QuestionQUESTION: My '76 MGB was keeping cool, with the needle never quite reaching the 'N' in the middle of the gauge. Then, for no particular reason, I added a bit of coolant while I was under the hood messing with various other stuff. Ever since, the gauge always reads hot. As soon as I start the car, well before it has a chance to warm up, the gauge is past the N, and it spends the rest of the time almost at the H. Would excessive coolant cause this, or must it have been caused by something else I did at the same time?
ANSWER: Hi Billy. The 76 B has a remote header tank, and it is important that it is kept half full, and no more. There is a strap round the tank. try to keep the level at the top of the strap. I don't fully understand why, but I have heard rumours about syphoning occurring when the header tank is over-filled.
Check that the radiator is full to the brim. With a cold engine, take the threaded bung out of the top of the rad. There should be no air inside the radiator.
After that, if the temperature gauge remains on the hot side, you need to investigate further. Could be a faulty voltage stabiliser. Is the fuel gauge reading high as well? They share the same stabiliser.
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QUESTION: My car doesn't have the external tank. I don't know that it never did, but the information I've found says the tank started in '77.
When the engine is cool and I pull the cap off the radiator, the coolant level is just a bit below the filler. Before I added more, of course, the level was lower.
AnswerHi Billy. Just checked my build records. You are correct. The 1800 MGB did not get a separate header tank until 77. The V8 had one from 73, hence my confusion.
Your radiator should have an air gap at the top of the tank. If you over-fill it, the coolant will expand as it gets hot and the excess will be lost via the overflow tube that exits by the filler neck. As it cools down, it will suck air in, so it will then have the right level after it cools down.
If your gauge has started to read high, there is something wrong. Could be a faulty filler cap that is no longer airtight. Could be the thermostat. Could be the voltage stabiliser that I mentioned earlier. Could be a loose fanbelt so the water pump pulley is slipping.
Another thought. It is summer. Have you recently turned the heater control to the off position? The heater radiator acts as a secondary cooler, so with it shut off the main radiator has to work harder. I have seen several MGBs with radiators that are past their best (fins corroded, slats bent shut, silted up internally ...). They continue to work OK but they have become marginal and rely on the heater matrix to prevent overheating. Perhaps you need a new radiator?
