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MG MGB Technical - SU fuel pump running warm then cutting out

This is so frustrating. 1972 B. fuel pump has started to act up. It cuts off and the engine dies. By going underneith and proding it (i want to prod it with a sledge hammer right now!) it will work but then dies again. I've cleaned it out and the points are clean, I noticed that the body was quite warm when i took it off just now, not hot but warmer than i expected. I'm loath to start playing with the points, electrical gubins (the earth and power terminals/wires are fine),
Please, anybody what should i do ?
Gerry Lodge

Gerry - The next time the car dies on you, instead of prodding the pump, remove the fuel tank filler cap. If you get a big whoosh when you do that, you have solved the problem - the tank vent is clogged - most likely due to rust in the union in the line that goes from the tank's vent to the charcoal canister. This union is in the line in the trunk where it runs from the vapor separator to the charcoal canister. It is in the line in the fender well where the vapor separator is located. If this is not the problem, the next most logical place is a clog in the inlet line to the pump, most likely in the pickup tube in the tank. The fact that the pump is getting hot to the touch indicates that the problem is the inlet side of the pump. See the article, Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Thanks for the input Dave. I have dismantled the pump and found the contacts were very corroded. Plus i had a spare parts pump to rob.

Fingers crossed.
Gerry Lodge

Why does everyone seem to, all of a sudden have trouble with the SU pumps? I am now getting worried, thinking, any minute now, mine will die! It is probably original 38 years old! I better get the parts ready to repair, I have been threatening to for a while. Mike
J.M. Doust

Mike - If you drive your car on a regular basis, that old SU pump will probably last another 38 years. It is primarily cars that aren't driven regularly that have a problem. Cheers - Dave
David DuBois

Thanks David, it has been up on the axles stands for the last 2 months. I am now at the end of the suspension rebuild so, back on the road. I am looking forward to the Hillclimbs I am to subject it to. Mike
J.M. Doust

Capacitor-quenched points, which all but the later cars had when they came out of the factory, do arc and you get spikes and pits which jam the points together, which was the problem I had (at probably 30 years). The later pumps had diodes, which made them polarity dependant, but the current (no pun intended) versions of those pumps use bi-directional transient voltage suppressors or metal oxide varistors which are noticeably better at quenching the arc than capacitors, and makes the pump polarity independent again. The points are easy to clean, reface and adjust (said pump has been working on my V8 ever since), and it would be worth-while fitting one of the later suppression devices at the same time. I don't know whether Dave can give us a spec for these, there are a huge range.
PaulH Solihull

The latest version of the SU fuel pumps coming from Burlen Fuel Systems are using a thermistor and while they allow a very high voltage spike, I have been told that the thermistor still sucks up most of the energy so that the points aren't damaged as much. I'm not sure of that explanation, and will be watching the pumps that I get over the next couple of years to see what the results are.

I have been using a bi-directional TVS to protect the MOSFET that I use in my optical triggering system for the pumps and they do clamp the reverse voltage generated when the current through the coil collapses. The one I am using P6KE24CA, which is rated at 24 volts, but the maximum clamping voltage is closer to 33 volts. I have put these on some pumps that people wanted to keep as points only pumps, but have not observed the effectiveness over time. As I remember, the TVS is what Peter Cole in England has been using on the pumps he has worked on. Cheers - Dave
ovbsmglp

This thread was discussed between 01/10/2011 and 03/10/2011

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