Heater blows cold
 

The heater blows cold in my 1998 disco. What would cause that? It use to work fine.

Low on water !

X2, this can be a serious condition, you either have a hose/clamp leak somewhere, or worse, a head gasket leak. Your year D1 has no heater on/off water valve, the coolant always circulates thru the heater core and dampers allow or mix air thru it based on you turning the knobs on the dash.

Also important to note that if coolant is low enough, thermostat will be uncovered and just in a steam cloud. The gauge will show normal, but the engine will grossly overheat. The pix shows the stat hole, and the gauge sensor to the right. The heat hose comes off the intake as well. So if so low there is no flow to heater....

The "output" to heater core is at top left of photo. So it does not take a lot before water is so low it also is not keeping thermostat immersed and the coolant sensor that sticks down thru the roof of the intake mainfold is just hanging in steam. You can see from the red line in the photo that it is not far between "no heat" and "no thermsotat working correctly" and "no gauge to warn you".

EDIT - as noted, if the stat has been run hot without benefit of water around it, who knows if it has been "changed". These type of stats are very cheap, like under $10, two bolts. Available in 192-195 (stock) and 180, and 160 - my experience is the alternate 160 is too cold.

1-make sure the coolant level is full
2-replace the thermostat

3. Keep an eyeball on the coolant level for the next few days. It it got low once, it may continue.

Any brand of coolant? Do I need new bolts for thermostat?

Any brand 50/50 premixed coolant, I like to use Peak, Prestone, Zerex or Wal-Mart brand (Peak).
The "X" inside the coolant tank needs to be covered.
No you do not need new bolts, but you do need a new gasket, DI tech section.

Thanks spike.

When I squeeze the hose from the radiator to the thermostat it is hot but doesn't feel like anything is in it.