why does my water temperature go down when i switch my heater on?

[GTI-JAY]

why does my water temperature go down when i switch my heater on? any suggestions? and is it good or bad that the temp goes down to 70 deg while driving long distances?? cheers

[sharki786]

my golf does the same when driving with heater on, the temp will drop to 70ish and when stationary the temp will rise back to 90

[clipperjay]

Dam it wrong place don't let them know I'm in here!  
The heater matrix is just another small radiator so if you can imagine when hot water from the block gets relived into the heater matrix the fan blow hot air into your cabin is cooling the water, as it goes back into the block cooler than going in, hence temps will go down a bit.
Its an old trick when your car used to overheat in traffic you would wind down all windows and sunroof then blast the heater on full it helps to keep the car cooler, but you die in the summer doing it  

[GTI-JAY]

so its normal then? just cant remember it doing that before i suppose it doesent hurt the car to have the water at a cooler temp dus it??

[Khare]

^^ but the heater matrix is not enough to drop 20C.

Your problem is the thermostat. Sharki's problem is the classic one, driving the temp will drop to about 70C, while stationary it will rise to normal temp. This is because there is no air flowing through the radiator cooling the system too much, which the thermostat prevents.

[GTI-JAY]

ok thanks il look into changing the thermostat 

[clipperjay]

Yes if your engine oil doesn't go over 100C.
Some people notice the MFA showing an increase in temps and it could be a sign of a water pump slowing down or a faulty thermosat as the water flow is not fast enough to push it through your radiator. Just keep an eye out for overheating very quickly as you want to catch that water pump stat sooner than later.
Plus a radiator flush might help to clean out any sludge in the system I recomend Holts two part flush. Remember to only use condensed water to minimise mineral build up!    

If the thermo change out, doesn't sort it, it will be the water pump mate!

[GTI-JAY]

ive never seen the needle move beyond 12 o clock think thats 90 deg?? lol but il bare that in mind for future ,  how hard is it to change ?? do the thermostat cost much?

[Wayne]

+1 sounds like a thermostat problem

[sharki786]

mine has always been like that for 3yrs (thats only if the heater is on, but if its off then the temp stays at 90), the oil temp sits between 80-90deg, i thought it was a good idea (keeping the engine cool)