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So it seems I'm losing coolant at a fairly fast rate. Had the car at the last track day in Feb (I think) and the scrutineers picked up I had none. Topped it up with water and had no problems.

Soon after I had the car in for the regular service and just told them to top up the coolant. Also no problems there.

Today I noticed I'm out of coolant again (well, in the reservoir anyway... there's still coolant in the radiator), and it's only been abotu 2000kms since the service when it was topped up full.

Does anyone have any suggestions for stuff to check to see where I could be losing it? I'm fairly sure it's not a leak in the bottle as there's no residue around the bottle, or along the piping from the reservoir for similar reasons. Where/what else can I check?

Just a suggestion.

I had a similar problem in a car I once owned.

Turned out to be a leaking cylinder head gasket.

Assuming there is no evidence of water in the oil or vice versa, the most effective way to find a leak is to do a pressure test.

JBT wrote:Assuming there is no evidence of water in the oil or vice versa, the most effective way to find a leak is to do a pressure test.

Yeah, that'll be on the cards if no one else has any other DIY suggestions.

marcusus wrote:Where/what else can I check?

short answer: EVERYWHERE

jack-up the car, take off the undertray and check around the radiator, bottom hoses, water-pump and the heater-hoses at the back of the engine.

quick guess would be that the gasket in the upper or lower radiator tanks are dying.

Oh, and check under the dash just in case it's leaking at the heater core. Feel around for damp carpet.

Take out the undertray. Turn on the engine, drive round the block and look around. Last time I had a leak, I fixed up one hose, went to Sliq's place and out came another one, all i saw was green coolant over my cam covers.

short answer: EVERYWHERE

What he said. Look for green water or white traces of burnt off coolant on or around all the coolant hoses and clamps. Use your nose as well as your eyes. Burning coolant has a very distinct smell. A pressure test will help sometimes but not always. My car had a very slow coolant leak for years. I had several pressure tests done but no leaks were found. The top radiator tank started to go off and I had that replaced hoping that was the cause, but no. A few months ago doing another job I noticed that the clamps on the small cooling hoses around the throttle body/oil cooler had white coolant residue on them. I replaced them and no more leaks or burnt coolant smells.

The advantage of using a pressure tester is that you can try it hot or cold and you don't have the issue of a running engine to contend with. This means you can look in and around all the suspect areas and you can depressurise and pressurise the system as required without a lot of the associated stuffing around and possible personal injuries.

The problem with coolant leaks is that sometimes you fix one point of leakage and then it will start leaking at the next weakest point.

What Garry and 1red5 said, also, if your 5 is old, then chances are, so is all the rubber hosing, I replaced all my 19yr old rubber hoses recently as a preventative measure.

Cool, good tips there. I'll give it all a bit of a go over on the weekend with both the advice on here and that on miata.net. Hopefully I can find what's causing it and get it resolved before the track day next weekend. Although the car ran no problems with only water in there last time, it's better to have it fixed properly anyway.

nothing to add on the techo tips but just recently I have been reading a little on coolants. Coolant does need to be changed periodically and if you are buying concentrate then add demineralised water in correct ratio. The glycol is also very nasty stuff which is supposed to be disposed of correctly and is poisonous and attractive to pets ( Garry remarked on the smell). I am now a bit of fan of wearing eye protection and gloves.

I think coolant is sweet. I had some thief steal some of my used coolant and tried to drink it. I hope he drank it all.

Welcome to Fuckedengineville. Population: 2, you and me.

Really? What's happened to your engine?

Yes coolant is sweet dogs, cats and little kids love it and die.

If your kids happen to drink it by mistake the cure is a bottle of vodka.

For me one of my heater hoses were split and I ended up losing a lot of coolant that way, I saw it as it dried up it leaves a white residue.