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MG MGB Technical - Coolant goes to tank & not back

Saturday on a 325 mile joy ride, my 78B started to get warm to the point that it worried me. I stopped and the fans ran a short time then went off and the radiator was cool to the touch. I loosened the cap on the expansion tank and coolant sucked back into the radiator. After adding more it ran cooler, but was starting to creep up just as I got home. could this be a bad cap?
Jimmy Chew

Look for the leak!

Classic symptoms of a lack of water in the system.
Chris at Octarine Services

the heater valve is always a leak favourite
Peter

Was the expansion tank full i.e. coolant coming out of the overflow? I've had that on a system with a remote expansion tank when the pump was sucking in air and pressurising the system, I've also heard of it happening when a bottom hose was loose enough to suck in air but not leak.
Paul Hunt

Yes the expansion tank was full and had blown a small amount on the frame. The only place I can find that might be a leak is a little corrosion on the heater valve. I plan to replace it and the radiator/expansion tank cap. Can you think of anything else?
Jimmy Chew

A head gasket leak will also cause pressurisation of the system and transfer of coolant from the radiator to the expansion tank. I eliminated this first on mine as it seemed the most obvious. In either case you will get bubbles of air surfacing in the expansion tank as well as rising coolant level (more than just expansion when warming up). If you use something like http://www.tooled-up.com/Product.asp?PID=134147 or http://www.sjdiscounttools.com/lis75500.html it draws air that is above the coolant though a liquid that changes colour if combustion gases are present.
Paul Hunt

I would agree with Paul that the system needs to be checked for both internal and external leaks. The test kits he references are commonly used by mechanics for testing for the presence of exhaust gasses in the coolant. Professional mechanics tell me that the kits work well, in most cases, but may not catch a small head gasket leak because there may not be sufficient volume of exhaust gas present. In the example here, we have a problem which appeared over a 300+ mile trip. We also know that some water was added to the cooling system, but not how much. So, some thoughts.

The amount of water in the engine needs to be checked by removing the bolt/cap on the thermostat housing (water elbow) and topping up the system. When filling from the expansion tank there are two problems--first, the very limited amount of water, about a quart, in the expansion tank may not fully replace the coolant which has been blown out of the system, and, second, for the water to flow from the expansion tank into the radiator and block requires that the system be sufficiently hot that the coolant will contract within the radiator and block creating a vacuum to draw the coolant from the expansion tank. Better to not take chances and fill the system fully so one is starting from a known standard.

The over heating situation is caused by lack of coolant. The coolant may be lost be either leakage, due to a bad connection/bad gasket/deteriorated part, or the coolant may be forced out of the system due to over pressurization within the system (blown head gasket). Both ways require different testing procedures.

The system needs to have a cooling system pressure tester attached to the expansion tank and the system tested to see if it will hold pressure properly. This is a device consisting of a hand pump having a pressure gauge attached to it with a rubber hose coming out of one end of the pump with the rubber hose terminating in a device which clamps onto the filler neck of a radiator or expansion tank. In use, it is attached to the system, pumped up to the pressure rating of the factory specified pressure cap, and allowed to stay there for several minutes to see what will happen. The system should hold the full pressure for at least one minute. If it does not, you should be able to determine where any external leaks are from the coolant being forced out of the system. Good idea to check all of the hose clamps for tightness, the hoses for cracks, the area around the water pump pulley for leakage and the expansion tank for leakage. (I have seen two expansion tanks develop leaks due to work hardening of the brass leading to cracking.)

I would also check the system, or have it checked, for the combustion gasses using the test system Paul has identified. The kit can be purchased, but I would recommend that the test be performed by a professional mechanic. The test kit is not something which will be used often and the better kits (more reliable) cost more than having the test performed by a professional. Having said that, I do have my own kit and do not feel it was money wasted because I can do the test whenever I want to, including as a part of my annual major tune up, as part of the system checks.

The presence of bubbles in the expansion tank can also be an indication of a blown head gasket. But, it can be an indication of the system expelling air after having been allowed to run low on coolant and then refilled. When looking for bubbles in the coolant, the cooling system needs to be filled properly, the pressure cap set on loosely, and the engine warmed up to its normal operating temperature. Then, slowly remove the not sealed pressure cap and look for bubbles in the coolant in the expansion tank. If you see bubbles, it is an indication that there may be a leaking head gasket. This test, along with the others, should allow you to determine how well the cooling system is functioning.

Les
Les Bengtson

This thread was discussed between 06/04/2009 and 08/04/2009

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