i have a 1994 discovery 300 tdi auto, yeaterday i had to replace the waterpump due the bearing going, so replace with a geniune one, today after getting up to temperature, put the heating on and was blowing cold air, then after driving a bit further the temperature guage went right to the top for about a minute, so went to pull over then all of a sudden the temp went back down to half way, and the heater was blowing hot air, its done this twice today! can anyone shed some light on this please?????

 

You have air in the system, you need to bleed all of the air out.
Fill the coolant resevoir, turn the heater on full hot, with the coolant cap off start the engine and run it up to temp.
Once full operating temp is reached and you have hot air coming out of the heater put the coolant cap back, shut off the engine and you are good to go.
Check the coolant level a few days after driving to make sure it is still full.

 

thankyou for the advice will do that this morning

 

have bled the cooling system and alls good thanks

 

Thank you for reporting back.
Glad to hear it all worked out for you.