Mechanical vs. Electric Fuel Pump
The fuel pump for the Willys F-head engine is mounted on the left side of the engine block and is mechanically operated by an eccentric on the camshaft. According to the Jeep Universal Service Manual, "The double-action fuel pump resembles two single-action pumps placed one above the other. A single fuel pump rocker arm actuates the two separate diaphragms. One diaphragm is part of the fuel delivery pump...the other diaphragm is part of the vacuum pump. The fuel pump consists of a metal body, a rubber diaphragm, rocker arm, valves, springs, gaskets, and a glass sediment bowl complete with strainer."
Jeff inquired on The CJ-3B Bulletin Board: "I have just made a change to an electric fuel pump from the mechanical one. I was trying to get my engine running and I finally found the problem in the carburetor -- it wasn't the pump. But why should I stay with the mechanical rather than the electric fuel pump?"
Wes K responded: "Perhaps the real question should be 'Why stay with the electric pump?' The majority of the electric fuel pumps in low performance vehicles got there for the same reason yours did. The owners were unable to properly troubleshoot their fuel systems and started throwing parts at it. The next best reason for installing an electric pump is in an engine swap where there is no room for the mechanical pump any more.
"The CJ-3B performed to its original specs just fine with the mechanical pump (left). The electric pump usually requires a pressure regulator so its output pressure can be matched to the input specs of the carb. The electric pump for safety reasons should have a kill switch (usually a simple oil pressure operated switch, but often an impact switch) which will turn the pump off if the engine quits or when you have a collision. Some electric pumps require a return line be added from the pump to the tank. Electric pumps should have a filter between them and the tank to protect them from sediment and other old rusty objects.
"I think it's easy to see that the original mechanical pump is the simplest installation and easiest to use."
Jeff: "It seems most Jeepers prefer the mechanical pump over the electric because it would give the carb the required amount of gas, as in not too much. My engine is backfiring some, especialy when I shut it off. Would the electric pump be the cause of this -- too much gas? If it is, how would I slow the pump? Or should I go back to mechanical and see if it works?"
Brian Hurt: "Most electric pumps put out more fuel than required by the carb. The one I had on my CJ-5 was cheap and worked fine. I swapped back to the mechanical pump because I have charging issues with the generator, and I was worried one trail ride that I wouldn't have the juice to keep the electric pump running. No battery, no electric pump = no gas pumped to the engine. It's just one less electrical thing to go wrong. And also, the cheap pumps do go bad. They just die. If your mechanical pump still works, hook it back up and keep the electric as a back up. If you stay with the electric pump I would suggest a fuel regulator after the pump, before the carb. Also, I never ran a filter before the pump. Anything big enough to cause damage to the pump won't fit down the fuel line. But don't hold me to that!"
Jim Hubbard: "I have had issues with electric pumps on several occasions. If while working on your Jeep you inadvertently leave the key on, the pump could and probally will fill your enging full of gas (called 'hydrostatic lock'). I have seen several fires caused by a needle valve sticking open and an electric pump continually feeding fuel to the fire. Failure is common with electric pumps, and if equipped with necessary safety switches, that compounds the problem. I just put a new pump on my 3B and from O-Reilly it was $54.00 with the vacuum pump. Be safe man, use the mechanical pump; when the engine dies the pump quits."
Eddie Stephens: "This is correct about the electric fuel pump flooding the engine -- been there, done that. I just purchased a CJ-3B with electric pump, and it will be the first thing to go."
See also Fuel Pump Tech Tips on CJ3B.info.
Fuel for a Cold Start
Ed Wilson wrote: "After a complete engine overhaul, carb rebuild, and addition of electronic ignition, I am still having difficult cold starts. An electric fuel pump was one remedy suggested. However, one advisor says the two pumps would work together, possibly using the electric unit only for startups, and one says I should bypass the mechanical pump.
"Also, a non-return valve in the fuel line was mentioned as a solution. I have not inspected the fuel pump yet, as it seems to work fine otherwise."
H. Woolridge: "You mentioned cold starts were a problem. Is the engine flooding? I had this problem with mine and an in-line fuel regulator from NAPA helped."
Brent: "I put an electric one on my M38 and have had good luck with it. I placed it inline, still through the mechanical pump. I bought a low pressure pump from the parts store -- it doesn't have a return line. The only problem is that it is a little loud."
Jon Hardgrove: "Have you checked to make sure the carb has no fuel (main reason for electric pump) when doing a cold start? Next time you do a cold start, remove the air cleaner, put on a set of goggles, manually open the choke, and work the throttle linkage to determine if the accelerator pump squirts a squirt of gas into the throttle area. If yes, electric pump probably not the answer. If no, start the engine (you now know there is gasoline in the carburetor), stop the engine and repeat the test. You now know if the accelerator pump is functional. If an electric pump is installed, remember 'big brother' wishes you to wire it from an oil pressure switch (safety), and to acquire a pump with approximately same pressure as an O.E. mechanical pump. Also, electric pumps are 'pushers'. The pump should be located as close to fuel tank as possible."
R. Jack: "My Jeep came to me with an electric fuel pump and fuel pressure regulator installed. I removed it and placed an original type pump. But my problem was rust in the gas tank. The tank outlet had no strainer, allowing a very small lump of rust to restrict the fuel line which was all visible when I cut open the gas tank before junking it. Additionally, check those GI jerry cans for rust in the bottom seam. Most were bad when surplussed."
Filter at the Carb
Jax: "I remember some info on an inline fuel filter that could screw in right into the carb fuel line inlet. I think it was a Fram but forgot the part number -- does anybody remember?"
Doc: "One of the first things I did after buying my Jeep was to get rid of a jury-rigged piece of copper tubing that ran from the fuel pump to the carb. Copper tubing will become brittle from vibration, leading to a dangerous situation from leaks. While I had the line off, one thing led to another and I rebuilt the carb.
"During the rebuild, I found lots of crud in the float bowl. I even found my nemesis, little specks of blue silicone gasket maker. I got to looking at the fuel filter screen inside the fuel pump and realized it couldn't catch the small stuff. While trying to find a fitting to plumb into the carb inlet fitting, I had a brainstorm.
"A Fram G3596 fuel filter (right) threaded right into the carb inlet. I happened to have one lying around since it also fits my F250 truck. The other side of the filter has a 5/16" flare fitting for which I formed a new piece of steel tubing to run to the fuel pump. This filter now catches most anything that gets past the coarse screen in the fuel pump. Whenever the filter is changed, cut the old one open with a tubing cutter. You'll be amazed to see what it will catch.
"The 5/16" line to match the filter is not the stock size. I had to change the output elbow fitting on the fuel pump to match."
Replacing the Fuel Line
Scott Blystone: "Since I've not seen this info elsewhere, here goes: To replace the fuel line from the tank to the fuel pump you need to go to virtually any parts store (NAPA, Advance, PEP boys etc) and buy the premade 60-inch 1/4" diam. steel fuel line. Every store I have been in has these premade.
- "Remove old line from vehicle. Cover both ends of new line with plastic baggie and twist ties to keep them clean until ready to thread in.
- Get under the Jeep from driver's side. The new line probably has two different nuts on the ends. One will be longer with more threaded area - that is the one you will connect to the tank. From under the car, aim the small nut end over the clutch pedal pivot bar, between the pedals and on the frame side of the steering gear.
- Periodically grasp line on both sides of clutch rod and pull gently to bend a little bit -- this will add a gentle curve needed to prevent line from touching floor panels.
- You may need to crawl out and go up to the engine. By grasping the line near the firewall and at the nut, you can flex it a bit to get around the steering gear without banging into the fender wall.
- OK, it's in, now for the bends. Get a line wrench the correct size and slip the box end over the nut on the fuel tank end. Slide the nut down about 3" from the end. Alternatively bend the line using the wrench on the nut, then slide the nut forward a bit towards the end and bend a bit more. This technique prevents you from kinking the line. A 90 degree bend 2-3" from the end will allow you to make the turn into the tank.
- To connect to the tank, put one layer of teflon tape on the nut, lie directly beneath the tank hole so you get the threads started straight, and tighten it up.
- Now, up to the engine. Here you use the wrench on the nut to bend the tubing three times. The tubing is probably lying against the frame rail.
- You need one bend 90 degrees up for about 5 inches from the rail, after the line is past the exhaust manifold (this keeps the fuel line as far from heat as possible).
- Then 90 degrees again so you are parallel with your fenders flat surface.
- Now angle the remaining end towards the fuel pump. You may need some final tweaking so your line enters the pumps without applying tension to it.
- If you still have fuel line clips on the frame, slip the line into them and fasten down.
"This is a great project - takes only 20 minutes or so and only costs a few bucks. On my CJ this was easier than changing a light bulb. I had to change the line because I damaged the nut gettting the old one back on after tank removal."
Don Norris took photos of his new fuel line. The first shot (left) shows the fuel tank connection and the clip on the hat channel.
The second photo shows the line passing through the toe board brace (120K JPEG).
The third photo shows the line strapped to the fender and bending toward the fuel pump (120K JPEG).
JC Jenkins adds: "When I replaced my fuel line, the first thing I did was to install a quality brass shut-off valve, so I could shut the gas off, if I ever wanted to remove the tank, and it serves as a cheap anti-theft device (turn it off, they won't go far). Also I have an in-line fuel filter right below the valve (we change the filter every year, this makes it easy), then we ran a fitting for 3/8-inch rubber fuel line (holds more gas, therefore stays cooler, less vapor lock in the rocks). I hope this helps: I know it isn't exactly 'stock', but it really works for us."
Jim Sammons comments: "The fuel line fitting coming out of the tank is 1/4-inch NPT. You can buy a fitting which has a 3/8-inch hose barb on the other end to run hose. However here in south Texas the brush has a habit of trying to remove the hose from under the tank when hunting, so I replaced the whole mess with 3/8-inch stainless tubing. The stuff is easy for me to come by since we use a lot of it in the oilfield. I ran it all the way to the fuel pump with an in line filter just before the pump. Be sure no matter what you use you wrap the threads with teflon tape which will seal the threads and make for easy removal if necessary."
Fuel Pump to Carburetor
Alan Haley: "My fuel line from the pump to the carb was a 3/16" tube that greatly resembled (and which I replaced with) a brake line. It runs from the pump, across the front of the engine just over the main shaft pulley, bends to follow the block back just under the oil filter and then immediately up. Bends once again over the top of the block and then back upon itself to connect with the carb. (See also Eric Lawson's photos in Engine Rebuild, Page 8 on CJ3B.info.) I made mine up a lot faster than it took to actually get it installed. I would suggest you take 10 dollars and buy a tubing bender to do this. You can get them at most auto supply stores.
"I had a huge amount of trouble getting the Jeep running this spring and spent a good amount of time on the fuel supply side of the engine. Some people said that running the line this close to the engine would cause the fuel to heat and interfere with pumping. I am positive that this course of travel was the original design and once I solved the real problems that I had (weak fuel pump and gas tank debris) the fuel supply was no problem. I would recommend that you install an easy-to-get-to inline filter."
Scott Blystone: "If there's no gas getting to the carburetor, there are two ways to approach it depending on how anxious you are to hear your CJ run:
- To get it running. Buy some rubber fuel line and hook it into the inlet of the carb. Attach the other end to some form of tank so it does not leak all over. Elevate tank about two feet over carb. Gravity will be sufficient to supply fuel to engine.
- To solve fuel delivery problem. This is one of two problems - either bad fuel pump or clogged line. I am assuming here that there is not gas leaking out somewhere in the system. First remove line from fuel tank to fuel pump inlet AT THE PUMP! Use a siphon pump, 60 cc syringe, etc., to try to draw fuel through line. This will determine if there is a clog in the line. If you cannot get fuel out, then blow air into it and try again. Often rust or debris from the tank clogs the line where it exits the tank. If blowing in allows gas to flow, then you need to remove fuel tank and clean it out. If fuel ran freely through this line to pump without clogs, then the problem is your fuel pump.
If you have the original pump (glass on top, vacuum lines) there are no rebuild kits available but you can still remove it and clean out gas passages, check for broken return spring on actuating lever, etc. If you cannot solve fuel pump problem, buy a new one - under $40 from numerous suppliers. Will not be original glass top but still has vacuum assist ports. It is possible that the fuel filter is so badly clogged as to not let gas pass through but I doubt it. You can take it out entirely for a few minutes without worry.Not a tough repair but a smelly one. My wife hates it when I come in from the barn smelling of gas."
Note: For seals and parts for rebuilding a fuel pump, try the Antique Auto Parts Cellar.
Thanks to Scott and the other contributors. The illustrations of the fuel system and the exploded pump come from the 1956 CJ-3B Parts List, and the F-head picture is from Facts about the Universal Jeep for Service Stations (and other Willys brochures.) -- Derek Redmond
See also Fuel Pump Tech Tips.
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How to Pressurize a Car's Fuel System - YouTube
The Fuel Return Line should be at least the same size, or one size larger than the supply. Make sure there are no sharp bends or kinks. The return line should direct fuel close to the bottom of the tank and away from the pickup.
Does size really matter when it comes to fuel lines? According to the fuel delivery system experts at Fuelab, it does. It's because fuel line size and length have a direct effect on the amount of fuel pressure drop your fuel system will experience.
How to Clean Old Brake & Fuel Lines with Aerosol Injected Cleaner
If it's below 60 psi, there could be a problem with your fuel pump or injectors. If the pressure is within specs, there might be a blockage in your fuel system. Try clearing any debris from the line and see if that solves the problem. Check the wiring and connector for the ECT sensor.
If the diameter is too big: the lines will take longer to pressurize. If your pump can't maintain the pressure, it can drop during times of high demand, leading to engine damage. It is an unnecessary expense, bigger is more expensive.
Your fuel line should be about the same. Too long and your engine may have trouble scavenging for fuel. This makes it difficult to tune the engine.
If you're just cruising around with a carbed SBC, you'll probably be fine with a 5/16" line up to about 500 HP because you'll never tax the system enough to starve the motor of fuel.
A 3/8”id fuel line can easily support 600 HP given sufficient “pump head”! Given a big enough pump a 3/8” steel line could support 1000 HP. The simple way to know is to install an electronic fuel pressure gauge. If the pressure falls as the engine RPM's go up you need more pump head.
The return only sees about 5 PSI of fuel flow. So no reason for it to be bigger. If your regulator is in the EFI unit and not back at the tank, then there should be no need for a larger return line as there should be minimum flow/pressure in the line.
Fuel supply restriction refers to a drop in pressure in the fuel line. This pressure drop is measured using a negative number because it is on the suction side of the supply pump or fuel lift. Issues like a clogged filter could cause pressure drop, although it can also be a result of the fuel line size.
How to Change Fuel Filter & Release Fuel Pressure▶️ How ... - YouTube
Fuel Pressure Test - YouTube
How to release fuel pressure from the fuel rail...quick and EASY - YouTube
How to Check Fuel Pressure Without A Schrader Valve (Andy's Garage