevents starting if it fails closed, maybe manual stop more reliable? Check valve would only help if opening pressure similar or greater than suction from tank/return lines but worth a try for a "leaker".
Update: This check valve, for example, has minimum open diff of 3psi to lift, greater with flow. This reduces lift pump pressure and retards timing so should be accounted for in LP selection. But valve not sucked open by return/tank until leak more than 92" above tank fuel level (3psi*27.7 "/psi H2O/.9 diesel SP). Not a problem for us.

http://www.siliconeintakes.com/liqu...jp8SiZWky_MQIog99tvN54j405n9CIKsaAsbqEALw_wcB

 

[PROFG]

Starting on first compression again but CMIIW (correct me if I'm wrong), opening Schrader while running should either leak fuel or stall engine, right? Did neither, how long should I wait?

edit acronym (I don't like uncommon acronyms used w/o clarification 1st use)

 

[Selahdoor]

It's impossible that our mechanical fuel pump does not have some sort of check valves.

Without any check valves, the diaphragm would simply push and pull on both lines every time it flexed.

You HAVE to have some method of allowing the fuel to flow only in one direction, or you don't have a fuel pump!

Now, you could try to claim it doesn't have check valves because what is in it is more like the valves in the head of an air compressor. But they are still check valves. They still only allow the flow of fuel in one direction.

And without them, the only thing the fuel pump would be doing is creating froth and/or pulses in the "free flow" of fuel that is being drawn by the IP.

The way the fuel pump works is the diaphragm pulls the fuel through the first check valve, when it flexes up. It CAN'T pull fuel through the second. Then when it flexes down, it pushes the fuel through the second check valve. It CAN'T push fuel through the first.

If you put an electric pump AFTER the mechanical pump, it CAN'T push fuel back into the mechanical pump, because there is a check valve in the fuel pump.

If you put an electric pump after the mechanical pump, and it is pumping fuel back into the mechanical pump, the check valve in the mechanical pump has catastrophically failed.

Check valves are not passive-aggresive. They let fuel go, or they don't. Period. Black and white. One way only.,

See, the check valve physically closes when pressure is put on one side. Put a tiny amount of pressure on one side, the fuel flows easily. Put all the pressure you want, on the other side, and absolutely no fuel should get through. If fuel gets past that, the check valve is leaking, and it's time for a new fuel pump.



Free flow is a pipe.

ANY check valve will 'freely flow' the fuel one direction. But I wouldn't call it "free flow". The flow is intended to go only one direction.

 

[Thewespaul]

It's impossible that our mechanical fuel pump does not have some sort of check valves.

Without any check valves, the diaphragm would simply push and pull on both lines every time it flexed.

You HAVE to have some method of allowing the fuel to flow only in one direction, or you don't have a fuel pump!

Now, you could try to claim it doesn't have check valves because what is in it is more like the valves in the head of an air compressor. But they are still check valves. They still only allow the flow of fuel in one direction.

And without them, the only thing the fuel pump would be doing is creating froth and/or pulses in the "free flow" of fuel that is being drawn by the IP.

The way the fuel pump works is the diaphragm pulls the fuel through the first check valve, when it flexes up. It CAN'T pull fuel through the second. Then when it flexes down, it pushes the fuel through the second check valve. It CAN'T push fuel through the first.

If you put an electric pump AFTER the mechanical pump, it CAN'T push fuel back into the mechanical pump, because there is a check valve in the fuel pump.

If you put an electric pump after the mechanical pump, and it is pumping fuel back into the mechanical pump, the check valve in the mechanical pump has catastrophically failed.

Check valves are not passive-aggresive. They let fuel go, or they don't. Period. Black and white. One way only.,

See, the check valve physically closes when pressure is put on one side. Put a tiny amount of pressure on one side, the fuel flows easily. Put all the pressure you want, on the other side, and absolutely no fuel should get through. If fuel gets past that, the check valve is leaking, and it's time for a new fuel pump.



Free flow is a pipe.

ANY check valve will 'freely flow' the fuel one direction. But I wouldn't call it "free flow". The flow is intended to go only one direction.

I’m sorry but you’re wrong here, take your fuel pump off, and blow through each end. There’s no check valve.

 

[PROFG]

If I replace mine will tear it down to see insides. Seems like would only work if a check valve was in line either before or after LP (closer than IP return).

 

[Thewespaul]

It works the same as many other diaphragm pumps, inlet is uncovered on the intake stroke, and output is uncovered during the exhaust stroke. Please take one apart and post pictures so we can put this to rest