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post #1 of (permalink) Old 01-06-2001, 08:52 PM
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Fuel filter

I was looking at my fuel filter on my 88 YJ and noticed that it had 3 lines connecting to it. One was the supply from the fuel pump, another went to the carb and then there was one on the side of it that went back towards the gas tank. I want to change the filter, but wanted to use a clear one. Do I need that third line comeing out of the side?

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post #2 of (permalink) Old 01-06-2001, 09:21 PM
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Re: Fuel filter

yes.. what that is.. is an overflow line.. basically.. when you get enough gas to fill the carb.. then the rest goes back to the gas tank.. keeps it from "bogging" out.. i guess would be the untechnical term used.. im no expert.. but i never ran this line with a v-8... and have never looked into.. or thought of why.. i just did it.. have too much to think about.. like cutting down my d60 rear 3".. whoo hooo almost done..


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post #3 of (permalink) Old 01-06-2001, 09:39 PM
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Re: Fuel filter

I have a CJ so this may not even be at all like your's so if it's not you can just disregaurd what I am saying but if it is than here you go. Some vehicles run and idle better with the return line. Some will vapor lock without it. I didn't use to have one and when I put a new fuel pump on it would put so much pressure on the needle and seat in the carb that it would flood. I would recomend leaving it since it is already there. I too wanted the clear filter so I purchased a smaller filter with the return line that goes in the place of the large regular one. Then I took the pipe cutters to the fuel line were it runs accross the valve cover right before it turns up to go to the factory fuel filter. I cut a section out and put in the clear filter. I like seeing what is coming through the fuel line. Now I have the benifit of both. If you look you might can find a T for the clear filter but I beleive most of the factory filters are calibrated to keep a certain pressure on the carb. Here is a picture I had and if you look closely you can see the clear glass filter right below the small filter that is were the original goes. If you wan't to see it better let me know and tommorrow I will take a better picture but its dark outside here and my digital has no flash.

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post #4 of (permalink) Old 01-07-2001, 02:20 AM
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Re: Fuel filter

[img]/wwwthreads_images/icons/tongue.gif[/img] Top Fuelie has it right. The three-port filter does have a slight restriction to hold good pressure on the carb and at the same time allow the fuel to be pumped in a circle and the excess going back to the tank. By doing that, the fuel stays cooler and the propensity to vapor lock is reduced, plus it is easier on the fuel pump to not run at a very low flow, which makes the diaphram operating lever flop around. IT IS IMPORTANT to have the third port rolled to a high position on the filter to encourage bubbles to find and flow into IT and not go to the carb.[img]/wwwthreads_images/icons/crazy.gif[/img] And one more thing.....as long as you are fond of clear filters, how about adding one to the vapor cannister line which connects to the carb bowl. That would intercept any stray bits of charcoal that are heading for the bowl of the carb.[img]/wwwthreads_images/icons/crazy.gif[/img]

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post #5 of (permalink) Old 01-07-2001, 03:29 AM
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Re: Fuel filter

Is a return line really all that important to the proper running of an engine? Let’s reason this out…

A fuel pump has three purposes:

1. Supply the engine with enough fuel under ALL conditions.
2. Maintain a proper pressure from the pump to the carb (or EFI).
3. Prevent fuel in the line from boiling and vapor lock.

If the pressure between the pump and the carb rises too much it holds the carb float needle off the seat. The result is the carb floods the engine at worst, and high fuel usage at best. A “average” gas engine in a Jeep will require about 10 gallons of gas to be delivered for each hour driven at highway speed. That works out to be about 2.5 to 7 psi range from high RPM to Idle. The psi usage is inversely proportional to the RPM. So, at idle the psi is higher. If you don’t want to flood the engine you’ve got to relieve the high pressure. The way to do this is to install a return fuel line.

I’ll use a mechanical pump as an example, however the same principle is in effect for an electric pump.

The mechanical fuel pump is limited as to its ability to deliver fuel by the area of the diaphragm and the speed of the engine and diaphragm return spring. Each of these factors are set in stone. The only variable is the age of the diaphragm and strength of the return spring. As the diaphragm ages the ability to produce a vacuum decreases. Eventually it will pinhole, tear, or otherwise fail. This usually happens far before the return spring breaks or looses spring.

The pump return spring is designed to only cause the pump to deliver fuel to the carb if it can overcome the pressure in the line to the carb. If the carb is not matched to the fuel pump then the fuel pump can overpower the carb with too much pressure.

To avoid the excess pressure engineers design and install a fuel pressure return line to relieve ANY excess pressure to the carb. Remember that this is a closed system, it's engineered. The return line’s line loss, length, diameter, etc. all are figured into the design.

To continue the example, when I installed my EFI system the system was running a mite too rich. Now, I’m a simple guy and too poor to buy a pressure gauge… but that don’t mean I’m dumb. I took the only variable I could control and played with it. The only variable was the return line. I removed the OEM line and replaced it with a larger diameter hose running into a 1gal gas can.

This should have reduced the pressure to the TBI because the diameter was bigger and the line was shorter. So, I started the engine and monitored the O2 sensor voltage in closed loop. Yep, it was right on. I installed the GM diagnostic monitor set to “road test mode” and watched what was happening as the engine idled. This also confirmed that the engine was running leaner.

The OEM return line was 1/4". I then took a few sections of 3/8" hard brake line and make a temporary return line and stuck it into the gas inlet and started the engine. Again the two monitors I used were showing the engine was running leaner. I returned the OEM line back to the carb and re-tested. The engine ran richer. The monitors confirmed this.

The rest of the day was spent running new 3/8” break line from the carb to the gas tank. I was surprised to find the OEM line was very rusted on the inside. That raised line loss, or resistance the pump had to over come. Also where the line ran up and over the frame I found the line had been crushed down. This was not visible, as it was under the line clamp. The constriction added more resistance that needed to be overcome by the pump. The pump could not overcome the resistance in the OEM line, and forced more fuel to the injectors. The injectors are stupid and just open and close for a length of time the computer tells them to open. So if you raise the pressure to the injector more fuel can get by the pintle for a given amount of on time. Lower the pressure, and the fuel passing through the injector decreases.

So in short, yep a properly engineered return fuel line is VERY important to the proper operation of the carb or EFI'd engine.

Safe Jeepin'

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