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MG MGB Technical - SU Fuel Pump Diagnosis Needed

Last weekend I inadvertently ran out of gas in my 67 MGB. Luckily I was only about a mile from home. I called my wife who brought me a can of gas. I put in a couple gallons and drove home. This weekend I tried to start my car. When I turned on the ignition switch, I got one click from my fuel pump, then nothing. I pulled the fuel line going into my weber carb. I again turned on my ignition switch..again one click from the fuel pump then nothing. No fuel flow. Could I have damaged the fuel pump by running out of gas? I've never torn one of these apart...are they repairable or are they just disposable? I've checked all the electrical connections and they are secure...thanks for any suggestions. -Rod
Rod in WA

Rod,
Sounds like the points have probably gone north in your pump. A whack from a hammer handle will probably temporarily revive it. You have not damaged it running out of gas. I've rebuilt SU's in the past, but I could never bring myself to trust them. I recommend a 1984/85 Honda Accord pump (made by Mitsubishi). They are $73.00 at NAPA and have push on hose fittings and nice rubber mounts. Physically they are about the same size as an SU and work the same way. They are quiet and are a cinch to install. I have one on my crossflow car and it keeps up nicely. I checked pressures and found just a shade over 2 psi static and at low revs and 1 1/2psi at WOT with two 45DCOE's sucking their hearts out. Bob put me on to them.
David
David

David, thanks for the tip...the SU pump I had was pretty new so I didn't think that the points would go out on it so fast. I may dig out my receipt to see if it still has a warranty. If not, I may just tear it apart to see what its guts looks like. Any special tricks to mounting the honda accord pump up next to the battery box? hanks. -Rod
Rod in WA

Rod,
The Honda pump has a pair of mounting points (ears) to enable the unit to be mounted on a flat, horizontal surface in it's intended application. That's great if you mount it in the trunk, but I installed mine in the stock location. The PO had done away with the SU and it's mount and installed one of those annoying electronic cubes, so I bent the Honda pump's ears 90 degrees and drilled two corresponding holes in the bottom of the circumference of the hole in the side of battery box. Using two 2" 6mm bolts and the rubber isolators and sleeves that came with the pump along with some flat washers, I installed the unit centered in the hole with the inlet and outlet nipples pointing towards the wheel. With the pump installed in this manner, the only noise heard is a click from it when the ignition is switched on. Drilling the holes at the bottom positions the pump as it is installed in it's intended host and may prevent a vapor lock condition that sometimes afflicts SU pumps if mounted in the wrong position, or it might work just fine in any position. Good luck.
David
David

Rod - See my article on SU fuel pumps on Les Bengtson's web site and the accompaning write up on my SU fuel pump services. The points on the new pump "should not" go bad in a short period of time, however, they sometimes do. This is particularly true if there is a clog in hte line between the tank and the fuel pump as this causes the pump to stall in the current on condition and if left on for any period of time will burn out the swamping resistor in the pump. The swanping resistor is part of the arch supressing circuit and when it burns out, the points will start arching excessively and soon burn out themself. It Doesn't matter what kind of a pump you use, it will fail if run for a sufficiently long period of time. I have had the Japanese "SU" fail on me and the Fawcet square, run all the time, make a lot of noise pumps fail on my truck. All are electro-mechanical units and will wear out in time. The SU pumps last about as long as any of the other pumps, but like anything else, you can get a bad one off of the shelf from time to time. Good luck - Dave
David DuBois

Well, it may not be the pump after all. I had a chance to get under the car tonight. I disconnected the fuel line at the pump that goes back to the tank. I removed the gas cap, then blew into the line until I could hear air bubbles in the tank. I hooked the line back up and turned on the ignition. The fuel pump started clicking away until, I assume, the fuel line from the tank to the pump was full, at which time it stopped clicking. I ran out of time tonight, but tomorrow I will try and blow out the line from the pump up to the carb. I'm now thinking I may have picked up something off of the bottom of the tank when I ran out of gas, and it is now lodged somewhere in the pump or the fuel line. If blowing out the line from the pump to the carb doesn't work, I'll pull the pump and dismantle. Dave DuBois...what is Les' website that I can lookup? Thanks. -Rod
Rod in WA

Rod - Sorry, forgot to include the URL. Here it is:
http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm
Where in Washington state are you located? Cheers - Dave
David DuBois

Rod - The pump will run untill the float bowls are full of fuel, at which time they stop until the fuel level drops far enough to open the needle valves, at which time the pump clicks again. If the engine is not running, a new pump, with no wear on the valves, will go for quite a long time before it clicks again. That sounds like what is happening on your MG now. Normally, nothing big enough to lodge in the pump can get through the line from the tank, so I would not suspect a problem there. The fact that after blowing out the line from the pump to the tank resulted in the pump running, I would not think that there is a further clog between the pump and the carbs. You have probably cleared your problem for now, but you will need to see about cleaning your fuel tank or getting a new one. As I stated before, a clog betweent he tank and the pump can damage the swamping in the pump (actually in the coil housing), causing the points to burn out prematurely. For that reason, you want to be sure that there is nothing in the tank that will cause that kind of a clog.
Good luck - Dave
David DuBois

Dave, I'm down in the Vancouver area, just north of Portland, Oregon. I'm looking forward to the all british field meet here in Portland on Labor Day weekend. I'm hoping to get back under the car tomorrow night and I will report back with my findings...thanks for the info. -Rod
Rod in WA

You say that running out of gas will not damage the pump, but once it has started pumping air, and until the engine actually dies which can be some minutes, the pump is chattering away like billy-oh. Whilst not necessarily damaging it, it does put a lot more strain on it, which could be enough to make a marginal pump fail. Clicking once with each turn of the key indicates the contacts have stuck closed i.e. are not opening when the diaphragm is drawn back. This releases when you turn off the ignition, which actually will pump a small amount of fuel through. Turning the ignition on and off again repeats the process. If a lump of dirt (amazing how dirt can float on top of fuel ...) had blocked the intake side of the pump it would continue to chatter with the ignition on, albeit slightly slower than when simply out of fuel. If the blockage were on the carb side of the pump the pump would have chattered a couple of times, then the back pressure would have kept the diaphragm pulled back, the contacts open, and switching the ignition on and off after that would cause no sound to come from the pump.

Removing the fuel line from the pump has had the effect of joggling it and releasing the sticky points, and it will probably now run for a while with no problems. But mark my words, it will fail again, at the most inopportune moment, so schedule a points cleaning session now.
Paul Hunt

This thread was discussed between 04/08/2003 and 05/08/2003

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