Mechanical Fuel Pump Repair
Fuel pump repair is a serious matter but nothing that is too difficult for even a beginner if you follow a few very important rules. To ignore any of these rules will invite certain failure and may even cost you your car.
First and most important is the use of the proper materials and supplies. The replacement diaphragm that you install must be made of a material called VITON. It is a sheet rubber that will withstand the modern gasoline and it is the ONLY material that will do so. Other materials, Nitrile, Buna N and Neoprene will eventually be destroyed by the modern gasoline and will end up in your carburetor as a sticky, gooey mess. When this happens it also becomes leaky enough to permit fuel to go directly into your crankcase where it may explode violently.
A second rule is, NEVER use Teflon thread sealing tape anyplace in the fuel system. It will cause untold trouble, trust me.
When rebuilding a fuel pump you should decide if this is for a driver or a show-car because the cosmetics are different for each. Let's stick with the driver for now. The usual problems with a fuel pump are leaky check valves, worn out diaphragm, possibly a broken spring or a leaky sediment bowl gasket. Of course there are other possibilities, but these are most often the cause of trouble.
Before you take the flange screws out, file a line across the edge of both flanges to indicate the position for re-assembly. Once you have the pump apart and cleaned inside and out you will be able to inspect it for any obvious damage as well as see how to disengage the actuating links that operate the diaphragm(s). If you have a duplex pump, where one diaphragm pumps gas and the other pumps air to create a vacuum for the wipers, both diaphragms should be replaced now. With the diaphragms removed check the seal under the diaphragms if there are seals there, and replace them if they show any sign of wear or are torn.
New repair kits are available from specialty suppliers, but be sure they are supplying VITON diaphragms attached to the piston(s). Some kits will include new replacement check valves for pumps that have removable valves. For pumps that do not have removable valves you can reseat the existing valve seats.
Remove the valve retainers and collect the small flat valve discs with their tiny springs and put them in a safe place. Get a wooden dowel slightly smaller than the hole above the valve seat but larger than the brass seat. Put a 3" long piece of the dowel into a drill press and lay a piece of medium grit sandpaper on the drill press table, grit side up. Run the drill and sand the dowel to get a perfectly flat face. Cement a small disc of very fine wet-or-dry sandpaper to the end of the dowel and, using a low speed, lightly contact the sandpaper to the valve seat. After a few seconds inspect the seat to see if it is bright all over. If there are dull sections repeat the operation until an even shine covers the entire seat.
Check the valve discs to see if there is wear at the seat contact area and if there is, turn the disc over to use the unworn side or sand the disc flat with very fine (600 grit) wet-or-dry paper placed on a flat piece of glass. Reassemble with the small springs and new fiber washers.
Next, check the seat where the sediment bowl fits by placing the bowl in place with no gasket. See if it rocks in any direction. Often the bowl seat will be warped from the strain of the retaining device. If the warping is slight the use of a thicker gasket will correct the seal but if warping is excessive, the seat must be flattened. (Straightening pot metal castings will be covered in an article in a future issue.). Check all the flange screws to be sure none of the holes are stripped and if they are, a HelicoilTM thread insert will correct this so all the screws can be tightened equally.
Install the piston(s) in the appropriate chambers by manipulating the links to engage the actuating arms as originally found. Don't forget to install the new seals, if required, and the springs that actually operate the pistons. When installing the covers observe the marks you filed to position the flanges correctly.
Tighten the screws only slightly snug at this time, as you may need to flex the diaphragm seals before you tighten the screws. The directions in the kit should tell you if this is required for your pump. Tighten the screws evenly, alternating from one side to the other and tightening each gradually until they are all tight. Install the inlet and outlet fittings using a paste type pipe sealant that contains Teflon, available at all parts stores.
A final check is to see if the job is successful. Install the sediment bowl with a new gasket. Try to blow into the inlet(s) and see that air will easily pass through the pump. Next try to suck on each inlet to be sure that no air will come through. Manually operate the actuating arm and listen for the sound of air chattering through the pump. If all these tests are passed you can be sure your pump rebuild is a success.
This article originally appeared in the November 2005 issue of Skinned Knuckles. It is copyrighted by SK Publishing and may not be reproduced, in whole or in part, without written permission from SK Publishing. See Skinned Knuckles for more vintage and classic car tips. Also see vintage car repairs and these other articles from Skinned Knuckles: