What temp should a TD5 thermostat open at?

Galen · May 7, 2010

I've flushed my cooling system out, put on a new thermostat and have taken the discovery for a spin.

It took about the same time as usual for the temp gauge to wander up toward the middle of its range and for the heater to start doing something useful, so nothing obviously untoward.

I've just parked up and lifted the bonnet to see if the water level is still ok and that there's no obvious leaks, and checked the radiator. The radiator is absolutely stone cold. There's some heat transfer at the feed pipe, but the main radiator and the outlet are cold.

The coolant in the header tank and all of the pipes coming off the engine are at or around 61C at the moment, so the coolant is taking heat from the engine.

What temperature do I need to get the coolant up to in order to get the thermostat open and coolant to flow through the radiator?

Headtester · May 7, 2010

From Rave:

"Thermostat - Main valve

The thermostat is closed at temperatures below approximately 82C (179F). When the coolant temperature reaches approximately 82C the thermostat starts to open and is fully open at approximately 96C (204F)."

However there is also the By-pass flow valve:

"The by-pass flow valve is held closed by a light spring. It operates to further aid heater warm-up. When the main valve is closed and the engine speed is below 1500 rev/min, the coolant pump does not produce sufficient flow and pressure to open the valve. In this condition the valve prevents coolant circulating through the by-pass circuit and forces the coolant through the heater matrix only. This provides a higher flow of warm coolant through the heater matrix to improve passenger comfort in cold conditions.
When the engine speed increases above 1500 rev/min the coolant pump produces a greater flow and pressure than the heater circuit can take. The pressure acts on the flow valve and overcomes the valve spring pressure, opening the valve and limiting the pressure in the heater circuit. The valve modulates to provide maximum coolant flow through the heater matrix and yet allowing excess coolant to flow into the by-pass circuit to provide the engines cooling needs at higher engine rev/min."

Hope this helps - I guess the key question is did you drive the car for a good run at reasonably high engine revs?

Galen · May 7, 2010

Headtester said:
Hope this helps - I guess the key question is did you drive the car for a good run at reasonably high engine revs?

Thanks for that.

Most of my test journey was at 2000 RPM and above, but not speedy as such.

I'll take it on a longer run and see how it fares.

Sean · May 7, 2010

block the air flow to the rad with a feed bag/carpet tile/bin liner - it'll heat up lots quicker and save you booting it about just to get it warmed up

Galen · May 7, 2010

Sean said:
block the air flow to the rad with a feed bag/carpet tile/bin liner - it'll heat up lots quicker and save you booting it about just to get it warmed up

I've already taken the fan off figuring that it would make a difference.

I've just taken it for a blast along a dual carriageway at 70MPH.
The top right (looking front the front) of the radiator is warm to touch. The rest is as cold as before.

The top hose is at 85C and is pressurised. It's not rock hard. I can compress it by hand, but it's not the easiest of things to do.

Disco-user · Jun 11, 2010

Hi did you get to the bottom of this?

I was running locally in my 2000 TD5 Disco the the temp gauge moved slightly above the mid position, I checked and the header tank was empty, since this happened the car is using/losing water.

I can find any obvious leaks but have also noticed the cold rad, and my hoses and soft so not certain its a head gasket issue. (the car had a replacement head about 3000 miles ago)

Galen · Jun 11, 2010

Disco-user said:
Hi did you get to the bottom of this?

My new thermostat opened at 84 Deg C.
The top hose seems to run warmer than the bottom hose.

From a refill my water level dropped approximately half of the header tank. Most likely due to trapped air being pushed out. Since topping up it's been fine and hasn't lost any coolant.

What temp should a TD5 thermostat open at?

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