How does the fuel return work?
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Cross-Country
Joined: Jul 2004
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How does the fuel return work?
Hey folks, this seems like a really basic question, but (unfortunately for me) most of what I know about diesels is with regard to small marine engines (Yanmar and others), and I now believe they work a bit differently.
I know that air in the fuel lines going to the engine will stop the engine, and I know about bleeding to fix it.
What I am used to is that air in the fuel return line from the high pressure pump back to the tank doesn't matter at all. (In fact, my boat has the fuel return plumbed in with the tank vent!)
Looking at how the fuel return lines go by all the injectors on my engine, I think I've found a different paradigm here--If the fuel return lines are an open hose going toward the tank, it seems that air could get in to the injectors, where it causes problems.
Did this just take me a year or two longer to realize than it should, or am I still wrong?
Thanks.
I know that air in the fuel lines going to the engine will stop the engine, and I know about bleeding to fix it.
What I am used to is that air in the fuel return line from the high pressure pump back to the tank doesn't matter at all. (In fact, my boat has the fuel return plumbed in with the tank vent!)
Looking at how the fuel return lines go by all the injectors on my engine, I think I've found a different paradigm here--If the fuel return lines are an open hose going toward the tank, it seems that air could get in to the injectors, where it causes problems.
Did this just take me a year or two longer to realize than it should, or am I still wrong?
Thanks.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
You are partially right, and sort of wrong.
What happens is the return line is also tied into the fuel filter, which is much higher than the fuel tanks.
What happens is when air gets into the return lines, it breaks the vacuum that hold the fuel up in the fuel filter.
This causes the fuel to drain back to the tank.
Now the fuel filter is not full, when you start the engine it runs fine for a few seconds.
The the IP gets the air that was in the filter, the engine quits and is very hard to start.
The fuel return line goes down in the tank a good way.
Normally you do not get air in the return lines from the fuel tank.
But any tiny hole, even one so small that it will not leak fuel out, lets air into the system.
What happens is the return line is also tied into the fuel filter, which is much higher than the fuel tanks.
What happens is when air gets into the return lines, it breaks the vacuum that hold the fuel up in the fuel filter.
This causes the fuel to drain back to the tank.
Now the fuel filter is not full, when you start the engine it runs fine for a few seconds.
The the IP gets the air that was in the filter, the engine quits and is very hard to start.
The fuel return line goes down in the tank a good way.
Normally you do not get air in the return lines from the fuel tank.
But any tiny hole, even one so small that it will not leak fuel out, lets air into the system.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
The ones in my tanks go almost to the bottom of the tank.
As far as starting, my guess is you have a very small air intrusion problem somewhere.
The lower the fuel is in the tanks, the more vacuum there is trying to keep the fuel up in the filter.
Parking with the rear of the truck downhill increases the vertical distance from the fuel level to the top of the filter.
Parking with the front downhill reduces the distance.
The greater the vertical lift is, the harder the lift pump has to work to supply fuel to the engine.
Right after I finished typing this I happened to think, you may have a hair line crack in the suction line in the tank.
Lots of fuel in the tank and the crack is below the surface.
Tank low on fuel and the crack is above the fuel level.
I have found that on steep hills, my truck will start sucking air going downhill when the tanks are low.
But I am talking about a 30% slope hill, which is the street beside my house.
Going downhill with 1/4 tank in either tank, it starts to die.
But if I keep it running, I can drive right back up with no trouble
As far as starting, my guess is you have a very small air intrusion problem somewhere.
The lower the fuel is in the tanks, the more vacuum there is trying to keep the fuel up in the filter.
Parking with the rear of the truck downhill increases the vertical distance from the fuel level to the top of the filter.
Parking with the front downhill reduces the distance.
The greater the vertical lift is, the harder the lift pump has to work to supply fuel to the engine.
Right after I finished typing this I happened to think, you may have a hair line crack in the suction line in the tank.
Lots of fuel in the tank and the crack is below the surface.
Tank low on fuel and the crack is above the fuel level.
I have found that on steep hills, my truck will start sucking air going downhill when the tanks are low.
But I am talking about a 30% slope hill, which is the street beside my house.
Going downhill with 1/4 tank in either tank, it starts to die.
But if I keep it running, I can drive right back up with no trouble
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Cross-Country
Joined: Jul 2004
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Alive again!
Originally Posted by Dave Sponaugle
You are partially right, and sort of wrong.
What happens is the return line is also tied into the fuel filter, which is much higher than the fuel tanks.
What happens is when air gets into the return lines, it breaks the vacuum that hold the fuel up in the fuel filter.
This causes the fuel to drain back to the tank.
Now the fuel filter is not full, when you start the engine it runs fine for a few seconds.
The the IP gets the air that was in the filter, the engine quits and is very hard to start.
The fuel return line goes down in the tank a good way.
Normally you do not get air in the return lines from the fuel tank.
But any tiny hole, even one so small that it will not leak fuel out, lets air into the system.
What happens is the return line is also tied into the fuel filter, which is much higher than the fuel tanks.
What happens is when air gets into the return lines, it breaks the vacuum that hold the fuel up in the fuel filter.
This causes the fuel to drain back to the tank.
Now the fuel filter is not full, when you start the engine it runs fine for a few seconds.
The the IP gets the air that was in the filter, the engine quits and is very hard to start.
The fuel return line goes down in the tank a good way.
Normally you do not get air in the return lines from the fuel tank.
But any tiny hole, even one so small that it will not leak fuel out, lets air into the system.
Yup, that describes exactly what happened to me (when I started having problems). I have now figured out pretty much what happened to me:
- I decided to do the "right" thing and replaced the fuel filter (and tried to drain water out of it) on my 1990 van after I had owned it a year or so.
- At that time (although I didn't know it) I probably created a small air leak at the filter.
- I decided to replace the old fuel return hoses. They WERE old and stiff, but probably still were OK.
- When I did this, the O-Rings (also 18 years old) were disturbed, so they started leaking--diesel squirting out around all 16 of them.
- I replaced the O-Rings as well, and the leaking stopped, and the engine turned over.
- Next I let the van sit and see if it holds prime overnight or not.
So far, I'm proving a lesson about old vehicles: If it isn't broke, don't touch it. Cause if you do, it will break, then you'll have to replace it for sure. (At least I didn't have to pull an injector!) It seems that every time I mention this discovery to somebody, they aren't very surpised
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Joined: Oct 2006
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Thanks. I think the crack theory is right. I thought the pickup tube just fell off, but now believe it is just cracked. while driving It runs out at about 1/4 tank, and with less than 1/3 tank it sometimes loses fuel from the filter while parked (but only when facing uphill). Maybe someday I'll have time drop the tanks...
To put things in perspective, here in Silicon Valley is probably flatter than most lakes in WV, so "uphill" means "pointing away from a storm drain" in a parking lot.
To put things in perspective, here in Silicon Valley is probably flatter than most lakes in WV, so "uphill" means "pointing away from a storm drain" in a parking lot.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
I have been to Silicon Valley.
We have parking lots steeper than the interstate going up to Donner Pass here.
One time I was in Reno headed to San Jose.
Donner Pass was closed because of snow and some local truckers were going around the pass on the old road.
So I asked if I could follow them.
Sure they said, they were both running flatbeds, so I asked no low clearances I have a 13'6" reefer.
No problem with low clearances so off we go with me in the middle.
Not sure about the road anymore, but I think we went down through Placerville.
That road reminded me of what we have here.
Steep grades, sharp corners and no bottom in sight on the outside corners.
The guy in back hollers on the radio I have a trailer problem, it is leaning to much going around the corners.
Nah, thats what they do when you haul swinging meat.
Then the lead truck freaked out, am I running to fast?
Doing just fine and I am behind schedule, let's go was all I could say.
It was an interesting trip, I wish it had been daylight because I bet there was some great scenery I missed that night.
I did get to see a lot of the Valley the next day though running to make pickups for the return trip.
We have parking lots steeper than the interstate going up to Donner Pass here.
One time I was in Reno headed to San Jose.
Donner Pass was closed because of snow and some local truckers were going around the pass on the old road.
So I asked if I could follow them.
Sure they said, they were both running flatbeds, so I asked no low clearances I have a 13'6" reefer.
No problem with low clearances so off we go with me in the middle.
Not sure about the road anymore, but I think we went down through Placerville.
That road reminded me of what we have here.
Steep grades, sharp corners and no bottom in sight on the outside corners.
The guy in back hollers on the radio I have a trailer problem, it is leaning to much going around the corners.
Nah, thats what they do when you haul swinging meat.
Then the lead truck freaked out, am I running to fast?
Doing just fine and I am behind schedule, let's go was all I could say.
It was an interesting trip, I wish it had been daylight because I bet there was some great scenery I missed that night.
I did get to see a lot of the Valley the next day though running to make pickups for the return trip.
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Hotshot
Joined: Feb 2004
Posts: 11,697
Likes: 3
From: Marengo, Iowa
Originally Posted by tbone91
Thanks. I think the crack theory is right. I thought the pickup tube just fell off, but now believe it is just cracked. while driving It runs out at about 1/4 tank, and with less than 1/3 tank it sometimes loses fuel from the filter while parked (but only when facing uphill). Maybe someday I'll have time drop the tanks...
To put things in perspective, here in Silicon Valley is probably flatter than most lakes in WV, so "uphill" means "pointing away from a storm drain" in a parking lot.
To put things in perspective, here in Silicon Valley is probably flatter than most lakes in WV, so "uphill" means "pointing away from a storm drain" in a parking lot.
On the return line on the sender, there is a little rubber cap that opens under pressure, but closed with none. If this cap fails, it can let the fuel drain down as well, because it will let air seep ever so slowly back up the line. I haven't seen many of these caps fail, but it is possible. If you get an air leak at the motor, it will make enough pressure on this cap to let the fuel leak back to the tank. The syphon effect then will pull it out of the filter as well. You can get at the front sender without dropping the tank, but the exhaust or the driveshaft have to be moved out of the way to do so. The rear isn't so easy.
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