I did a search and didn't find anything quite the same (though https://www.rx7club.com/showthread.php?t=914328&highlight=Coolant was sort of close).

I have an FD that, cold starting, the add coolant light and buzzer come on. I topped it up, but it only took about 200ml (1 cup roughly). This happened a few times- then I was off finding something and left the car running and noticed the buzzer went off when it hit regular running temperature. I tried again this morning and noticed the buzzer and light went off and stayed off as soon as it got up to temperature.

Could this mean the coolant mix is wrong/unsuitable? It's not freezing weather or anything, just a regular morning.

I see a lot of people suggesting air in the coolant system where there are buzzer issues- is this likely to be the case if the buzzer goes off as soon as the car is warm?

Ideas or help much appreciated

 

Definitely not the coolant mix. The coolant buzzer does one thing and one thing only - it buzzes when the coolant is low.


Think about your scenario a bit. When the car is cold, the thermostat is closed so the coolant does not circulate through the radiator and instead gets re-routed through the engine so that it warms up quicker. Once the coolant is up to operating temps, the thermostat opens and coolant flows into the radiator. Hence, if your coolant is low to begin with, it will likely become lower once the thermostat opens.

Air in the coolant system could be a cause of your issue but I suspect that you might have a small leak from one (or more) hose clamps that may be old and are no longer sealing properly. Or the coolant hose itself could have become hard enough where it doesn't seal sufficiently.

At this point in the car's age, you will want to replace all of the coolant hoses and hose clamps if you haven't already. Here in the US, we can get the full coolant hose kit with replacement stock hose clamps for under $200. The stock hose clamps are the way to go.

There is a very long informative thread in the 3rd gen FAQ about the cooling system. Do give it a read to educate yourself further.

 

See, this confuses me. I would assume that would mean that the buzzer would come back on later, but it doesn't.

I am pretty sure the hoses have been replaced recently (like within the past 3 months), so will check the clamps and see what I can find, plus read that coolant thread.

 

The senario is a little different. Only speculation.....but it MIGHT be a case of the coolant level being on the low side when cold but not quite enough to set-off the sensor's buzzer. Then as it warms (but before the thermostat opens and before it rises to the level of the overflow outlet) the coolant expands (from heat) sufficiently to stop the buzzer. Then the buzzer goes off again when the thermostat opens and the overall level falls again.

What mods do you have? AST in place? Do you hear any gurgling or boiling sounds when it's hot and you shut it down?

 

I'm Not an FD guy,but I read a bit awhile back that the FD's Cooling system is a Finicky little Brat!
The AST on the system seems to be the culprit of a lot of Woes that the FD owners encounter.
I would search that out and see what you can dig up on that subject.
I hope that helps.

 

SgtBlue did a good job elaborating on some thoughts that I had.

A coolant system pressure test would be a good way to look for any leaks and tired clamps and brittle hoses that are causing leaks or sucking in air when cooling off. See the fAQ

 

With the possible exception of the AST and a couple of inexpensive hoses going to the turbos that should be changed occasionally, the system isn't much different than any other car. The plastic OEM AST can fail with heat and age, but if replaced with a metal one it's not a problem.

 

Just an update on this...
The car did start to lose more water (not that I could see any on the ground or anything, but in topping it up it was starting to take quite a lot). The pressure test showed a leak in the water pump.
So now the problem is diagnosed, means we can get to work fixing it!

Thanks all for your assistance