Thermostat not opening

K

Kaz

Member
So on my tdi, it seems the thermostat is not opening on my 15 mile motorway journey - the needle on temp cause does not move above 60, and on touch the top pipe is hot, and lower rad pipe is cool - I have replaced the thermostat but still got the same issue - any ideas?
 
You did replace the thermostat correctly with a good quality OEM one I take it? Do you have correct coolant that is not past its useful life?
 
Actually your problem is the opposite of your title. The thermostat is never fully closed. Thermostat never opening would cause significant overheating to the engine and as the temp sensor is on the engine you would notice 110+ . Always open the fluid is circulating through the complete system and temp stays low. Other possibility is even though you say motorway if your speed is low the air cooling takes more heat out than the engine can generate to a point. Current ambient temps should rule that out though.
 
What engine is it?.mines a TDI 90 ATL and I changed both the thermostat and temp sensor.But temp would limp its way to 70/80 degrees.But that changed when I replaced the egr valve (ATL has a egr cooler).Even with climate on its gets to temp fairly quickly and stays just under 90 degrees.But as @Rogan said for £6 sensor and a little effort it's definitely worth a try
 
Kaz said:
So on my tdi, it seems the thermostat is not opening on my 15 mile motorway journey - the needle on temp cause does not move above 60, and on touch the top pipe is hot, and lower rad pipe is cool - I have replaced the thermostat but still got the same issue - any ideas?
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Have you got VCDS if so you can see actual sender temps for both sensors I believe,
could also be trapped air, did you fill it up recently
 
audifan said:
Actually your problem is the opposite of your title. The thermostat is never fully closed. Thermostat never opening would cause significant overheating to the engine and as the temp sensor is on the engine you would notice 110+ . Always open the fluid is circulating through the complete system and temp stays low. Other possibility is even though you say motorway if your speed is low the air cooling takes more heat out than the engine can generate to a point. Current ambient temps should rule that out though.
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I say it’s not opening as the bottom rad hose is cool to the touch but top is much hotter - almost scalding

Engine is amf - what sensor is it I need?
 
Kaz said:
I say it’s not opening as the bottom rad hose is cool to the touch but top is much hotter - almost scalding

Engine is amf - what sensor is it I need?
Click to expand...
Sorry, don't know what sensor you need, but definitely think it would help if you can find someone local with VCDS who can give you a hand.
 
Kaz said:
I say it’s not opening as the bottom rad hose is cool to the touch but top is much hotter - almost scalding

Engine is amf - what sensor is it I need?
Click to expand...
@Rogan linked to the temperature sender in post #4. As @steve_c noted, it's worth a scan to check temps though the temperature sender is a pretty quick cheap swap. It has two separate sensors within giving temp to Ecu and dash.
 
What thermostat did you fit and did you fit it into the block? Was it specifically for the A2? The temp sensor is fitted to the right end of the enging block below the tandem pump. it is a 4 wire sensor. There are threads on here about it. What colour is the coolant? With the engine off remove the expansion cap, start the engine. Do you see coolant flowing from the little upper hose into the expansion tank, it wont be a huge flow but it should be coming through.
It would not be the first time that cheap thermostats dont work or open too late.
 
Is the fan staying on? Could also be faulty blue sensor at the bottom of the radiator. Alas you just have to be methodical and replace one at a time
 
If "the bottom rad hose is cool to the touch but top is much hotter - almost scalding" then there's no circulation. Probably thermostat (only use OEM), definitely not sensor related, in my opinion. A scan is the first thing to do/get done. Mac.
 
That is why I asked if there is a flow back into the expansion tank. No flow either a blockage or pump issue.
Unfortunately when we ask questions to help diagnose problems and the answers are either partial or none and we are not around the vehicle it is frustrating when we are trying to help solve problems.
 
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