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MG MGB Technical - Fuel Pump Running On

Stopped and filled up the fuel tank on my '67 GT after work today. Several miles later, stopped again for a few minutes to shop. When I got back in the car and turned the key, could hear the fuel pump running top speed like there's no tomorrow. Started the car and got back on the road, and the fuel pump kept it up through two red lights. It finally quit a few miles down the road. What could cause this? I checked the fuel lines when I got home and everything looks tight and secure.
Glenn G

Glenn, only two things cause this. One is a fuel leak on the engine side, such as a stuck needle valve in the carbs, or an air leak on the tank side. When you say it quit, do you mean it stopped working or just went back to the normal click clunk rhythm. If it went back to normal then it is more likely to be a fuel leak that fixed itself. Needle valves most likely.

Tony
Tony Oliver

Hi Glen,

I agree with Tony.

Herb
Herb Adler

Do you have a fuel filter?
I believe that all cars should have one of those simple paper in a plastic container type.
It should be in the engine bay where it is visible and also easy to change.
If you have a filter then I would suspect a small particle has lodged in the pump inlet valve.
When this happens the pump discharges back into the tank on the pressure stroke and so keeps running.
This problem can happen when filling the tank stirs up some rust particles.

Mick
M F Anderson

I agree with the above, my experience when the pump ran continuosly backs this up as that was the time the diaphragm failed in the fuel pressure regulator (aftermarket device, here is trhe pic again). As soon as I bypassed it the pump stopped


Stan Best

It depends if it is pumping fuel out of the overflows or not. If it is, it is a float valve leaking, usually from dirt, possibly from a sunk float.

If it's not pumping fuel, then it is either sucking air from a perforated supply-side pipe, which could be inside the tank, the pump diaphragm is detached from the solenoid (less likely if it ran for several miles), or one of the pump one-way valves is stuck open.
PaulH Solihull

When I said it quit, I meant that the fuel pump went back to normal speed. The pump does go nuts when a carb inlet valve gets stuck, but I didn't smell the usual strong gas smell this time. I did smell gas, but not as much as if it were coming out full blast. So I agree that it must have something to do with air in the fuel supply line or a stuck valve in the pump itself. I did make a sharp left turn shortly before the pump overrevved. Will have to try that again. I think I will install a filter, only I'll put it upstream of the fuel pump. Thanks for the responses. -G.
Glenn G

Glenn,

Do not put a filter upstream of the pump (between the pump and the tank).
SU pumps do not like that. If the filter has a high resistance to flow, or becomes clogged, it will hold the pump in a position with the solenoid energised.
This will overheat the coil and/or the diode, probably leading to total pump failure.
Only put a paper filter in the engine bay.

Mick
M F Anderson

Glenn, there is a filter built into the inlet valve seating. It is a simple brass gauze that stops most things getting into the inlet valve. To clean it you have to strip the pump down and remove the inlet valve.

As Mick says, a filter before the pump is a bad idea unless you get a clear plastic type and you check it on a very regular basis. Of all the pumps I have serviced, and thats a lot, I have never seen the internal filter blocked.

Tony
Tony Oliver

This thread was discussed between 16/02/2011 and 17/02/2011

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