There are lots of places for coolant to leak, but XM's have a habit of splitting the reservoir along the front for some reason. The best fix is a new reservoir and they are still available, but the part now supplied is not the same as what the car originally came with. Unless you can find NOS of the original style, can repair what you have or want to make something up from scratch, you will need to buy a few extras with the new reservoir.
The new reservoir is a different design and does not have an overflow tube like the old one, so the overflow is no longer used and it just runs out a 'spillway' at the back of the cap. Earlier cars need only one port, so the second port has to be blocked, using either the factory rubber plug or by tapping for an M6 bolt.
The new type cap is common to various other models and is of a different pressure and design to the old one. The old one fits, but the new style is designed to reduce the chance of a scalding injury as it lets you undo it about 45 degrees to release the pressure and then turn it further, which is stiff and needs a little downward pressure, to remove it.
The level sensor is like the one on a C5, but shorter, so you can't pinch one from a C5. It needs to be replaced because the mounting for it is totally different and the old one doesn't fit the new reservoir. It's a $100+ part locally, so you either buy it, try to cut down a used C5 part or fit a blanking plug as fitted to some mid-production C5's that had the sensor removed in a recall.
To change to the new type reservoir, the parts needed are:
New style reservoir 1307.HS
Level sensor 1306.F0 (The parts list suggests 1306.86, but it's supercedes to 1306.F0)
Reservoir cap 1307.C7 (Inexpensive)
Pipe cap/plug 1436.17 (I think. I used an M6 bolt and some hose instead)
(The parts diagram does tell you to also order the sensor and cap, but it's easily overlooked.)
You don't need to drop the coolant to change it. It will probably be low anyway, but just keep the hoses elevated and little coolant will be lost.
The new reservoir is a different design and does not have an overflow tube like the old one, so the overflow is no longer used and it just runs out a 'spillway' at the back of the cap. Earlier cars need only one port, so the second port has to be blocked, using either the factory rubber plug or by tapping for an M6 bolt.
The new type cap is common to various other models and is of a different pressure and design to the old one. The old one fits, but the new style is designed to reduce the chance of a scalding injury as it lets you undo it about 45 degrees to release the pressure and then turn it further, which is stiff and needs a little downward pressure, to remove it.
The level sensor is like the one on a C5, but shorter, so you can't pinch one from a C5. It needs to be replaced because the mounting for it is totally different and the old one doesn't fit the new reservoir. It's a $100+ part locally, so you either buy it, try to cut down a used C5 part or fit a blanking plug as fitted to some mid-production C5's that had the sensor removed in a recall.
To change to the new type reservoir, the parts needed are:
New style reservoir 1307.HS
Level sensor 1306.F0 (The parts list suggests 1306.86, but it's supercedes to 1306.F0)
Reservoir cap 1307.C7 (Inexpensive)
Pipe cap/plug 1436.17 (I think. I used an M6 bolt and some hose instead)
(The parts diagram does tell you to also order the sensor and cap, but it's easily overlooked.)
You don't need to drop the coolant to change it. It will probably be low anyway, but just keep the hoses elevated and little coolant will be lost.
