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Trouble Shooting the Fuel Delivery System

By Brent Price & Bob Sprowl 1/7/02; Written for the North Carolina Ford Truck Owners.

This is a list of steps for trouble shooting fuel delivery problems.   This generally applies to gasoline engines with a carburetor and the traditional mechanical fuel pump, but much of this information can apply to any vehicle.

1. Do the easy checks first.

a. Make sure you have enough fresh gas in the tank, and that the tank does not leak.   Put more than a couple gallons in it.   Some vehicles will not pick up the fuel with less than 2 gallons in the tank.

b. Trace the fuel supply line from carburetor to tank.   This often overlooked step may save time in replacing unnecessary parts.   Visually look for a clogged filter, crimped together line, busted rubber hose connections, rubber line that is extra soft and collapsing together or so brittle that it has pin hole leaks, etc.   The rubber hoses are often the problem.   There may be a short rear rubber hose at the back near the rear tank.   There may be an inline filter on the frame rail, and a rubber hose connection on the frame rail as well.   A feedline with an air leak or simple crack will not pump.   Use only fuel grade rubber hoses; it should be marked on the outside.

. Fuel pump checks

a. Test the fuel supply at the carburetor.   Unplug the line at or near the carburetor, place into a suitable container, and have someone crank over the engine.   Keeping your eyes away, you should get healthy spurts.   If yes, then fuel system up to that point is probably ok.   (See step 6 below for checking fuel pressure and volume, if necessary).   You must have a restriction past this point if there is a fuel delivery problem.   Check the filter at the carburetor, carburetor float, fuel inlet needle valve restriction, etc.

b. If there was no spurt when you removed the line at the carburetor, then remove the inlet feedline to the fuel pump.   Have a plug ready to block off the line from the tank, or the tank may completely drain on you.   Rig up an alternate feedline into a jug or can of gasoline.   Leave the line unhooked at the carburetor and placed into suitable container.   When cranked over, the fuel pump should pull from the jug and spurt into the container.   If yes, then the problem is on the supply line from tank to fuel pump, and pump is ok.   If no spurt, then problem is most likely with the pump.

c. To check the pump by itself, remove it from engine, but keep the inlet to the jug hooked up.   Most importantly, keep your head and eyes from directly underneath the pump, as fuel will drip as removed. Work the pump arm by hand a few hard strokes (this is a good procedure for verifying a new pump as well!).   If fuel now happens to spurt out, whereas it did not spurt when turning over the engine and with pump mounted to engine, then something more serious is wrong such as worn pump arm, the cam eccentric is loose or broken, broken timing chain, broken cam, etc.   Use flashlight and fingers to feel around inside the timing cover for any clues.

3. Fuel lines from pump rearward:

a. If you have a hand vacuum pump, you can pull a vacuum on the line and see if fuel will draw.   Try at the fuel pump inlet first, then go back to the connection at the tank next.  That way you can isolate where along the feedline the problem is occurring.

b. With the gas cap off, use compressed air to lightly blow back from the fuel pump line rearward.  Though this may clear a clogged line, this fix may be temporary assuming debris is present.   You will still need to drop and clean the tank for a sure fix


c. The problem with older tanks may be with the tank pickup tube.   The pickup tube extends into the tank, and often has a coarse filter on its end (?73 and up trucks do).   The filter could be clogged, or the pickup tube could be corroded and the end broken off or otherwise blocked.   Tank removal is often necessary.

c. If you remove the connection at the tank, use compressed air and blow through the line anyway since this is a convenient time to clear the line from fuel pump rearward.

4. Filters

a. An inline fuel filter should be mounted between the fuel pump and the carburetor.   A clear inline filter (with replaceable inserts) works well near the carburetor, and is in plain sight whenever you raise the hood.   You only need one inline filter, to keep flow restriction to a minimum.

b. A fine fuel filter placed on the inlet (supply) side of the fuel pump can cause cavitation (vapor bubbles) if it is restrictive or becomes restrictive.   A coarse screen-type filter is usually placed on the tank pickup tube from the factory, and this protects the fuel pump from large debris.   The diaphragm fuel pump is resistant to small debris, but needs a clear low restriction path to the tank for proper suction.

c. Many Fords came with a small cylindrical screw-in filter at the carburetor.   You should be able to blow freely through it with your mouth.   It is cheap insurance, so replace if in doubt.

d. Some carburetors (Holley, Quadrajet, etc.)   may have a small filter immediately in the carburetor housing where the line connects.   It may be a sintered metal, paper fluted, or screen design.   You do not need this filter if you have the separate inline one.

5. Tanks

a. Most fuel tanks in older vehicles have a small level of water and rust and debris present.   In general, this crap stays low and goes unnoticed until it builds up.   A problem can occur after running the tank very low which causes this stuff to get sucked up.   If filters are clogging frequently, you may have no choice but to drop the tank.

b. The pickup tube in the tank usually has a coarse screen to filter out large debris and protect the fuel pump. This is good insurance to have.

c. Small amounts of water and routine condensation can be addressed by using gas treatments and by maintaining close to full tank levels.

d. If the tank is usable but dirty, it can be boiled out and cleaned by a radiator shop.   Use a can of available restoration grade tank cleaner and then sealant for the inside (POR, Eastwood, etc.).

6. Volume and Pressure Tests

a. A quick test can be performed with the line disconnected at the carburetor and using a catch can.   You should get a minimum of one pint of gasoline in 30 seconds.

b. A more refined test is to splice in a temporary fuel pressure gauge at the carburetor and after the inline filter.   Use a gauge that is gasoline compatible and reads to around 15 psi max for best resolution (carburetors only).   You should get 4-6 psi while the engine is running.   Rev engine up, pressure should not increase or drop off more than about 1 psi, and not below 4 psi.   If over 6 psi, strongly consider an inline adjustable fuel regulator and set it at 5 psi using your temporary gauge.   On carburetors, too high of pressure causes needle and float level problems.

c. Note electric fuel pumps on carburetted engines also should meet the 4-6 psi supply.

d. For fuel injection vehicles, fuel pressures are typically 35 psi or so.   Check the manufacturer?s specifications.   Use a test gauge that goes to around 50 psi, do not use the low pressure gauge.

7. Replacing a fuel pump

a. When reinstalling the fuel pump, it helps to get the cam eccentric properly turned.   Else the pump arm has to be compressed when installing, which makes it difficult to get the bolts started.   Before putting the gasket sealant on the fuel pump mounting flange, hold the pump in place and bump the starter in small increments to get to the optimum position.   A little pressure on the arm is normal.   Remember to wipe some light grease or oil on the dry pump arm to reduce wear until the oil starts flowing.

b. Use a stock fuel pump for most applications.   A significant change in fuel pressure, high or low, will cause carburetor problems, float level changes, bowl overflows or leaks, etc.   In some cases, you may need to readjust you float settings.

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