I don't understand the physics

[jdb]

92 7.3 IDI non-turbo with fuel filter to #1 cylinder cap delete.

I have read about, and experienced, very rough engine running when the return lines are leaking.

I don't understand the physics of this. High pressure fuel is fed to the injectors to run the engine. Low pressure surplus returns to the tank. With the fuel filter to #1 cylinder delete, there is no feedback.

How does air intrusion in the low pressure return lines going back to the tank affect how the engine runs?

 

[IDIoit]

it does not. it only effects how the engine starts.
once the air is worked out, it will run fine, but continue to leak.

 

[jdb]

it does not. it only effects how the engine starts.
once the air is worked out, it will run fine, but continue to leak.

Yes, it's true that it is only when it starts (lots of smoke, bunny hopping). But I still don't understand how air in the return lines makes this happen. What am I missing?

 

[IDIoit]

think of it as a non automotive water pump in a 5 gallon bucket.
the bucket is full of water. and the pump I pumping water outside of said bucket.
what happens when the water level gets below the pick up?
not a steady stream of water.

well in our case, its fuel.
when air gets introduced into the system, we do not have the required amount of fuel.

 

[jdb]

I apologize for being dense, but I still don't get it. The supply is drawn from the bottom of the tank ... no air there (I have plenty of fuel in the tank). My fuel system diagram shows the return lines going back to the tank, so presumably any air in those return lines just gets added to the air that is above the fuel in the tank. So what am I missing?

 

[needlenose]

I think the reason why a leak only affects start up is that once the system is inactive, the "weight" of the remaining fuel in the return line will pull air into the system through the leak as it continues to slowly flow back into the tank. Once pressurized, these air bubbles jack with the injector timing making it hard to start.

If there are no leaks, any fuel movement out the return will simply pull more fuel from the tank like a siphon.

At least, that's the way I understand it.

 

[jaluhn83]

A return line leak should not cause rough running except for starting. The return line leaking air allows the fuel retained in the system to drain back (same idea as an air leak in a siphon making it not work). When you start it then there is a certain amount of air that has to be purged from the system.

If you have rough running continually or after about 1-2 min of running when starting it's likely not the return lines, but something else. A leak on the suction side of the fuel system could cause this problem.

 

[FordGuy100]

Fluids such as water, and diesel fuel (among many, many others) can be modeled as in-compressible liquids. Now this definition isn't exactly true, as they do compress, very slightly. When everything is full of diesel fuel (pump, injectors, return caps), when the injection pump compresses the liquid, it only very slightly changes the total volume of fuel. Because of this, what the IP pushes into the lines, must come out of the lines, volume wise (with some losses).

Now, you have return lines full of air, air in the system. The IP now pushes fuel, but since their is air, which is compressible, all the air does is change volume. Unlike liquids, you can compress air and have large changes in volume (think air compressor). The IP is trying to push fuel, but it can't, because there is all this darn air in the system and the IP/fuel its pushing can't compress the air in the system enough to have the system purge itself. The little bit of fuel being pushed only slightly changes the lines air pressure, not enough to actually pop the injector.

Of course, it does, and then it starts on a few cylinders. Then it finally pushes all that air out and fills the lines with fuel.

You have a water bottle filled with half water, half air. Another is filled completely with air. Now, with the same amount of force, squeeze each. What do you observe? The half empty bottle is easy to squeeze in comparison, but for hydraulics and fluid flow conditions like we are talking about with the fuel system, that just means its hard for the fuel to push the air out of the system. Whereas the bottle filled with all water, when you squeeze the sides, it will actually expand the dimensions of the bottle in other directions because of the fluid force


Its scary to think about how much money I've spent to have to learn that

 

[jwalterus]

You have a water bottle filled with half water, half air. Another is filled completely with air.......... Whereas the bottle filled with all water...........


Its scary to think about how much money I've spent to have to learn that

You need a refund......

 

[Shadetreemechanic]

Yes, it's true that it is only when it starts (lots of smoke, bunny hopping). But I still don't understand how air in the return lines makes this happen. What am I missing?

I think you may be missing that air in the return lines migrates to the pump, so you have air in the injection pump as well.
That air in the pump, does all these things that others described well.

 

[laserjock]

It also drains the supply fuel back to the tank over time so you have to reprime everything. Depends on the severity of the leak and duration it sits as to how far the system bleeds down. Gravity is a cruel and heartless *****.

 

[jdb]

Thanks for everyone's input on this. This all is starting to makes sense to me, and FordGuy100's description of compressible fluids is very useful.

However:
1) is it true that a leak in the return line at an injector allows the hard (high pressure) line to the injector to drain down, which it seems to me would only occur if it could leak through the injector itself; and
2) because the engine starts fairly easily but runs rough, it would seem that only one or two injectors are not firing properly, so this leak would only drain down the hard line at one or two injectors and not the neighbouring ones?

If as shadetreemechanic says, "air in the return lines migrates to the pump, so you have air in the injection pump as well. That air in the pump, does all these things that others described well", would that not result in difficulty starting (since air would be fed to every injector, not just one or two)?

 

[OLDBULL8]

No it's not.

1) is it true that a leak in the return line at an injector allows the hard (high pressure) line to the injector to drain down, which it seems to me would only occur if it could leak through the injector itself; and

It is highly unlikely that a air leak in the return line at any injector would cause the hard line fuel to drain down, any air leak at an injector would cause the fuel in the return line to drain back to tank, it would have no effect on the hard line. There is a check valve in the return line at the top of the IP. If you have eliminated the return line from filter to return line, you could drain the whole return system, but not the filter to IP line.

2) because the engine starts fairly easily but runs rough, it would seem that only one or two injectors are not firing properly, so this leak would only drain down the hard line at one or two injectors and not the neighbouring ones?

Your trying to interject two different things. Defective injectors and air intrusion. Defective injectors would make the engine run rough, after a start and still continue to run that way, even after being warmed up. Air intrusion, would make it hard to start, depending on the amount of air intrusion, it would have to be on the suction side, the engine would eventually smooth out and run OK.

The physics is: Air always goes to the highest point.

 

[jdb]

My observation is that the truck smokes badly (smells like diesel) and runs poorly on start, but runs ok (although still smells a bit like diesel) when warmed up.

I have eliminated all visible fuel leaks in the return lines and caps.

I put clear hose between the lift pump and the fuel filter. No air entrained.

I have clear hose on the return line to IP connection. No air entrained (see photo).

You must be registered for see images attach

I believe that this line leads to the IP check valve that you show.

Does this mean the problem is with the injectors themselves and not the return lines?

 

[pelky350]

If no air is in the system is it possible your cold timing advance isn't working causing it to start kinda hard/rough or possibly your overall timing is a bit retarded causing it to smoke and smell like diesel while running?