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Unread 09-07-2011, 10:05 AM   #1
boardmaker
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More questions: fuel return

First, I want to thank those who have replied to my many questions. I am slowly working my way through the jeep and your comments have really helped me understand how things work. I have been thinking today about the fuel return line. Is this really needed? What are the ramifications of plugging this (or just going with a single output nipple filter) and then removing the return line?

Thanks.

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Unread 09-07-2011, 12:18 PM   #2
92 Green YJ
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yes it is needed. especially on an FI vehicle. This returns any unburned fuel back to your tank to be cycled thru the system again.
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Unread 09-07-2011, 01:20 PM   #3
boardmaker
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I don't think we are talking about the same line. This fuel line comes out of the filter. I am not sure how unburned fuel could ever get the this point.

It seems kind of wierd to me that there are two output nipples on the fuel filter. Why would you want to draw fuel from the tank, through the filter, and then send it back to the tank? I have to be missing something here.
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Unread 09-07-2011, 01:28 PM   #4
vadslram
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Because if your pump puts out more fuel than your engine can burn it has to go somewhere.
Why would you want to disconnect it?
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Unread 09-07-2011, 01:36 PM   #5
Que89YJ
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The return line is needed because pressure is maintain while volume is changed. The amount of fuel used at idle is much less then when you are rolling 70Mph down the freeway. The pressure put out by the pump is maintained between 31-39Psi even though you are using a much larger volume. The way thisd is done is by dumping the unused fuel back to the tank at lower use times like idle.
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Unread 09-07-2011, 01:50 PM   #6
Andywear
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Yeah, just plug it and let us know how it turns out.

While your at it remove the 3 extra lug nuts you have holding your wheel on.


J/K... just take off the 2 extra nuts
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Unread 09-07-2011, 02:00 PM   #7
dtn8tr
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Quote:
Originally Posted by Que89YJ View Post
The return line is needed because pressure is maintain while volume is changed. The amount of fuel used at idle is much less then when you are rolling 70Mph down the freeway. The pressure put out by the pump is maintained between 31-39Psi even though you are using a much larger volume. The way thisd is done is by dumping the unused fuel back to the tank at lower use times like idle.
Que...aren't you confusing that with the FI engines? The standard mechanical fuel pump on a 4.2l puts out 6-9 psi. The rest of the explanation makes sense.
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Unread 09-07-2011, 02:03 PM   #8
Que89YJ
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LOL yep I sure did! Doh! Never mind.
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Unread 09-07-2011, 02:08 PM   #9
notmyj
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With a mechanical fuel pump it will always be pumping fuel when the engine is running. Because it is a diaphram pump, you can't dead head it as it will pop the diaphram. When the bowl in the Carb fills up like when at idle, and the pump is supplying more fuel than the engine using, the needle and seat in the Carb will close thus not allowing any more fuel into the bowl of the Carb. Since the engine is still running, the pump is still pumping. There fore it has to go somewhere. So it puts it back to the filter and creating a loop for the fuel to travel so as not to dead head the pump and cause damage. The pump should have a pop off valve in it that regulates the pressure to 9 psi. Once that pressure is reached, the pop off valve opens up sends the fuel back to the filter.

Long story short, yes, you need it.
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Unread 09-07-2011, 04:29 PM   #10
Alabamaranger
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[QUOTE=notmyj;12137934]With a mechanical fuel pump it will always be pumping fuel when the engine is running. Because it is a diaphram pump, you can't dead head it as it will pop the diaphram. QUOTE]

It's been done for years - Ford, GM, Chrysler with carbs. One line into the fuel pump and one line out to the carb. No fuel return lines. When fuel pressure builds, it holds the diaphragm of the pump down. So at any rpm, if the carburetor doesn't need fuel, the back pressure holds the diaphragm down and the linkage just operates without pushing any fuel. On the down side, when in traffic and with the fuel just sitting there in the hot fuel pump and lines, it could vapor lock.

With the addition of a fuel return, the fuel is always moving so less likely for a vapor lock. I beleive the early CJ's did not have fuel return lines.
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Unread 09-07-2011, 09:06 PM   #11
boardmaker
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Thanks guys. I learned a lot on this one.
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Unread 09-07-2011, 09:49 PM   #12
Old4X
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Alabamaranger has the correct answer. The fuel line, filter and carb sit right on top of the exhaust manifold. The constand flow of fuel cools the fuel line and filter. Often the carb boils the fuel out when shutting down a hot engine, but the line still has fuel, so re-starts aren't too difficult.

The 3 line fuel filter also acts like a fuel pressure regulator, reducing and stabilizing fuel pressure to the needle and seat in the carb.
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