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MG TD TF 1500 - Fuel filter draining back

I've replaced the low-level sender on the fuel tank. It was probably the original, or close to it, and had been leaking around the gasket for what looks like years (see pic). (Oddly enough, even after all those years, it looks as if the unit could have been cleaned-up and put back in place!)

Although the fuel tank looked worse than it actually was and was remarkably clean inside, for added protection I've added a glass-bodied filter in the fuel line, just before the pump. There are no visible leaks anywhere (no smell of petrol either), the pump primes fine and the the car runs without any hesitation. But the fuel quickly drains back down into the feed pipe, immediately the pump has switched off, when I would have expected it to have kept the prime and remained full.

Am I correct in thinking that the filter should remain full, or is there a bleed mechanism in the pump, which will allow the filter to drain as I've described?

Regards, from a rather cold and wet SE of England (although we *have* been 12 weeks without any appreciable rain, so I really shouldn't grumble)

- Tom.



Tom Bennett - 53TD 24232

Only thing I'd be concerned about Tom, is the filter being on the suction side and I know where your coming from, to prevent debris from entering the pump, but if the filter gets plugged, it could starve the pump and possibly cause it to burn out. I prefer the filters on the pressure side to help prevent this from happening. Hopefully all will be well in your case. With the clear filter, you can at least observe it's condition. PJ
Paul J

Tom, You might want to try leaving the gas cap loose just to see if there was some negative pressure in the tank causing the fuel to quickly drain back. If it solves the problem, then perhaps your cap is not fully vented. Cheers Phil
Phil Atrill

Tom, I'm not being a wise guy, but are you sure that the line is empty? I have seen clear-bowl filters that show an air space under normal conditions. The fuel pump is a clear path through when it's not powered. Air would have to be coming in from the float bowl side of the fuel system. Bud
Bud Krueger (TD10855)

Thanks very much for the replies.

Paul, I'll watch the build-up of debris in the filter and I will move it when I've got the fittings. I wanted to get the car running again, as it's due its annual MOT test in the next couple of weeks, and I was concerned I might have a lot of sediment after removing the fuel tank, so I went for the quick option and put the filter in-line with the plain copper pipe feed to the pump. It's mounted vertically, BTW

Phil: The cap to the tank was open and I've only got a couple of gallons in it (which would help, but might also explain some of the draw-back)

Bud: All suggestions are welcome. Yes, all the fuel in the filter goes back down into the feed line, as soon as the pump stops. It's not a residual air bubble that I'm seeing. The pump quickly draws fuel into the filter, and on to the carbs, as soon as it starts. It's not running excessively long (IME) each time it's activated - I've not counted the clicks (it's also difficult to hear them when the engine is running) but I'd guess less than 10 per run. There's no sign (or smell) of leaking petrol and no hesitation when the engine is running (although I've not been out on the road, just had it revving when stationary). Putting all this together I figured I couldn't have much of a leak. When I first noticed the air, I disconnected the line after the pump and ran some fuel through, thinking it was just air trapped in the system but it has made no difference.

If the system is designed that way, then I won't worry, but if the draw-back down towards the tank shouldn't be happening and it's likely to overwork the fuel pump, then I'll have to investigate further.

The system is obviously open to the atmosphere beyond the filter but the absence of any sign of a leak, and the fact that the pump isn't operating excessively, is mystifying me.

- Tom.

Tom Bennett - 53TD 24232

If the fuel level in the tank is below the filter, it will drain back,

Don Harmer

Tom - The valves in the SU fuel pump are check valves, to keep the fuel flowing in one direction only. From you description, I would suspect an air leak between the pump and the tank or in the pump itself, or an inlet valve that is not seating properly. A poorly seating valve disk is not easy to repair, particularly if the failed portion is the seat itself, in that the seat is part of the overall pump body casting. That said, it is also not a fatal flaw, as you noted that the pump is still working well and will probably continue to do so for many years. The air leak, if it exists is something that is easy to cure. The very first thing to do is to make sure all the fittings and joints on the pump are properly secure. Start with the inlet, outlet fittings and the filter plug. Using a 3/8 Whitworth spanner of socket tighten each fitting and plug that is fitted to the pump body. I normally do this with the body held in a vice and put considerable torque on each item until it is firmly seated against the gaskets. Next, make sure that the six screws that hold the coil housing, diaphragm, sandwich plate and pump body together are tightly torqued (I use a torque value of 12 inch/pounds, but if you don't have a torque screwdriver, just make sure that the screws are firmly seated). After doing the above, Disconnect the fuel line from the carburetors, direct it into a glass jar and turn the ignition on. Observe the flow of fuel into the jar - if it is full of bubbles, then there is an air leak on the inlet side of the pump. To determine if the air leak is somewhere along the line from the tank to the pump, remove the line from the inlet side of the pump and use a flexible line to connect the pump inlet to a container of fuel and repeat the test. If the stream of fuel from the pump outlet still contains bubbles, the pump is the culprit. If there are no bubbles, the problem is the line from the tank to the pump (possibly the 1/4 BSP compression fitting that attaches to the pump).

On the subject of a filter between the tank and the pump, Paul is correct, this is a potential pump damaging situation. The problem is that the damage is very subtle in that it is all internal, but will cause premature points failure. See the article, SU Fuel Pumps Facts and Myths, in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

This thread was discussed on 12/06/2011

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