Way Outta Control
Join Date: Sep 1999
Location: The Palouse
Thanked 3 Times in 3 Posts
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.