How is fuel pressure maintained?

[danda]

Ok, I'm trying to understand how the fuel system works, especially the low pressure side.

So the pump draws from the tank, through the fuel filter, and into the IP.

The fuel filter housing, IP, and injectors are each connected to the return line. The return line returns back to the tank.

Somehow 3-5 PSI is maintained in the system, or so I've read.

Yet if there is no restriction in the return line, then how is the pressure maintained, eg between fuel filter and IP? In particular, as I understand it, the pressure should be maintained even with the engine off, when the low-pressure pump is not active.

Ok, I believe that the IP and injectors should require high pressures for fuel to get from the primary path into the return line. But what about the line between the fuel filter housing and the number #1 injector return cap? What prevents pressure from bleeding off there?

 

[Nero]

If I remember correctly there's a pressure relief valve right before the elbow of the fuel return fitting on the IP, that's how it maintains case pressure.

 

[The_Josh_Bear]

The pressure going into the filter is 5 psi on a good mechanical pump. From there some of the fuel(a tiny bit) is allowed to bypass to the return on the #1 injector return so that air may escape there if you are late switching tanks or whatever. It works well when the check valve is operating. The check ball type doesn't really wear out but the duck bill type will break eventually, then it lets the fuel siphon back to the tank when shut down.

From the main outlet to the IP it is drawn in by the impeller built into the IP where it is taken and pressurized and sent to the injectors.
The fuel exiting the IP is very important as it cools it down and must be under a very little back pressure, less than 2psi from reading I've done. I doubt very much you'll ever read even 1psi on the return though if it's working.

So there should be about zero back pressure in the return line and 5psi inlet to the IP. The filter will not maintain a pressurized state due to the return to #1 and even if that's sealed, the IP tolerances aren't tight enough to hold it. They aren't designed for that. Like your oil pump it's designed to move fluid when moving, but is not a perfect seal when stopped.

Hope that helps!

 

[franklin2]

On most fuel systems the lift pump has way more capacity than the fuel system needs. The fuel pump has a spring inside against the diaphragm that is calibrated for 5 psi. If there is less than 5 psi the spring pushes the diaphragm in the pump down and it pumps fuel in when the pump arm pushes back against the spring. When the system builds to 5 psi, there is 5 psi above the diaphragm in the pump because of the spring, and there is 5 psi below the diaphragm because of the fuel pressure in the system, so the diaphragm doesn't move and pump.

All the little bypasses and orifices in the system ahead of the pump do not add up to more than the pump can comfortably supply with some reserve, and still meet the demands of the engine.

 

[danda]

The filter will not maintain a pressurized state due to the return to #1 and even if that's sealed, the IP tolerances aren't tight enough to hold it.

Ok, good to know. So then if I open the schrader valve after the truck has been sitting 24 hours, I should not expect fuel to come squirting out, right? maybe some dribble, but not a pressurized squirt, as it would if it had held 5psi pressure.

 

[IDIBRONCO]

Somehow 3-5 PSI is maintained in the system, or so I've read.

This is not correct. The 3-5 PSI is only between the lift pump and the injector pump. The highest pressure is (naturally) between the injector pump and the injectors. There should be little to no pressure in the return system (ideally about .5-1 PSI). There should be negative pressure (AKA suction, in the case of a fluid) between the fuel tank and the lift pump.

 

[danda]

The 3-5 PSI is only between the lift pump and the injector pump

right. understood. so my question now, is what should happen when the engine is not running? Should that 3-5 PSI remain in the line between the 2 pumps, or it leaks out, eg within 24 hours?

I ask because I am suffering from hard starts, and in order to diagnose effectively I think I need to understand how it should be working. For example, if I depress (or measure) the schrader valve after 24 hours, would a correctly operating setup still have 3-5 psi pressure or no?

 

[Jesus Freak]

There's a check valve in the lift pump. If it's compromised the fuel will leak back to the tank. I doubt you'll have pressure but there should be fuel there. That's what happened to mine one time.

 

[franklin2]

right. understood. so my question now, is what should happen when the engine is not running? Should that 3-5 PSI remain in the line between the 2 pumps, or it leaks out, eg within 24 hours?

I ask because I am suffering from hard starts, and in order to diagnose effectively I think I need to understand how it should be working. For example, if I depress (or measure) the schrader valve after 24 hours, would a correctly operating setup still have 3-5 psi pressure or no?

Do you have a thread going on your starting problem? A description of how it acts when it starts (or doesn't start) can help us help you figure out the problem.

If you have a air/leaking problem on top of the engine, the engine will start normally and then stop. Then there is a bunch of cranking till the air is worked out and it starts.

If you have some other problem, the engine may not hit or try to start at all. That could be a glowplug problem or some other problem.

Not starting hot can be another problem.

 

[danda]

yeah I have a thread about it here:

Air intrusion on 1990 7.3L f350 with banks turbo

Ok guys, so I've got the classic hard start due to air intrusion. I bought this truck used in 2000 with 113k miles on it and have use it very infrequently, so mostly it has sat. A few years back it developed the hard start symptoms and I replaced the glow plugs and return lines, which did not...

www.oilburners.net

I haven't had time to mess with it the past few days, so figured I'd start this thread to maybe learn a little in the meantime.

I'm still wondering: if one of you guys with a correctly functioning fuel system presses the schrader valve after its been sitting 24+ hours, what happens? does fuel spurt out, or just a dribble, or what? ie, is pressure maintained or not? I think the answer will be that it is not maintained. how about immediately after shutdown, or 5 minutes after?

 

[Jesus Freak]

So I checked it. My truck has sat since yesterday afternoon and there's nothing as far as pressure at the valve. I cranked my truck up (it cranks almost immediately) let it run for a few minutes, shut it down. Waited a couple minutes, checked it for pressure and there was pretty much nothing.

 

[franklin2]

yeah I have a thread about it here:

Air intrusion on 1990 7.3L f350 with banks turbo

Ok guys, so I've got the classic hard start due to air intrusion. I bought this truck used in 2000 with 113k miles on it and have use it very infrequently, so mostly it has sat. A few years back it developed the hard start symptoms and I replaced the glow plugs and return lines, which did not...

www.oilburners.net

I haven't had time to mess with it the past few days, so figured I'd start this thread to maybe learn a little in the meantime.

I'm still wondering: if one of you guys with a correctly functioning fuel system presses the schrader valve after its been sitting 24+ hours, what happens? does fuel spurt out, or just a dribble, or what? ie, is pressure maintained or not? I think the answer will be that it is not maintained. how about immediately after shutdown, or 5 minutes after?

I read your other thread. You need to fix that leaking injector properly, whatever it takes. No washer or rigging, it should go together properly and not leak. You may need another set of eyes to look at it to see what is wrong with it.

 

[danda]

So I checked it.

awesome, thank-you! So at least I can eliminate "pressure at schrader after sitting" as any kind of diagnostic, and I think from the various answers I have a better understanding of how the fuel system works.

 

[danda]

You need to fix that leaking injector properly, whatever it takes

yeah, that's my priority next time I can work on it. will update the other thread after.

 

[The_Josh_Bear]

Anytime you suspect fuel not being ready to prime the engine just pull the fuel filter before cranking. It should be very close to full when everything is working. Classic fuel drainback will leave the filter half full. Knowing this will save a lot of wasted time and usually money.