These cars do not overheat when everything is working properly. They quickly warm up to about 180F (needle horizontal on the gauge) and stay there, and the coolant reservoir will stabilize at about half or three-quarters full. However, these cars are also old enough that often everything isn't working properly.
The most common causes of overheating are:
- Air in the system, cured by bleeding as described below. If you're finding that you're losing coolant and have to bleed repeatedly, you may have a small leak somewhere.
- Clogged radiator, not unusual in an old car. The cure is to remove it and have it rodded out by a radiator shop, or replace it with a new aftermarket unit.
- Bad thermostat.
- Water pump impeller vanes are rotted away, or water pump belt is slipping.
Refusal to warm up
This is nearly always caused by a stuck thermostat.
Filling the system
Like any reasonable car of the era, the X1/9 uses a 50/50 mix of antifreeze and distilled water. You can top up with tap water in emergency, but if you add more than a pint or so you'll want to drain and refill with the right stuff when you get a chance.
To fill the system:
- Open the bleeder valve. It's on top of the radiator on the left side of the car; you reach it from the front of the front trunk, through a small access hole in teh corner. It takes an 8mm hex socket, and just a turn or so should open it.
- Turn the heater control to full on to open the lines to the heater
- Slowly fill the system through the overflow tank in the engine bay. When coolant starts to drip from the bleeder, close the bleeder, top up the coolant, put the cap back on.
The first time you start the car, watch the temp gauge carefully. If there's any sign of overheating, stop the car, let it cool, and bleed as described below.
There will still be air trapped in the system, so you'll want to bleed it once or twice after you've driven it a few times.
The system is designed that normal water circulation will sweep trapped air into the radiator. There's no circulation through the heating system when the heater valve is closed, and no flow to the radiator unless the thermostat is open, so to do a thorough job of bleeding you'll have to have run the car with the heater on and until it reaches full operating temp once or twice.
To bleed, you start with the motor cold and the car parked on level ground. Remove the cap on the coolant reservoir, open the bleeder, and listen to air hiss out. When it starts to flow coolant instead of air, close the bleeder and top up the coolant level in the reservoir.