Thread: Yet another 300tdi overheating question...

Check that the small hose from the thermostat housing to the 3 way valve is flowing normally, ie not blocked at the 3 way. This hose has the important function of allowing hot coolant from the head to reach the thermostat in good time to let it open at its normal temperature. A closed thermostat on its own flows very little coolant so its bulb won't get hot coolant from the head until it's reached the concern zone.

While ferreting around for tools to check the 3-way coolant valve I found the old thermostat - clearly stamped "82". So obviously the old 'stat opened earlier than the new one - hence the temp difference. The big question is:
Should I run an 82 thermostat instead of an 88? Considering the known overheating issues of 300tdi engines, surely the 82 is the better option? Or is my (very) limited mechanical knowledge missing something?

no air locks anywhere restricting water flow? mine was heating up very quickly too when i first finished my head works, which included pretty much brand new cooling system. it had airlock in it and also small hole at the bottom of the recored radiator i put in. there is a write up on here on filling up from expansion tank using coke bottle.

where is your engine coolant temp sensor mounted? as beeutey said, i replaced the three way hoses too on advice from people on here and new cap on overflow tank.

something stopping normal flow until it heats up properly by the sounds.

Originally Posted by meabas

While ferreting around for tools to check the 3-way coolant valve I found the old thermostat - clearly stamped "82". So obviously the old 'stat opened earlier than the new one - hence the temp difference. The big question is:
Should I run an 82 thermostat instead of an 88? Considering the known overheating issues of 300tdi engines, surely the 82 is the better option? Or is my (very) limited mechanical knowledge missing something?

just saw this as i posted. there are some 82C ones around. the 88C is the normal for d1 tdi. cant remember what the 82C or was it 78C ones were for. but i would stick to 88C. unless PO put the other one in for a reason.

I have attached a pic of the location of my TM1 sensor - you can see the red heat-shrink in the centre of the pic. I have not changed the position of the sensor since it was first installed and it seems reasonably accurate - running around 85-90 with the 82 thermo that was previously installed and at around 87-92 with the new 88 installed.
I have checked and flushed the 3-way coolant valve - no blockages.
I'm the process of checking and removing any air-locks in the system.

An 82c thermostat wont make the engine run any cooler, it will just open earlier. 95C is too low to set the alarm, more like 105 from what others on the forum have suggested. It drops back to operating temp quickly so i wouldnt be concerned.

Originally Posted by meabas

I have attached a pic of the location of my TM1 sensor - you can see the red heat-shrink in the centre of the pic. I have not changed the position of the sensor since it was first installed and it seems reasonably accurate - running around 85-90 with the 82 thermo that was previously installed and at around 87-92 with the new 88 installed.
I have checked and flushed the 3-way coolant valve - no blockages.
I'm the process of checking and removing any air-locks in the system.

87-92 sounds perfect. mine runs at about that. i think the head can get up to about 120 at the max end of the scale. my temp gauge is at the head and ive set my alarm to 110C. as loanrangie said, shouldnt make your engine run cooler having the other thermostat. should just open up earlier and take a bit longer to warm up i guess.

Read somewhere that the 82 degree one is for 200tdi, 88 for 300 tdi. having said that though, I run an 82 degree thermostat in my 300tdi.

Tom.