Needing a bit of advice and sorry for the long post!
I have a 1997 mk2 import VS with around 65k on it which I bought around March this year.
Car just sailed through the MOT with no advisories and is tip top with no rust on the rails or sills.
1 issue I am having (or it may not be) is the coolant was low when I checked it after the MOT and the engine felt quite hot. The temp gauge always stays at around 12oClock and the heaters blow hot after around 5 mins of driving.
I waited for it to cool down and looked in the radiator and that was quite low and there seemed to be a lot of rust in it.
I then looked in the header tank and there was rusty sludge gathering at the bottom corner where the little pipe goes it.
I poked a stick in it and pulled some out and rubbed it between my fingers and it quickly dried and felt powdery and not oily. There is no mayo under the oil cap and no white smoke from the exhaust.
I topped it up with coolant, firstly in the radiator and then the header and went slightly over the max line.
I drove it once since (2 days after topping) and the level was at the low level line and rose after 30 mins driving. When I stopped the engine felt hot again.
So I have the car booked in for a coolant flush and refill today to tray and flush out the rust.
Anything spring to mind what may be the issue so I can suggest this to the garage?
Hopefully it’s just bad maintenance in the past and a good flush and clean out of the system will sort it out.
I had a similar problem a couple of years ago, not on a 5, where I had a problem with a core plug. It allowed oil through to the cooling system, resulting in a brown sludge throughout. No coolant ever got into oil, so that always looked good. By the time we decided to get the head off and find out what was going on, the coolant system was full of oil. A pressure test found a small jet of oil coming through a core plug. It’s all fixed now, but I can’t get the system clean no matter how many flushes it has. My mechanic even tried running washing powder through the coolant system for a few days to break down the oil. 2 years on, the system is still dirty, but it runs well and no problems since.
My 5 had a flush through with holts 2 part rad flush around the same time. I’m not paranoid, but you never know… That’s been clean as a whistle ever since.
As long as there has been no overheat and there is no sign of head gasket failure, your issue should only be coolant circuit related.
Get the garage to check the radiator flow, so run water through top hose port and expect to see similar flow from bottom hose port. MX5 rads do have a tendency to partially block, especially where there is obvious corrosion.
Water is clearly getting out of the system somewhere, so it is either leaking or being expelled through the overflow. Your garage should check all hoses and unions as well as the thermostat and radiator cap.
You should always see clean, usually coolant coloured fluid in your cooling system. Any sign of rust or other contamination indicates a poorly maintained vehicle. Once the problem has been resolved I would spend some time cleaning that reservoir bottle up so you can see immediately what is in the system.
I hope this is an isolated problem and you get it sorted easily.
The garage will be able to determine if the radiator is in good condition. New radiator cap and thermostat wouldn’t hurt, but stick with genuine Mazda, not aftermarket. I switched to waterless coolant last year, haven’t looked back. For me, the main benefit was lack of pressurisation, handy on a high miles 21 year old engine. The garage that changed it uses in their MX5 Race team, and reckoned they were getting better times with it, because the temperatures were held better than with water. Getting it to burp properly was trickey though. Cost probably added about £50-60 to a normal flush and change with a long life HOAT antifreeze.
Just be aware, what you have in the header tank could well be ‘rad weld’, if and when you flush it out you could have a pin hole coolant leak somewhere.
Rad weld does feel dry and powdery when rubbed between your fingers.
Rather than putting washing powder in the coolant, which isn’t really powerful enough and must have foamed up (might it also cause the car to spin?), after flushing the system, try central heating cleaning fluid.
I’ve used it before in another car to disperse rust, it works very well.
Hopefully not caused by head gasket failure. After the system has been flushed out, a new radiator has been fitted and the system refilled, bled and settled down, look for excess bubbling in the coolant (and / or more oil) with the engine running. If there is, it’s a sign that pressure from a combustion chamber is blowing into a coolant channel.