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MG MGA - MGA radiator overflow when engine stopped.

I'm running an 85 degree c old style bellows and collar thermostat with a new 7lb radiator cap on my 1958 1500 MGA. engine runs at about 190 max on the gauge and everything is fine until the engine is stopped, then the temp climbs towards 212, which is to be expected, but well before boiling point is reached, coolant is expelled from the overflow pipe. This continues to happen at nearly every engine stop until the rad needs topping up. Im am not overfilling the radiator (1/2" below the seal orifice or 1" of coolant in the filler neck)and the block has been cleared of sludge and scale in the water jacket, so what's going on there?!
Lindsay Sampford

I think you could be overfilling it. I fitted a header tank to mine to stop that happening. what happens if you let it find its own level does it stop passing water? :-)
Robert (Bob) Midget Turbo

I think you will find that is quite normal on the MGA standard system. When you switch off the engine, it is still very hot and there is no mechanical means of cooling the water. As you say, the temperature is seen to rise and the water expands; there is only place for it to go and that is overboard. It will continue to vent after successive trips until a suitable radiator level is reached. i.e. there is enough residual space in the radiator to cater for the additional expansion. Some of us have attached overflow recovery tanks to solve this, as is the standard on all modern cars.

In summary, nothing wrong with your car, just a case of there only being so much room in the radiator for expanded water.

Steve Gyles

A 50/50 mix of coolant will normally expand by about 1-cup when at 200F. If you do not allow for this amount of space in the system to start with, it will always expell this amount when at temperature. It is just physics. Nothing is abnormal about it. As already stated, when the car stops, cooling stops and the coolant gets a bit warmer. This makes the coolant expand more, spitting out a bit more when stopped.

If I fill my radiator to 1/2" below the filler seal, I will always loose coolant. It is a never-ending maintenance chore. I leave just about 1/2 inch at the bottom of the neck. All season long, every time I check it, it is at the same level. I don't get the embarrasing (and ecologically unethical) dribble of antifreeze on the ground under the car when I stop.

If you feel you need to fill the coolant this much, changing to 100% water, drops the expansion by more than half. You can fill the radiator a bit more without forcing it out at running temp. Plus you will get better cooling capability. But you lose a bit of margin in boiling capability.

Overflow (recovery) tanks are a good thing.

C Schaefer

Lindsay - As the others have stated, you are experiencing a very normal phenomenon of the older style cooling systems. See the article, Expansion Tanks, Coolant Recovery Systems, And How The Cooling System Works in the Other Tech Articles section of my web site at: for a detailed explanation of what you are experiencing and how to resolve it. Cheers - Dave
David DuBois

Thanks to you all for your answers. The thing that was concerning me was the fact that, left to its own devices, the coolant would get well below the level it should be at. I am running a 50% antifreeze (OAT) mixture so with the expansion you say I can expect from this strength I guess I'll have to live with it. Is it neccessary to change the 1" filler neck on the radiator to the more modern 3/4" neck in order to fit a coolant recovery system?
Lindsay Sampford

I find that the level at which my car stops expelling excess coolant is about 1/4 inch of coolant in the horizonal portion of the filler neck. If I fill it any fuller it will spit it out when the most people are around to tell me my radiator is leaking!

Ed Bell

As others have stated your fluid level may be too high. I put in enough water/antifreeze (50/50 mixture) so the level is about a 3rd of an inch in the filler neck.
One thing not mentioned earlier: you could be getting the wrong radiator cap. I purchased a new one (years ago) from NAPA. All looked good, but the radiator cap was just about 1/16th of and inch too short. I suppose it had the correct pressure, but wasn't long enough to properly seal. I purchased another from Moss and it was just right.

I would suggest first to try the fluid level at about 1/3 to 1/4 inch showing in the bottom of the filler neck. If you are still leaving a trail of water behind you as you enter the garage, then call Moss for a new radiator cap.

Ray Ammeter

Lindsay - As long as you have the correct cap for the 1" filler neck, it is not necessary to change to the shorter neck. As for the amount of coolant you have in the system, on our 66 MGB, with a 3/4" filler neck, the coolant level would be clear out of sight before it would stop pushing coolant out. Since I installed the coolant recovery system, the coolant is always right at the top of the filler neck if I remove the cap. I installed the recovery system because I could not see the level of the coolant and didn't know if it was above the level of the top of the tubes or not. Now it is not a worry. All new cars today either come with a coolant recovery system installed or an external expansion tank mounted high on the inner fender. Cheers - Dave
David DuBois

I have noticed water expelling from the radiator exactly as Lindsay describes. This is why I fitted a coolant recovery tank. I often wondered why so much water overflows from the radiator given that it is not boiling. If you take the temperature rise of the water from say 60 degrees ambient to 190, and the coefficient of expansion of water, this does not I think equate to the volume expelled. As long as the temperature is below 212 the discharge can only be expansion based and not from vapour pressure -puzzled?
J H Cole


My B started expelling coolant (33%anti-freeze) from the filler neck just yesterday. Last week I re-filled with fresh coolant, found the car still running a little too cool (it had been really too cool for a time and I had put it down to the coolant I was then using), replaced the thermostat to find the cool-running problem corrected - but overflow from the filler neck mentioned.

I had the rediator cap pressure tested today - its a 10lb/70bar cap. Pumped to 70bar and we watched the needle decline to zero in a silky smooth slide. That cap looked perfectly good and was less than 2 years old.

Have a new cap fitted tonight and will test run tomorrow. I think the earlier too-cool running was due to a failed thermostat opening too soon. With that corrected temperature was building to levels previously not experienced and the failed cap was letting the hotter, expanding coolant escape.

My suggestion would be to have the cap tested to see if that is contributing. Of course, there may be a number of factors in the case of your car (eg those mentioned above) as well, just as there were with mine.

Roger T

There are two things to check to make sure your cap is operating normally. Verify that the cap and the radiator neck match. Pressure test the cap to see that it both holds the pressure and vents AT the proper pressure.
The included picture shows me testing a 13# cap. It released pressure when pumped above 13# but holds at 13#.

R J Brown

Thanks for all you input folks. Seems like it's normal for an MGA to "wet itself" on arriving at its destination, so a neat solution to this embarrassing problem (especially when your coolant is bright red and makes the place look like a crime scene!)is called for. I will start a new thread entitled "MGA coolant recovery system" to see if anyone out there has come up with a tidy and reasonably cheap way of installing one on an A.
Lindsay Sampford

This thread was discussed between 17/05/2009 and 18/05/2009

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