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MG TD TF 1500 - Solid state fuel pumps

Late last year I converted to negative ground on my TF. A negative ground Moss pump was installed and worked well for seven months. Two months ago, my car started running rough after about 15 minutes of running well with hard driving in 95 degree weather. This drove me crazy for numerous test drives and finally the car just stopped. The fuel pump was dead.
Moss replaced the pump (2 year warranty) and the new pump was installed. There was no change. After about 15 or 20 min. the car starts running rough with some mild backfiring. Now the second new pump runs rapidly with no fuel flowing. The fuel line to the tank is clear. No problem blowing through it and I can hear the air bubbling in the gas tank. I have owned a TD, TC and this TF for over 40 years so I am not a newbie. At first, I thought the problem was heat related and have installed a new Lucas coil, Champion RN5C plugs and four different condensers. All to no avail. Now I am convinced that this is a fuel problem and suspect that the fuel pump is heating up and not pumping sufficient fuel. Has anyone experienced problems with Moss solid state fuel pumps. Any ideas will be greatly appreciated. Thanks in advance.

Gary McGovern
G B McGovern

I have had this happen several times, on several cars. If there is any chance for air to enter the line between the tank and the pump, the pump will not draw a vacuum, but will suck air Ito the line. The problem is worse when there is less fuel in the tank, or you try to drive up hill.
I have had the copper line crack, fittings crack, and buggered up fittings causing an air leak. They do not show up as a fuel leak. In a pinch you can wrap tape over the problem area. Typical places to look are near the tank where the pipe bends, at the P clamps at hold the pipe, and within a foot of the pump.
Another thought is the tank may not be venting. Remove the fuel cap and see if the problem resolves.
Be well,
David
D. Sander

Gary, Make sure the gas line is not picking up excessive heat from the muffler or exhaust system. This 10% Ethanol gas, if that's what you use, vaporizes at a lower temperature than straight gas and if the line picks up too much heat it can create a vapor inside. Is your TF an early model with the pump on the fire wall or later with the pump on the rear chassis? PJ
Paul161

Gary - Both David and Paul are giving sound advice. I would lean toward an air leak more than vaporization, but either situation can cause the symptoms that you describe. The first thing I would suggest, is to remove the fuel line from the rear carburetor and direct it into a container marked at the one pint level. Turn the ignition on and time how long it takes for the pump to produce 1 pint. If you have a low pressure pump (mounted on the firewall) you should get 1 pint in 50 seconds. If you have a high pressure pump (mounted under the car on the frame) you should get 1 pint in 55 seconds). There should be no bubbles in the stream of fuel in either case.

If a pint of fuel is being delivered takes appreciably longer than the times posted above, then you have an obstruction between the tank and the carburetors.

If there is an appreciable amount of bubbles in the stream of fuel being pumped, you have an air leak somewhere between the tank and the carburetors - the most usual place is the fittings at the tank, the pump or at the carburetors. See the articles, Air Leak Troubleshooting and Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section of my Homepage at: http://homepages.donobi.net/sufuelpumps/

As for excessive heat causing vapor lock in the fuel delivery system - if the pump is located in the engine compartment, you could try removing the bonnet and drive the car on a sunny day and see if the problem goes away. If the pump is mounted under the car, try a heat shield between the pump/fuel line and the muffler/fuel line.

If you need further assistance, e-mail me at SUfuelpumps@donobi.net. Cheers - Dave
DW DuBois

Paul and D Sander,
Thank you for your replies. As it turns out, this old guy caused the problem. I have had the pump on and off numerous times with this chugging problem. Yesterday when I reinstalled the pump, I had not screwed in the bottom brass filter fitting tight enough and it was indeed sucking air. Fuel pump now working as it should. I might add that this is a very early TF with the pump mounted on the tool box. I have stuck with this since 1973 due to ease of access.

I will now start checking for possible air leaks in the line to the tank. Paul, I have never ever had a problem with vapor lock, but anything is possible. When are you moving back to SC?

Thanks,

Gary
G B McGovern

The pump on my TF is under the rear floor so after tinkering with the SU, lying on my back on a cold concrete floor I fitted a Facit. It was cheaper than an electronic conversion for the SU and runs faultlessly. Only problem is the continuous ticking sound - it doesn't appear to switch off when the floats are full but there has never been any flooding of the carbs.

Jan T
J Targosz

I had the rapid firing fuel pump issue on one of my Morris Minors this summer when the ambient temperature was over 90F. The Morris uses the same SU pump as the MGTD and it is mounted on the engine bulkhead. Car never actually stopped running but the pump was working very hard. When hot I tested by putting the fuel line from the pump to the carb into a jar and noted a lot of bubbles in the fuel. When cooler, no bubbles. My solution which apparently has worked, was to insulate the full length of metal fuel line with pieces of split open rubber fuel line.
John Quilter

Jan, I spent many years working on fuel pumps from a cold floor, too. Then I found that access is easy if you take out the sidescreen compartment floor. When I mentioned it here many, many people told me it was already very well known....

David
DavePro

Paul, I have never ever had a problem with vapor lock, but anything is possible. When are you moving back to SC?

Thanks,
Gary

LOL, funny you say that as we've been thinking about it, all my family is there. PJ
Paul161


Gary I'm glad you found the problem. A fellow club member here in Columbia had an issue of not getting fuel to the new pump. Found out the ferrule on the copper line where it goes into the fuel pump would not seat therefore sucking in air. Cut the line and installed new ferrule solved the problem. Sorry I did not mention that at Dillard. ? It was great to see you and Pam!

Ed
ECS Stanfield

Regarding Dave DuBois comment on timing the fuel pump flow.

New SU electronic pump fills a 16 oz glass jar in 36 seconds. This test has been run three times and all were exactly the same. Can too much pressure be a problem. Pump is mounted in the engine compartment.

Ed, it was also great to see you guys as well as Choo Choo and Milly. I let Kay Powell know that you are alive and kickin'.

Gary
G B McGovern

Gary - What I quoted was the minimum fuel delivery specification for the pumps as published by Burlen Fuel Systems. My experience is that when the pump is operating as it should, the fuel delivery is considerably larger than the specification (particularly when they are new). As for the pressure that the pump puts out, that is determined by the volute spring that is behind the diaphragm. The low pressure pumps (mounted in the engine compartment) is 1.5 psi. The high pressure for the pumps mounted under the car is either 2.6 psi or 3.8 psi (I have never figured out how either of those pressures are determined for a given car, but in my experience, it doesn't seem to make much of a difference and have not had any complaints about the output pressure from pumps that I have restored regardless of which (high pressure) spring is used. Cheers - Dave
DW DuBois

Dave, thanks for answering the question concerning too much flow. For what it is worth, after 30 minutes of driving, the flow from the pump increases to 45 seconds with a hot engine compartment. I have checked this twice. Still checking the fuel lines for air leaks. I have been under the car, so like all the others, my back is sore.

Regards,

Gary
G B McGovern

This thread was discussed between 06/10/2016 and 09/10/2016

MG TD TF 1500 index

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