TDI water temp fluctuating

[Stevoraith]

On a recent long journey in the wifes car (first time I've driven it far in about a year) I noticed that the water temp guage which normally sits rock solid in the middle at 90degC was fluctuating between 70 and 80C.

 

The car is a 2011 vRS TDI (so 2.0 TDI CR 170).

We've had it since new, cambelt and water pump replaced April '16 so water pump is 5 years old and will be replaced when the cambelt is due again in a couple of months.

 

I took it to work yesterday so I could pay attention to the temps on a decent journey.

 

I was able to check the oil temp as I have maxidot and it rose steadily to around 77C and stayed there throughout the journey.

 

The water temp rose to around 75C, then fell again to 70 and then just sort of oscillated between 70 and 80 while I was doing a steady 70mph in 6th (~2k revs).

 

I then did a couple of miles in 4th at 70mph (around 3k revs) which didn't make much difference to the temp so I put it back into 6th.... at which point the temp increased to around 85degC before falling back down again.

Towards the end of the journey (30miles, 35minutes) I gave it a bootfull of revs off a few roundabouts and then after a long uphill section the temp had finally risen to 90C.

On the downhill section of the same hill it started to cool down again.

Ambient temp was around 2degC.

 

Has anyone experienced anything similar?

I was hoping it is just a sensor/sender rather than a real issue but I have no idea.  Obvioulsy cannot tell if the water genuinely is cooler than normal or if it just thinks it is.

I mention the water pump but I would assume if that was failing that it'd be hotter than usual, not cooler.

 

Keen to get it diagnosed ASAP as it will be getting cambelt and water pump done in April so if it is something which needs changed in the coolant loop then I'll save a bit in labour/coolant if I get it changed at the same time.

I had my local specialist 'check it' when it was in for a service in November but all they did was read the codes and said nothing came back.

 

Any help/thoughts would be appreciated.

[MikeTheThinker]

Just in case, have you checked the cooling water levels and condition (froth, soap, oil)?

[Stevoraith]

Coolant needed a small top-up (less than 100ml) but otherwise looks in good condition.

 

That was last October when I first noticed the issue, haven't checked it again since 

[Westbury63]

38 minutes ago, Stevoraith said:

I was hoping it is just a sensor/sender rather than a real issue but I have no idea.

 

i'd say you've hit the nail on the head already. The temperature sensors are know to fail and are easy to replace cheaply. That is what I would try first. 

 

HTH

[Stevoraith]

Any idea which sensor is most likely?  

 

A quick search on here previoulsy threw up a number of sensors in various places, some of which were apparently a pain in the backside to change.

[Westbury63]

In my MK1 Otci I replaced 3 of these over 12yrs of ownership. I don't know your 2.0 TDi CR engine, but the temp sensor was part of the thermostat housing.

 

[Stevoraith]

59 minutes ago, Westbury63 said:

In my MK1 Otci I replaced 3 of these over 12yrs of ownership. I don't know your 2.0 TDi CR engine, but the temp sensor was part of the thermostat housing.

 

Thanks mate, similar symptoms when you replaced yours?

[Westbury63]

1 hour ago, Stevoraith said:

Thanks mate, similar symptoms when you replaced yours?

No worries. The needle would do all manner of weird things some times. I've still got a spare in the garage even though my Mk1 went 3yrs ago!

 

I woud have thought Youtube would show where the part is located, the 2.0 engine is in so many vag cars. Replacement will be <£10 if you can diy.

[langers2k]

Given both sensors are reading low, it's more likely to be a thermostat.

[Stevoraith]

25 minutes ago, langers2k said:

Given both sensors are reading low, it's more likely to be a thermostat.

Is the oil temp reading low?

 

I genuinely have no idea, I never look at it.  I assumed because it wasn't fluctuating like the water temp it was probably ok.

 

Any idea what normal operating range is for the oil temp?

 

My BMW has an oil temp gauge, I'll make a point of looking at that on the way home from work now! 

[langers2k]

Yup, oil should be 90'C plus really.

 

Just to point out, the coolant gauge lies. It'll show bang on 90'C for a wide range of temperatures, from 75'C to 105'C (or something similar) if I remember correctly. Apparently modern drivers expect the gauge to stay still rather than move with engine load....

 

Anyway, I had similar issues with my CR140 and a new thermostat fixed it. I logged the temperature before and after to check which I posted here:

 

If you have a DSG, there is a separate thermostat for the DSG that's worth replacing first as it's much cheaper and easier to access

[Stevoraith]

This is excellent, thank you.

If I had seen that topic when I originally searched I wouldn't have had to start this one!

 

Sounds like exactly the symptoms I'm seeing.  I have a manual so it sounds like there's just the one thermostat to change.

Also since the car isn't being used much just now with lockdown I'll just leave it until the cambelt water and pump are being changed in a couple of months as it sounds like that should save some labour and the cooling system will be being refilled and bled anyway.

 

As a matter of interest, I paid closer attention to the oil temperature gauge in my 335d on my way home from work tonight.  I was aware it generally sat around the mid-point (which is 100C) but I can now confirm that it fluctuates around that (maybe +/-5degC) dependant load.

 

Thanks once again for your help.

Edited 30 January, 2021 by Stevoraith

[Gissin]

100% thermostat 

[-mike-]

Can I be cheeky and jump in here to ask the best way to diagnose the thermostat? I have access to vcds this weekend and have read something regarding the radiator output temperature and the engine temperature. Am I right in thinking that if the radiator outlet and engine temperature rise together from low temperatures that the thermostat is stuck open? The car gets up to 90 no problem (on drive home from travelling 200 miles to bring it home), but temperature will drop at light load to the point that the fake temp gauge needle actually drops so I assume quite a temp difference. When on the throttle it's fine and keeps temp but will drop off 90 at low load.  Apologies for the thread hijack if this isn't allowed but I figured that one place is better than multiple topics!

 

Car is a 2010 Octavia TDI 140 (CR)

Edited 18 May, 2022 by -mike-

[Guest]

Sounds like a stuck open stat.

 

I had a similar experience with a new purchase, turns out the cylinder head had been done and a grinder through the stat to prevent another cooked engine.

 

Might be worth investigation especially if the heater doesn't blow hot.