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MG TD TF 1500 - Fuel Pump Runon

A trip of 75 miles yesterday found the TF running like a new car. There were no problems until near the trip's end, when stopped at an intersection I could hear the fuel pump running like a mad thing.

Up with the hood/bonnet and no leaks could be found. The same was found under the entire car. No leaks at all.

Once home (car ran like a top the entire way), I searched the archives and found that similar conditions pointed to the fuel pump itself. I won't be able to get to it for a few days, but what would everyone expect that I will find when I get the pump on the work bench and start opening it up?

Gene Burgess

Gene - How hot was the ambient temperature when you experienced this problem? If the ambient temperature was high, you could have been experiencing a mild form of vapor lock. Do you still get the constant running of the pump after everything in the engine compartment has cooled off? If not, that would be an indication that what you are experiencing vapor lock. Before tearing into the pump, use a piece of flexible line and run the outlet of the pump to the bottom of a large container. Turn the ignition on (disconnect the white wire from the coil, so you don't overheat it while running this test) and observe the fuel stream for bubbles. If you get a constant stream of bubbles with the flow of fuel, you have an air leak on the inlet side of the pump. If there are no bubbles (after the initial surge), then there is nothing wrong with your pump or the fuel lines and I would not suggest disassembling it. If you do get continual bubbles, try tightening the screws at the base of the coil housing that hold the pump body and coil housing together and see if this eliminates the bubbles. If not, the next thing is to disconnect the fuel line to the inlet side of the pump and tighten the inlet fitting, then reconnect the fuel line and make sure that it is good and tight and then rerun the "bubble test". If you are still getting bubbles, then you may have a failed gasket between the body of the pump and the sandwich plate or a hole or split in the diaphragm.

If there are no bubbles or pump run on now that everything is cooled off, you may have experienced either a form of vapor lock as I stated before or other transient problem, such as a piece of dirt in the inlet or outlet check valves that kept the disk from seating properly. If the problem has gone away, there is no reason to disassemble the pump or anything else. If it reoccurs, only to disappears again, I would suggest two things: 1) if it only happens during hot weather, find some way to better ventilate the engine compartment (not a bigger fan or electric fan, but perhaps remove the top bonnet panels when the temperatures are very hot or insulate the fuel line where it enters the engine compartment). 2) check your fuel tank for any debris that may be getting past the in tank filter (don't install an auxiliary filter between the tank and the pump as that can cause damage to the pump if it becomes clogged). Cheers - Dave

David DuBois

You were right on the money. Today I put the key in the ignition, gave it a twist, and the fuel pump fired right up, slowed to a steady click and stopped. In the nearly ten years I've had this LBC I've never experienced any kind of vapour lock at all. This is a new experience.

So now I'm looking for ideas as to what to use to insulate the fuel line to the carbs. Does anyone have suggestions?

And Dave, I know how vapour lock can affect the carbs, but just how does it affect the fuel pump, and still allow the car to run as if the problem wasn't there at all? I'm puzzled.
Gene Burgess

Fuel evaporates, pump keeps filling the bowls?

Are you running stainless fuel lines or rubber? That was one of the "other" things I did about the same time I went to Jet-Hot on the exhaust manifold.
David 55 TF1500 #7427
David Sheward

Gene - "In the nearly ten years I've had this LBC I've never experienced any kind of vapour lock at all. This is a new experience."

It is a new experience for most of us. Today's fuels are blended much different than it used to be just a few years ago. In an effort to clean up emissions, the fuels today are blended to vaporize easier and have new and exotic materials in it, not the least of which is ethanol. It is also blended for vehicles with fuel injection (when was the last carbureted car produced?). In the fuel injections system, the fuel is brought from the tank to the injectors under pressure (up to 90 psi) and there is no problem with it vaporizing where it is not supposed to. the absolutely worst time for having a problem of vapor lock, either the type you just experienced or the vaporization of fuel in the carburetor when the engine is shut down for a short period of time after being driven is the first really hot day in early spring, before the fuel companies have changed over to summer grade fuel from winter grade. The winter grad fuels are formulated to vaporize at an even loser temperature to improve cold starting.

I didn't ask you what year your TF is, but if it is an early 1250 TF, it has the fuel pump in the engine compartment, which is the worst place to have a fuel pump as far as vapor lock is concerned. The fuel is being drawn from the tank by applying a reduced pressure (a vacuum), which serves to make the fuel all the more prone to vaporize (this, by the way, was why MG moved the pump to the back, near the tank where the fuel is sent to the engine compartment under pressure so it is less prone to vaporizing). Combine this with the modern fuels of today and you have a receipt for vapor lock.

I believe that what you were experiencing was a classic vapor lock situation, that was enough to make the pump work very hard to supply fuel to the carburetors, but not sufficiently bad cause the engine to start stumbling due to fuel starvation. A few degrees more heat and you probably would have had an unhappy engine.

The situation that you refer to, of vaporization in the carburetor, is also vapor lock but caused by a different situation. This happens when the engine is shut off and left to sit for 10 - 30 minutes. The fuel in the passage from the float bowl and the jet becomes vaporized, which puts extra pressure on the float, not allowing fuel into the float chamber. This is due to heat soak and can be prevented simply by propping the bonnet up on the right side so that the heat in the engine compartment to escape, without over heating the carburetors (by the way, a heat shield does absolutely no good for preventing this type of vapor lock, because there is no air flow through the engine compartment to carry the heat away). Even if the bonnet is not propped up, you can usually clear this situation by pulling the choke out when you start the engine and keep it out until the vapor is pulled through the carburetor).

There are two things that you can do to minimize vapor lock problems. 1) make sure that the grill slats are open enough to allow full air flow through the engine compartment. You want a clear view of your radiator through the grill slats rather than a nice display of the chrome job (or paint job in the case of TDs) on them. 2) is to follow Dave S's example and have the Jet-Hot coating on and inside the manifold to move the heat through it as fast as possible. Cheers - Dave
David DuBois

One other thing that could be happening,,,, Asuming that you have a pump with points in it,, (and not the electronic one that makes the clicking noise) It could be possible that the pump needs adjustment... I don't have the manuel here at the office, but if it is out of adjustment, the mechanism in side will keep "flipping over".. This happened to me a long time ago, and an adjustment cured it..

Steve Wincze

I am in a unique position to back up what Dave D has said : "fuels are blended much different than it used to be just a few years ago". My TF sat for over 2 years. Have always used Shell hi-grade in Izzy. This year I drained old gas and refilled so I got a real "taste" of what the new "blend" does. Izzy"s plugs looked very healthy when parked (nice and "gray").
On restart this year was running way rich, (even before cleaning my float bowels and possibly incorrectly setting level to 3/8" factory setting instead of recommended 7/16".)
Years ago I fought the vapor-lock issue and from this BBS followed recommendations from several people to solve the problem.
I made several changes that made sense. Problem is, I did them all at once so it is a little hard to tell what did the "most" good!

If I were to guess what did the most good (in order) I would say:
1: Carbs rebuilt by John Twist
2: Jet-Hot exhaust manifold treatment
3: Going to stainless fuel lines (old ones were rubber)
4: 1" spacers @ carb bodies to intake manifold. (reduce heat transfer from block. my car had no spacers on it when purchased...suspect PO took them off to make it eaiser to remove air cleaners!)
I totally agree that heat shield did nothing, or very little. Took it back off.
The only drawback from any of the above is the spacers make it harder to put front air cleaner on....but I cheat and remove a bonnet side panel bolt that makes that a much easier job!
The only thing I keep telling myself I should do but haven't yet is to add an electric fan and/or switch out the old 4 blade for perhaps a 6 or 8 blade one.
David 55 TF1500 #7427
David Sheward

Dave S. - "The only thing I keep telling myself I should do but haven't yet is to add an electric fan and/or switch out the old 4 blade for perhaps a 6 or 8 blade one."

A 6 or 8 blade fan will do very little good to solve the vapor lock problem and none at all to solve the heat soak problem. By the time the car is traveling at about 20 mph or greater, far more air is being pushed through the radiator and engine compartment than any fan the will fit in a car's engine compartment will ever push. I replaced the original fan in our TD for a 7 bladed plastic fan from the late MGBs, but not to try and push more air (it didn't change the operating temperature at all), but to avoid the possibility of a heavy, steel fan blade from exiting through the bonnet if it should break off at speed. An electric fan would probably be of some use in preventing (or at least limiting) the heat soak problem in that it could be set up like in modern cars, so that it will run whenever the coolant is above preset temperature. This would move air through the engine compartment when the car is not running and keep the temperature at a level where the fuel in the carburetor passage from vaporizing. I have toyed with the idea of an electric fan, set up this way, but toying is about as far as it has gotten. Cheers - Dave D.
David DuBois

Thanks to all for the insights. I'm presently running copper fuel lines and my factory original fuel pump is in the rear of the car. My TF has the standard chrome grill slats up front, but the lower/under the front fender/bonnet side grills are presently off of the car.

It sounds like I need to track down a fuel pump manual, and when I put the LBC away for the winter that I should be considering getting the exhaust manifold off to Jet-Hot.

Gene Burgess

Gene ,
I did not make that very clear,(sorry),..copper lines from tank to pump, mine is "newer" style above rear axle, copper from pump to fitting in bonnet, then stainless to rear carb and stainless to front carb. When I purchased the car "to carbs" was rubber and I heard from someone on this post that the rubber lines could contribute to vapor lock problems.

David D. : Good points ...I have heard quite a few say they run MGB fan and just assumed more "blades" would push more air.(kind of like running 12 vane water pump instead of 6) Have not had vapor problems for years, (after doing the above). Haven't had a chance to check plugs since last parked, (been remodeling kitchen) but I "shut down" under load and coasted into garage like Gene had said in a previous post.
David Sheward

I put an electric fan on with a manual switch.
But I mounted it backwards. When I shut down I turn it on and it blows the HOT out the front and sucks the HOT from the engine. I will be putting a timer on it so if I forget to shut it off when stopped I will not run the battery down. Works Well

Curiously, I would have thought that the copper and stainless steel lines would contribute to vapour lock more so than the rubber lines?

Gene Burgess

I would like to add my $.02 cents worth. First of all I worked for a major oil company (Manager Automotive Events, Union 76 Racing) back 7 or 8 years ago. Back then there were no less than 16 different EPA formulations for "Regular Gas". On top of that these formulations changed seasonaly. How many of us top off the tank in November, and burn that gas in June? Me for one.

Secondly, when I was having problems with my '51 TD, I sent my fuel pump to Dave DuBois for rebuilding with his optical sensor, Since then... Zero problems.

Two lessons I learned... Make sure your fuel is blended for the current season and make sure your fuel pump is working properly.
J. McLeod

This thread was discussed between 12/07/2009 and 15/07/2009

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