Smiths heater control valve
 

Odd fault after a wholly satisfactory 280 mile run in warm weather, on returning to my garage steam was arising from bonnet and lots of coolant being spilt. Opening bonnet revealed steam and water spray from side of heater box. Stopped this by selecting hot on cabin heater control. Allowed radiator to cool down before topping up with 1/2 gallon coolant. Restarted engine and noted slight drip from heater control valve. Does anyone know what other cars use this standard looking 3 port valve so that I can source a replacement? In the meantime is there any harm in diconnecting the two hoses to the valve and blanking them off to isolate the valve and heater. I do not need any more heat in the car in summer!!!!

Chris,
There is no harm in bypassing the heater valve.

What car do you have?
Regards,
Kevin

If it's a 408 or uses the same valve, I just bought a Jaguar C16559 for their Mk2 from SNGBarratt that seems identical. You need to order a new donut washer too to fit between it and the heater.

David

Good find David. It's the same as the one on my 411 S5 (see photo), before I fitted air con. It seems the price can vary quite a bit, so it's worth shopping around.

Chris, make sure you don't undo the two small bolts that fix the bracket onto the heater body - the nuts behind them are not captive!

If my memory serves me correctly, the job will be easier using small BA spanners.

Heater valve
 

Kevin and David, my car is a 409 Mk2. The valve looks identical to the 411 one pictured so will order the Jaguar one and a donut as advised. Meanwhile I intend to disconnect the hoses and rejoin them with a piece of 1/2in alloy tube thus maintaining flow back to block via inlet manifold connection. Is this OK?

That's what I did on my 410 and it worked fine.
I found getting a decent seal between the valve and the tail/pipe off the heater matrix was not straightforward and it was suggested by marque specialists that a blob of sealant is commonly used to good effect.
This what I did and no ill effects or leaks since.

Chris, yes that will be fine.

There one thing about your original post that is niggling me - that's the description of steam and lots of coolant being expelled. I was going to suggest you check that the thermostat is working properly, but if it wasn't opening there is no way you could have done a 280 mile trip without the engine overheating.

heater control valve
 

After the failure of my 409 Smiths heater control valve, I have bypassed it by joining the connecting rubber hoses with a piece of 1/2 tubing. Tested by running around the town and temperatures normal at indicated 90 deg C. Tried to remove faulty valve by disconnecting cable and heater baffle linkages then removing the 2 2BA nuts holding the valve flange to the thin steel bracket attached to the front of the heater box. Valve stuck solid on bracket despite gentle persuasion to break gasket seal with a sharp chisel. Warned not to remove bracket screws as nuts behind inside heater box are not captive. Is there some additional hose connection inside the heater box to the valve? I thought it was like a Jaguar heater valve with just a chamfered face on the rear of the flange sealed with a rubber donut type washer? Any ideas gratefully received. The nearest new valve I could find with what I think are the correct arrangement of coolant connections is for an Austin Healey 3000.

From memory the only thing on the back of it is the tail of the heater matrix.
I/we took the whole heater box out as we were redoing the seals, cleaning up the fan motor, refurbishing the matrix and so on, so it was easier to manipulate when out of car. I'd suggest being gently brutal twisting the control valve left and right but perhaps wait for a second opinion.
Good news that you've identified the problem though.
When working through the car last year I tended to get a lot of my parts from SLJ, Pegasus or Bristol Classic Car Parts (in no particular order).