Possible air intrusion when parked uphill?

[Grantly222]

Hi
Longtime lurker first time poster. I have an 86 f250 6.9 with 92,000 original miles. When I park it uphill and leave it for a few days it seems to lose prime. Sputters and then no fire. Cranks and cranks until I open the shraeder valve and get some fuel in the line again. What’s weird is no problem if parked down hill. No other surging or rough running. Fuel selection valve maybe? Has anyone experienced this? Thanks all!

 

[Cubey]

My RV is doing the same thing, despite an electric fuel pump (which should help purge the air), a new fuel selector valve 6 months ago, newer o-rings 2.5 years ago, with no visible leaks anywhere. I'm stumped, in my case.

 

[Grantly222]

I’m hoping someone has chased this down before. I’m also wondering if it could be the shraeder valve …

 

[Cubey]

I’m hoping someone has chased this down before. I’m also wondering if it could be the shraeder valve …

I keep a valve cap on mine so I doubt it's that in my case. My electric fuel pump would instantly fix that even of it was.

 

[Selahdoor]

You have air getting in, somewhere.

It's probably not big enough to leak fuel, but it is letting air in.

Then your entire system is leaking down, back into the tank. From both the supply and return lines.

If you park nose downhill, the lines stay full, because as I said, the leak is not big enough to leak fuel.

First thing you can do is to put a check valve in the fuel line, just before the light pump.

You are going to have to eventually find the leak, or go on dealing with the problem.

Although I have to say, I was having the same problem, and couldn't find the leak.

Even though I installed clear hoses up top to see if I could catch any bubbles.

I put in an electric lift pump to augment the mechanical lift pump. And I put a check valve inline with each pump.

I haven't had the problem since.

The electric pump is wired to come on with the ignition switch being in the run or start positions. So as soon as I turn on the switch and am waiting for the glow plugs, that pump is already pumping. Which makes up for any small losses there might have been.

 

[Cubey]

The electric pump is wired to come on with the ignition switch being in the run or start positions. So as soon as I turn on the switch and am waiting for the glow plugs, that pump is already pumping. Which makes up for any small losses there might have been.

And yet I have the mech pump deleted and an epump and it still does it to me. So I'm guessing it's in the return system, not the supply? The pump isn't dead since fuel comes out the valve when the pump is running without the engine running. One would think it would refill the return lines via the filter to return line connection. It's hard to fix something when you don't know where the problem is.

 

[Old Goat]

Do you still have the factory water filter on the Firewall on the drivers side next to the brake Booster?
I have read it can be a source of an air leak.
When I bought my 86, it was still there but the 2 rubber hoses were connected or looped together to bypass it. I later replace these hoses and they were cracked especially on the ends where they were clamped onto the steel lines.

Have you ever changed your Injector Return lines? How old are they?
You could have an "O" Ring not making a good seal and cause your fuel to leak back to the tank.
You can either buy a return line kit, or
buy a bag of 50 "O" Rings from Amazon or McMaster-Carr for around $7. Viton size 111,
and change them out.

Pull the caps off as a unit, remove the old "O" Rings, make sure the grooves on the injector is clean. grease up the new ring and slip it on the first groove. Grease the 2nd one and roll over the first ring. Once all are on, and greased up snap the caps on as an assembly.

There are 3 hard line fuel seals refered to as "Olives. One is on the hard line between the Spin on Fuel Filter and the IP. and the other 2 I believe are where the return lines come together at the rear of the engine. I understand they are a PITA to replace.
I had a leak where the Hard line connects to the IP, Mine was dripping but suppose it could suck air?
I replaced it with push lock hose. Russ Repair has the Olives and this kit also.

Fuel system components - Page 1 - Russ Repair

russrepair.com

Goat.

 

[gandalf]

I’m hoping someone has chased this down before. I’m also wondering if it could be the shraeder valve …

That's always a possibility, and easy and cheap to repair. The center section of the schraeder valve screws (s c r e w s) in and out. You need the right valve cap to do this. The schraeder valve is just like the tire air valve on a bicycle. I'd go to a bicycle shop for the proper valve cap and replacement valve center section.

 

[Grantly222]

Thanks Goat
The water separator is bypassed.
I’m not sure on the other things you mention, I do know the IP was serviced / replaced in about 2002 but the truck saw very few miles between then and now. Still, that’s a long time ago. Thanks again for the reply.

 

[franklin2]

This is the way to think about the complete fuel system on top of the engine.

Ever take a straw and stick it in your drink, put your thumb over the end of the straw and pull the straw out and the drink stays in the straw until you move your thumb? That is exactly how this fuel system works. The pump pulls fuel from the tank, pushes it through the filter, some of it goes back down the return and some of it is used by the injection pump. The return on back of the engine is just a open 5/16 line going downhill back to the tank.

When you shut the truck off, the complete fuel system is supposed to be tightly sealed on the engine. So the fuel is actually hanging in the return line going down to the tank. But any tiny little air leak will let air in and let the fuel slowly drain back to the tank. Parking the nose downhill makes it harder for the fuel to make it back to the tank, so that covers the problem up.

An electric fuel pump will not fix this problem. the electric pump will make it easier to charge the system back up. But you will have to turn the pump on and wait a while, till the pump pushes all that air out of the system and refills it with fuel. Some of the return fittings are not very large, so this can take some time. You might want to turn the pump on and wait 5 minutes and see if that helps it.

So far I have been able to find and fix my leaks. It can sit for months and not leak down. 1st round was the o-rings in the return caps around the injectors. Any slight dampness with dirt sticking to it is a sign of a very very small fuel leak, which can be a very large air leak.

2nd round was a very small leak around the fuel heater.

 

[Cubey]

An electric fuel pump will not fix this problem. the electric pump will make it easier to charge the system back up. But you will have to turn the pump on and wait a while, till the pump pushes all that air out of the system and refills it with fuel. Some of the return fittings are not very large, so this can take some time. You might want to turn the pump on and wait 5 minutes and see if that helps it.

So far I have been able to find and fix my leaks. It can sit for months and not leak down.

My RV was keeping it's prime when parked level for months last summer. During winter in Arizona (low elevation) it started giving the symptom on fairly level ground.

I guess I'll try the fuel pump thing next time it won't start on the first try. I can set the key to accessory (turned backwards from off/lock) to turn on the fuel pump for a few minutes to see if that purges the air without the glow plugs firing.

 

[Black dawg]

If the injector return lines havent been done, probably a good place to start. I have had results using a stethoscope with an open ended hose to find air leaks.

 

[rempfer]

This is the way to think about the complete fuel system on top of the engine.

Ever take a straw and stick it in your drink, put your thumb over the end of the straw and pull the straw out and the drink stays in the straw until you move your thumb? That is exactly how this fuel system works. The pump pulls fuel from the tank, pushes it through the filter, some of it goes back down the return and some of it is used by the injection pump. The return on back of the engine is just a open 5/16 line going downhill back to the tank.

When you shut the truck off, the complete fuel system is supposed to be tightly sealed on the engine. So the fuel is actually hanging in the return line going down to the tank. But any tiny little air leak will let air in and let the fuel slowly drain back to the tank. Parking the nose downhill makes it harder for the fuel to make it back to the tank, so that covers the problem up.

An electric fuel pump will not fix this problem. the electric pump will make it easier to charge the system back up. But you will have to turn the pump on and wait a while, till the pump pushes all that air out of the system and refills it with fuel. Some of the return fittings are not very large, so this can take some time. You might want to turn the pump on and wait 5 minutes and see if that helps it.

So far I have been able to find and fix my leaks. It can sit for months and not leak down. 1st round was the o-rings in the return caps around the injectors. Any slight dampness with dirt sticking to it is a sign of a very very small fuel leak, which can be a very large air leak.

2nd round was a very small leak around the fuel heater.

Three questions if you please:

1. So if I understand correctly, the lift pump normally supplies more fuel than the injection pump requires and that excess fuel simply flows back to the tank through a return line. One would think an alternative to this constant circulation of the excess/unconsumed fuel back to the tank would have been to incorporate a pressure-regulated bypass in the lift pump itself, such that only the quantity of fuel demanded by the injection pump would flow from the lift pump to the injection pump. Could you comment on this?

2. Why are there return lines on the injectors at all? Does the injection pump not meter the quantity of fuel delivered to the injector? Does it rely only on pressure regulation, always delivering more fuel than will actually be required at each power stroke? If so, then is unconsumed fuel continually flowing out of each injector requiring capture and return to the tank as with the lift pump (#1) situation above? Again, please comment.

3. If my understanding is reasonably accurate, I've seen posts offering modifications to eliminate the return line(s), stating something to the effect that doing the mod "solves the air intrusion problem completely." How could eliminating the return lines possibly work?

Thank you in advance.

 

[Cubey]

The injection pump does meter it's fuel through a metering valve and sends the excess into the return line system.

Regarding the return lines on the injectors, being that the injectors are lubricated and pressure popped with diesel instead of oil, it's probably better to have too much fuel going to them than not enough, so the excess need to go somewhere... aka the return lines. Having too much fuel without anywhere for it to go would probably not be a good thing for the injectors, the steel high pressure lines, and the seals in the IP.

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[1987695spd]

Hi
Longtime lurker first time poster. I have an 86 f250 6.9 with 92,000 original miles. When I park it uphill and leave it for a few days it seems to lose prime. Sputters and then no fire. Cranks and cranks until I open the shraeder valve and get some fuel in the line again. What’s weird is no problem if parked down hill. No other surging or rough running. Fuel selection valve maybe? Has anyone experienced this? Thanks all!

In my case it was the water separator, when pointed uphill air would seep in through drain, I'm guessing that when parked down hill any leakage would result in fuel being siphoned from the tank in stead of air from the drain valve. Never figured it out until I noticed fuel spot where I parked. Leak got worse replaced water separator, the problem went away. The leak was not noticable the last day.