Radiator knocking noise

Hi can anyone advise!!!

My 1981 S3 SWB 2.25 petrol engine starts and run and the tempurture guage doesnt show it to be overheating, but when its gets to normal tempature around half way up the guage, water starts to leak out of the top of the radiator valve, having recently replaced the heater valve and flexi heater pipes I had to refill the water. At first I thought I had over filled the radiator so drained some off by dipping my fingers in the header tank with the cap off. The water was only warm and not hot or boiling! Than a knocking noise developed from the radiator, this noise continues even when the engine has stopped. Also I get engine over run when switching off when this noise is present. I susspect the carburator needs setting up but thought this was about right. The engine doesnt seem to suffer from loss of power as it revs o.k and has torque when pulling away. There didnt appear to be any water in the oil and there is no milky gung under the rocker cover. Any ideas?

Did the problem only occur after you had replaced the heater value and pipes? It could be that you have an air lock in the cooling system.

IIRC the heater hoses from the block to the heater unit on an S3 are a little bit higher than the top of the radiator. Last time I did this I undid a hose clamp on the heater hose and let the air escape from there.

Hi Andy,

Its almost certainly an airlock I susspect. I have only just noticed this and was going to go all over the cooling system at the weekend. I was going to flush the whole system just to make sure everything is good?

Take care.
The "knocking noise" could be steam dissolving in water due to a combination of local over-heating and poor coolant circulation. It's a noise quite common in wood-burning boilers if they heat up faster than the water can take the heat away.

Are you SURE the thermostat is the correct type, and that it is working?

Have YOU had THAT thermostat in a pan of water with a thermometer and heated it up?

If NOT, how can YOU be sure it works? You can't ...

CharlesY

Thanks CharlesY, I dont know the history of the engine and thats why I'm going to systimatically check the whole thing to be as sure as I can be. I didnt know what could be causing this which is why I sought the wisdom of this forum so appreciate all the suggestions. After all its only a cooling system which consist of a pump, theromstate and radiator plus the heating(famous last words and all that. Once I know whats what I'll provide an update.

You missed out the most important bit - THE BOILER!

You need to be certain that the coolant is circulating so that boiling is prevented, especially what they call "hot spot" boiling.

There are two circulation routes ...

1. from the engine (boiler) through the thermostat to the radiator to transfer heat to the air

2. when the thermostat is closed, through the BY=PASS pipes back the the engine "boiler". If this continues for more than a few minutes the coolant will boil for sure.

You need to be sure the thermostat starts to open at about the right temperature, AND that wthin a couple of degrees more it is wide open.

CharlesY

O.K this was caused by two problems, the first and main cause was a duff thermostat and the second by a failed heater hose which I hadn't replace earlier. Now the motor runs without overheating, thanks for all the help.

Regards.