The problem is as follows:

Air is getting into the cooling system from somewhere, and the coolant reservoir bottle gets full to overflowing.
There is therefore not enough coolant in the cooling system and the engine then begins to overheat, which is not good.
A side-effect is, of course, that the heater does not work.

I have taken the car down to TGM and they pressure tested it and did a head gasket leak test, both of which it passed.

I have also changed the radiator cap which has not fixed the problem.

I also got Honda to change the coolant completely yesterday in an effort to fix it.

I can keep bleeding the system using Billman's excellent guide here but this is only a temporary fix.

Does anyone have any ideas what the cause might be, please?

Thanks

 

No coolant loss?

 

No (other than out of the top of the reservoir).

This has been going on for a while now and there are no signs of any coolant on the garage floor. My "fix" this evening involved decanting the coolant from the reservoir back into the radiator.

 

Has the head become porous? I know you have said you have been pressure tested but your issue reads like k series issues that are common in certain lotus derivs.

 

You need to test for gases in the coolant to have a better idea.

 

If the coolant is ending up in the expansion bottle then it's either over pressurizing and forcing it out, gasket or thermostat,, or not holding pressure and it's expanding out as it heats up, has it got oem rad and rad cap.

 

Do any of your coolant hoses look perished?

 

All, thanks for the responses
Chesh. It's been both pressure and gas tested at TGM.
Ray. OEM Radiator (replaced a few years back) and new OEM rad cap. Is it worth getting the thermostat checked, too?
japcrap (Alex). Hoses look ok although they are pretty old now of course. Would the pressure tests not cover this, though?

FWIW it had been ok for a few weeks (before and after TGM) but was made worse again by the replacement coolant yesterday. I suspect the system may not have been properly bled. I'll give it a proper go tomorrow with a the car on an upslope and see if that helps.

 

if it's only started again after the coolant replace then that is more than likely the problem,

 

When you said that air is getting into the system I thought that if a coolant hose was perished then that could be the entry point. The hose/s could be perished enough to let the air in but not so much that they allow enough air to escape to fail the pressure test. Just a theory really based on my knowledge of automotive rubber hoses. Quite easy to visually check them and should be cheap to replace.

If they all look ok I'd be checking the radiator and thermostat next.

Did Tom say that they couldn't find any problems so try somewhere else essentially?