Calling armchair mechanics...

[vrsmxtb]

I have a post going in the car specific forum but not having too much luck (apart from the very helpful anicecupoftea) there, I'd really appreciate some opinions on a cooling system problem I have.

It's on that PH fave, a Mk1 MX5.

Basically I've been losing coolant, mostly from the overflow tank overfilling. On one long journey in warm weather I also experienced overheating with the temp guage moving high (they are notorious for only moving once temps are silly high)

I have changed all of the following now, in this order:

Rad cap
Thermostat
Radiator / Water Pump / Two of the main rad hoses
Coolant flush with Holts radcleaner and backflush through block with hose.
Engine coolant Temperature Sensor

I thought I'd made a breakthrough when someone pointed out that my cooling fan wasn't coming on, so changed the coolant sensor and it cuts in fine now. But after a drive on a warm day with a fair amount of traffic idling I came back to the car the next day to find the expansion tank had lost all water again.

I've had two sniff tests done, both negative - hoever i'm concerned as my dipstick oil level seems very high when checking from cold, plus I get quite a lot of water droplets from the exhaust once the engine is warm. There are no signs to the best of my knowledge of oil in water or vice-versa though...

I ran it without the 'stat and with the cooling fan bridged permanently on for a while and it seemed to behave itself in that time.

It's really doing my nut in, could it be a head gasket leak that only opens up at high engine temperatures? It used to definitely overflow before, but now the expansion tank level still rises after a drive but not to the top, yet still seems to lose the water from the tank back to the radiator after a longer/enthusiastic/warm weather/motorway speed drive, although the temp gauge stays at normal levels at all times now. No obvious puddles under the car either.

Help?!

[OldSkoolRS]

I'd be tempted to try another thermostat as if you've overheated with the current one it may be damaged. They're cheap enough and usually not hard to fit. The other thing is to check on the position it has to fitted: Some have a little hole that needs to be in a particular position (usually highest as it's to help with airlocks I understand). Otherwise I'd mention that you should usually top up the coolant to a the line with the engine cold (unless the MX5 manual says differently) and the level will rise with the engine hot.

Failing that, try changing the door hinges as you seem to have replaced everything else.

EDIT: You don't have damp carpets at all do you? As it could be the heater matrix leaking.

[vrsmxtb]

OldSkoolRS said:

I'd be tempted to try another thermostat as if you've overheated with the current one it may be damaged. They're cheap enough and usually not hard to fit. The other thing is to check on the position it has to fitted: Some have a little hole that needs to be in a particular position (usually highest as it's to help with airlocks I understand). Otherwise I'd mention that you should usually top up the coolant to a the line with the engine cold (unless the MX5 manual says differently) and the level will rise with the engine hot.

Failing that, try changing the door hinges as you seem to have replaced everything else.

EDIT: You don't have damp carpets at all do you? As it could be the heater matrix leaking.

Thermostat is a good shout and one I was thinking of doing again as I don't think the one that was replaced is OEM spec. It overheated on the old 'stat, so this one shouldn't be damaged. The small valve is at the top for sure.

No damp inside, and the heater matrix is on an open circuit so would get coolant loss consistently, not just after high engine temp drives.

All things seem to point to weird head gasket leaks or cracked head/block now, all other bits replaced!?

[Ozzie Osmond]

I'm skeptical about the thermostat. You could just remove it and see what happens - which would either confirm or eliminate beyond all doubt. But since they're chap it would be tempting to just stick in a new one once you've taken the cover off.

As for oil level, why not just do an oil change and then watch carefully?