Hi

I have an A4 B8 2009. I have changed thermostat and sensor and still, my thermostat does not rise. On long runs, it might rise to 60/65 but very rarely. It takes an age for warm air to come in the heater (maybe 15-20 miles) and when it finally does come, it's nowhere near as hot as it should be even when set to HI. What am I missing?

Hello Ray,

Just one simple question:- What make replacement thermostat did you use? 

Kind regards,

Gareth. 

The replacement was a Serck thermostat

Hello Ray,

Never heard of Seark thermostats -  radiators,  yes. 

By what you say, so would seriously suspect this thermostat. I have just changed a thermostat on a 1.6, A3 but chose a genuine VAG one in preference to an aftermarket one at a third of the price, due to experience of the potential for variable quality with aftermarket ones.

Not sure if you changed this yourself or had it fitted by someone else, but if you fitted it, I work on the principle that I save on labour so can 'afford' to use genuine parts for critical applications. 

Kind regards,

Gareth.

I finally got round to changing the thermostat to a genuine one but unfortunately made no difference. Temperature stays just under 50 degrees. 

Sorry to hear that Ray.

Have you had the actual running of the coolant checked to compare it with the indicated temperature?

Kind regards,

Gareth.

 

Yes, have had it checked since and the car is running cold as indicated. They didn't know why and no faults were showing up. Very confusing.

Well Ray, this is confusing to me, so let’s hope someone more knowledgeable on here can throw some light on it. 

As I see it, stating the bleeding obvious, if the thermostat has been correctly fitted and is properly serviceable, then it must retain the initial heated coolant in the engine block, until that coolant reaches around 80+ degrees i.e. near normal on the gauge. The heated coolant then opens the thermostat, and allows the coolant to be passed through the radiator to be cooled, while the block heats more coolant - and so on. Eventually all coolant reaches the operating temperature. 

For the coolant not to heat up?? Suggests the thermostat is ‘permanently’ open. How could that happen? If it opens at too low a temperature - but two faulty ones? That would be a bit of a coincidence! but.....? I would now want to test it’s operation by heating water in a pan and actually checking the opening water temperature with a thermometer. Got to positively eliminate that first! 

Just a thought - no more than that. Could some pressure build up in the engine block force the thermostat to open early? Food for thought?? 

Could you let us know how you get on Ray.

Kind regards,

Gareth.

Thanks, Gareth. Unfortunately tried that too, on both thermostats we replaced and both worked fine in boiling water. Not sure about a pressure build up but how would I test/find that?

Guessed you might have tried that Ray, but being pedantic, did you drop them in boiling water and found they opened when you did that? 

Probably not. My test is to gently lift the opening section very slightly (important words being gently and very slightly) and insert a length of fine cotton under the seat so that it is jammed under the seat. You can then dangle the thermostat in the water to be heated and note the temperature at which the thermostat falls off - I.e. when it opens. 

Apologies if that is what you did. Boiling water is fine if you want to see if it opens, but not if it opens too early. 

Pressure in the system? Worth feeling how hard the hoses are when running. No need for me to mention all safety requirements?? 

If above proves to be OK, then I’m struggling Ray. 

 Anyone else have any advice?

Kind regards,

Gareth. 

I've seen instances where a bad radiator cap can cause issues with the cooling system.  They're designed to keep the cooling system pressurized, but to allow the system to vent into the overflow if it exceeds a certain pressure.  I'm not sure how to test one, but it might be worth looking into.  Maybe yours is allowing the system to over-pressurize.

I’m with you Michael with this theory, but what confuses me is that I would not expect the coolant to pressurise the system to any great extent before the thermostat opens, and if I am right with this, then you would expect the gauge to get close to normal - say after approx. 1mile - before dropping back. 

Coolant reservoir caps can be an ignored source of issues, and yes, I guess they can fail to release excess pressure, but more often (I think) they fail by not allowing the pressure to build up. Although thermostats are rated to open around mid 80s degrees, the full operating coolant temperature can reach around 100 degrees ( the boiling point of water). This is fine, since the actual boiling ( bubbling) point of water is increased with pressure so the slight increase in pressure - around 4psi - will suppress the ‘boiling’ (bubbling) point. If the caps fails to allow this slight boiling. 

Kind regards,

Gareth.

I suspect you're right about the gauge moving towards the normal range before dropping down.  But the only other thing I can think of, assuming the thermostat is working correctly, is maybe a previous owner bypassed the thermostat by modifying the coolant hoses in some fashion.  I've seen this done as a temporary fix, but only in extraordinary situations.

Google 'Inline thermostat' for the Audi A4 B8 2.0tdi. Its at the bulkhead end of the engine inside one of the coolant hose (lower). A pain to get to as it's buried underneath other pipes and hoses but can be reached. Once replaced  temp gauge will reach 90°c and stay there guaranteed,  just done mine after changing thermostat,  coolant etc. Hope this helps someone having issues with engine not reaching the correct temp especially in winter.

Hello ‘t’,

Are you saying there are two thermostats on this vehicle? 

Kind regards,

Gareth.