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[John_Clayton]

My Omega starts boiling after about 3 miles. The temperature gauge rises to maximum and the warning light comes on. The hoses all feel hot, as does the top of the radiator. I'm assuming it's a circulation problem. Is it likely to be the mechanical water pump? I believe these should be changed with the timing belts but this wasn't done at the last belt change. Is there an issue with the pumps? Could it be anything else? I'll be repairing it myself soon but am interested in possible causes so i know what to look for. The only thing I've noticed recently is that the electric fans are often running when the engine's turned off after a journey. The temp. gauge hasn't shown any problem until now, however.

[Lazydocker]

Could be the water pump impeller at fault. Could be the stat has failed closed (although that is rare) or could be a blockage in the system

[tango]

do you still get hot air
if no then most likely a circulation problem caused by pump failiure

[Bionic]

My fans run when I turn off too and its quite normal. You dont say if you are losing coolant? I suppose you have already checked to see if the coolant tank is pressurising or there is excessive vapour from the exhaust? Both are signs of head gasket problems. Mine went with no warning at all!
If not then its probably as you suspect. I doubt that its the water pump, they are changed with the belt because the bearings and impeller seal are probably worn, not because the impeller is kaput.
Another problem is that it might have air lock and needs to be bled off.
Bit more info would help....

[John_Clayton]

Thanks for the suggestions to date. I noticed the water pump appeared to be leaking slightly so changed that but the problem still persists.

To answer your questions - yes I do get heat from the heater, no there doesn't appear to be excessive water vapour from the exhaust and, yes I am losing a lot of coolant. It appeared to be exiting from the cap on the expansion tank but, on closer inspection, I now find it's coming out of a crack at the base of the filler neck. I tried some Araldite as a temporary fix but that didn't work so I'll get a new tank.

I'm also going to change the coolant temperature sensor as the radiator fan doesn't start, even with the temp. up around 100 deg. F. I've tested the fan and it works OK with power applied directly to it.

The only other relevant part I haven't yet looked at is the auxiliary electric water pump. What is its purpose and could it cause my symptoms?

[henryd]

If your expansion tank is leaking then change that before doing anything else,the pressure in the system raises the boiling point of the coolant