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MG MGB Technical - installing new fuel pump

I need to change my existing fuel pump which has packed up, any info will help in fitting the new pump and any likely problems to look out for.
Model MGBGT 1078 YEAR
many thanks
Stuart
si robathan

Stuart-
When installing a new or a freshly rebuilt fuel pump, it is prudent to replace the rubber fuel hose (flexible pipe) that connects it to the fuel tank. Should the fuel hose be found to be deteriorated and cracked, the fuel pump may be partially pumping air into the system and not properly pressurizing. This would be symptomized by very rapid clicking of the fuel pump. It should not click more than once every 20 seconds with the ignition on but the engine stopped. However, kinking the intake fuel hose between the fuel tank and the fuel pump acts like another valve (albeit a no-way valve in such a case) in the fuel line, so the problem could be with the intake valve inside of the fuel pump sticking in the open position. This condition is easy to diagnose without disassembling the fuel pump. Simply disconnect the intake fuel hose (flexible pipe) from the fuel tank and place it in a small transparent container of fuel. If you see the level going down and up repeatedly while the fuel pump is clicking like mad, then the intake valve is stuck in the open position.

One of the more interesting and somewhat quaint features of the MGB’s fuel pump is the use banjo unions. Sadly, these usually cannot be refurbished and must be replaced whenever they start to leak. Be aware that the new AZX 1300 Series SU fuel pumps now use an O-ring (Burlen Fuel Systems Part # AUB 654; Moss Motors Part # 370-655) between the ports of the fuel pump and the banjo fitting. If your fuel pump has a recessed area on the intake and outlet ports, you will need to use an O-ring instead of the fiber washer that was used on the earlier fuel pumps. The order of assembly for the banjo fitting on these new pumps is as follows: Fit the O-ring into the recessed area of the inlet and outlet ports of the fuel pump. The banjo fitting goes on top of the O-ring with its flat surface against the O-ring, a standard fiber washer goes into the recessed area on the top of the banjo fitting, and finally the banjo bolt is inserted through the fitting into the port, and is then tightened until the fitting is good and snug against the port. Be warned that if you install the fiber washer on the inside of the banjo connection, you will have a suction leak. If the fuel pump chatters away at a relatively high rate, caused by the points carrier repeatedly throwing over, it is an indication that it is drawing air into the intake side of the fuel pump. Here is an 'Ol-Timey-Mechanic’s fix for the problem of a leaking banjo union at the fuel pump: Take a new set of fiber washers and wrap them radially with Teflon plumber's tape (the type that is used for sealing pipe threads). Wrap the tape around, overlapping the wraps, in much the same wrapping manner as the paper that was once used to wrap new tires with. You should then have no more leakage after you install these modified washers.

The fuel pump should always be mounted with its outlet on the top. The outlet is the port that is closest to the top-hat style cover on the body of the pump. Note that it is not necessary for the holes on the inside of the banjo to be lined up with the outflow of the banjo union as there is a groove around the interior of the banjo fitting so that it will conveniently pass fuel when it is mounted in any orientation.

It should be noted that the SU fuel pump is vented. The purpose of these vents is to relieve the pressure built up behind the diaphragm each time the coil is energized, pulling the diaphragm upward. Without these vents, the air trapped behind the diaphragm would be compressed, acting as a stiff spring that that would then resist the movement of the diaphragm. If both of these vents were to become clogged, the pump will then become inconsistent in its operation. The main vent, which allows air to move both in and out, is located at the bottom of the coil housing. The vent on the end cover (if one is present), incorporates a check valve that allows air only to exit, but not to enter. It is not there for the purpose of admitting cooling air to the electronics, it is merely an additional vent that expels any pressure caused by the upward travel of the diaphragm.

A piece of 1/8" tubing should be attached to the vent line fitting at the base of the coil housing, and be terminated with a ‘T’ fitting (it can just as well be an ‘L’ fitting) inside of the boot (trunk). This tubing can be made of any material as its sole purpose is to vent air into and out of the space behind the fuel pump diaphragm where there are no fumes present. The termination inside of the boot (trunk) is for the purpose of providing a relatively dry termination of the vent line, and to keep road debris and water out of the vent line and thus keep moisture out of the pump. In no case should the vent line be left to dangle under the car, as water splashed under the car can been drawn into the coil housing, causing severe rusting inside of the pump to the point where it will cease to function. If you do not feel comfortable with the vent being inside of the boot (trunk), it can be run to a high spot under the car (outside, top of the battery box comes to mind) and attached there. In any case, there should be no fumes coming from this vent line into the boot (trunk), as it is only venting the air from the area of the pump that is above the diaphragm. Except in the highly unlikely case of the diaphragm having developed a leak, no fumes should be present in this area.

Definitely fit an inline fuse that takes the Original Equipment tubular MGB fuses into the fuel pump circuit. This should be fitted as close to the power supply as possible, not close to the fuel pump as one might think, as the purpose of the fuse is not to protect the fuel pump itself, but rather to protect the wiring against short circuiting. A very convenient for the fuse position is where the rear wiring loom (harness) joins the main wiring loom (harness) in the mass of connectors by the fusebox. Put a couple of solder bullets (not the blue or red crimp type as neither are the correct size), and with an additional single bullet connector you can insert the fuse between the two harnesses. When the points close and full current runs through the coil, it is drawing just over 4 amps (3 Ohms x 4 Amps = 12 Volts). Since the initial current draw might be a surge, use a 10 Amp fuse. You can also use a standard 17 Amp-rated, 35 Amp blow fuse. I know that the fuel pump does not need a fuse as big as this, but this would be to protect the wiring and that size is perfectly adequate for the purpose. This will spare the necessity of carrying multiple fuses around, and you should have two spare standard ones in the fusebox anyway. Be sure that the white positive (+) wire goes onto the terminal on the end cover and that the black ground (earth) wire goes onto the terminal on the flange of the solenoid coil housing. If the fuel pump works, but the engine then stops, remove the fuel cap from the fuel tank. If you hear a hissing noise, and / or if the fuel pump works without the fuel cap, then the vent inside of the fuel cap is blocked, forcing the fuel pump to pull against a partial vacuum in the fuel tank.

Stephen Strange

A new SU pump will have the recesses in the inlet and outlet ports as Steve states above. If the pump comes without the required 'O' rings, you can get them from your local NAPA auto parts store under part number 2-017 (sometimes listed with the 2 as just a 017). For pictures of how the banjo fittings are assembled, see the article, New SU Fuel Pumps with 'O Ring Seals in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Also see the article, SU Fuel Pump Vents in the same section of my web site for illustrations of the vents for the pumps.
Cheers - Dave
David DuBois

Stephen
Many thanks and I will keep your observations very much in mind.
One point which may be obvious , can I access all the pipework and connections to the fuel tank from the boot or do I need to work from below, ie by jacking up the car or even using a ramp. I hope that I can work through the boot as to jack up the car will be a problem. Thanks for your advice so far, it is most helpful
PS I have already put an in-line fuse in the wiring. When I first got the car, 6 months ago, the pump was connected direct to the battery!! not a good idea!!
si robathan

Stuart-
Except for the vent lines in the boot, everything has to be done under the car. Fun, fun, fun!

Be sure to have a look at Dave's website mentioned above. The images are quite usful.
Stephen Strange

1078? 1978 I trust! The benefit of the rubber bumper cars is that the pump is higher than the top of the tank, and as long as you keep the unions that high no fuel will siphon out, unlike the chrome bumper. The electrics end is in the boot, as is the worm clip clamping the rubber seal onto the pump body, which is its mounting.

A rubber bumper pump change really is a doddle. Slacken the unions first and remove the breather tube from the body from underneath first, then undo the electrics and worm clip in the boot, then back underneath with the new pump to hand to remove the unions, pull the pump out, slot the new one in the rubber seal, and loosely refit the unions. Then back in the boot to tighten the clip and reattach the power wire, then underneath to tighten the unions and refit the breather tube. It's not more than a ten-minute job.

Late rubber bumper hoses seem pretty flimsy, with non-reusable clamps, so laying in a length of 5/16" petrol hose and suitable clamps is a good idea.
PaulH Solihull

This thread was discussed between 28/08/2010 and 30/08/2010

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