Possible sticking thermostat?

[newkid]

My mums 1.6 diesel has had just about every water component changed over the last 12 months, water pipes to stainless steel, new water pump, new radiator and switch. Today I bleed the system till no more bubbles are coming out the new radiator, the heaters are getting slightly warm and the new needle after idling for 30 minutes gets to just about the area marked with the white line. If you go for a short drive the needle gets were the red warning light is. Once you stop and it sits idling again the temperature needle drops back down to the white marker. I've checked both pipes at the radiator and even straight after the short drive both are stone cold, the pipes in the engine bay are hot but not to hot to touch. Could it be the thermostat (the only part I haven't changed) is sticking open and causing the system not to stay at optimum temperature.

Cheers.

[kevtherev]

If the thermostat stays open then the temperature would never reach optimum.
It would languish at the cooler end of the scale

I've checked both pipes at the radiator and even straight after the short drive both are stone cold

this suggests the thermostat has not opened, or you simply have not drove it long enough to get the coolant up to temp

The heater circuit has nothing to do with the radiator circuit, it comes directly off the engine at No1 cyl. through the coolant distributor
If the heater matrix is not getting hot then there is a blockage (gunk) or the valve has not opened properly... or the thermostat is stuck open

If you go for a short drive the needle gets were the red warning light is

This is confusing as it describes where the needle should sit under normal conditions.
If the needle drops lower then the thermostat isn't closing

[newkid]

Not sure if this helps Kev, both pipes at the radiator were stone cold, no heat at all. The 2 thinner pipes at the heater matrix were hot. when driving and the needle in the middle the heater blew hot, when we stopped and starting idling again the heater still blew hot but not as hot and the needle was dropping, if u gave it a rev the heater got hotter.

[kevtherev]

Again the heater matrix is drawing coolant from the engine, if the heat drops away, then either the flow through it is too slow or the thermostat is fully open or it's gunked up...

So if the rad hoses are cold then the thermostat hasn't opened or the rad is blocked.

there are two confusing pieces of information here.
Time to eliminate the obvious
Take the stat out and test it

[kevtherev]

test
opens at 87 deg iirc

[newkid]

Im a little lost here, the radiator is brand new so I would presume that eliminates it from being gunked up, also bleeding the radiator was extremely easy, with the revs held at about 2k the bubbles pushed out and then we had a steady flow of coolant.

[Oldiebut goodie]

The heater circuit has nothing to do with the radiator circuit, it comes directly off the engine at No1 cyl. through the coolant distributor

Forward end of the cylinder head - it's a diseasel.
Sounds like the thermostat is stuck open to me also.
Be prepared for a possible replacement of the plastic thermostat housing as several people have found that they are distorted with age and heat and it is nigh on impossible to get it to seal correctly again. Mine was oval when removed and prevented the o-ring sealing fully, others have found that the face is distorted out of true and also causes trouble with the seal. Available from Brickwerks. £5.86 http://www.brickwerks.co.uk/index.php/t ... iesel.html

[newkid]

Ok ill buy a new thermostat and try that, the housing should be ok as it came of fine a few months back when I changed the waterpump for a brand new item but ill get one just incase.

Cheers for the help, i'll let you know how I get on.

[Plasticman]

also hose correct at the rad ? and have you bled it the correct way
mm

[newkid]

Needing a quick answer if possible.

Finally got round to changing the thermostat and the thermostat housing today, both came with new rubber o rings, but when I removed the old one I could only see one o ring, do both get used or just the one?

Thanks