Lotus Elan and Lotus Elan Plus 2 Forums

On my rebuilt Elan S4 she runs fine with the heater on but after a while the heater goes cold!!
I suspected an air lock but leaving the rad cap off overnight made no difference.
I have run the engine up with no rad cap on and water flows across fine.
Should the radiator cap be the highest point in the system?
Mine seems to be lower (looking from the front) than the hose from the thermostat housing!!!
Could this mean I have air in the system which cannot get out and is ending up in the heater matrix?
Any thoughts gratefully accepted.
Ian

Hi Ian

Air can get trapped in the system but this may not be your problem

The heater relies on the pressure differential between the bottom of the thermostat housing and the pump suction to achieve water flow through it. If the thermostat fully opens then that pressure drop is minimal and the flow through the heater reduces substantially. If the heater core is blocked with deposits then that reduced flow may be insufficient.

If you disconnect the heater hoses and flush out the heater core you will probably improve the performance.

To run the radiator and engine cooling system full with no air in the system, at least on later cars, Lotus originally used a recovery cap and bottle so that as then engine heats up and the coolant expands the air comes out of the system and separates out in the bottle and when the engine cools only coolant is sucked back in. These recovery caps and bottles get lost overtime on many of the cars

Some people run the engine with the front of the car jacked up to make the radiator cap the highest point in the system to make it easier for the air to come out when they run the engine and get it hot the first time to get all the air out.

There are various other mods including the cliveyboy system ( cliveyboy.com) which puts the cap at the top of the thermostat housing or the 26R header tank in the top hose approach (sold by TTR and others). I have also seen air bleeds fitted to the top of the standard thermostat housing. All of these mods may help but in practice the standard system in good condition works Ok in most circumstances

cheers
Rohan

Thanks for your reply Rohan - jacked her up at the front as you suggested, started engine and managed to get half a kettle of water into her!! Just been for a run and all seems fine. Heater works. Don't really need it on as it's roasting here today but my temp gauge is away being repaired, and it's a way of checking that all is working as it should.
Thanks
Ian