Thermostat or temp sensor?

[mikeyr]

Have a 206 with the 2.0 GTI lump in it with an engine that isn't getting any warmer than 70c on a 20 minute commute (certainly nowhere near the 90c it should sit at). Fuel economy is awful too. No warning lights and car itself seems to be running fine.

Internet mechanics suggests it could be the temp sensor or the thermostat stuck open. These seem the most likely culprits?

I have a bluetooth OBD device and the torque app so presumably I could check the engine temperature on that. If it matches the temp gauge on the dash then that would point the finger in the direction of the thermostat itself right?

Not mechanically minded but don't mind a bit of spannering if it is a simple enough fix but want to check my logic through first!

[JamesD1]

i had a temp sensor go in my seat Leon, the temp gauge would fluctuate and mpg drop a bit. it was very easy to replace just needed to unplug it and remove a washer then do the reverse.

as for thermostat you need to try the heating / aircon

[Rovinghawk]

mikeyr said:

I have a bluetooth OBD device and the torque app so presumably I could check the engine temperature on that. If it matches the temp gauge on the dash then that would point the finger in the direction of the thermostat itself right?

Not mechanically minded but don't mind a bit of spannering if it is a simple enough fix but want to check my logic through first!

If the sensor is faulty then both the gauge & the OBD will have the same reading as they get the info from the same source.

[mikeyr]

Knew there would be a fault in my logic!

What's the best way to determine which culprit to go after first then?

[Slidingpillar]

If after 5 miles you can comfortably put your hand on the top hose - thermostat.
If you can't - sender unit.

[ShaunTheSheep]

Cheapest easiest test i can think of would be block off the radiator with some cardboard slotted in front of it - you don't have to be exact just wedge it in anywhere blocking the airflow over the rad. Keep clear of any fans etc. though.

If that improves the situation then it's the thermostat. Should have zero impact on a dodgy temp sensor though i'd have thought.

EDIT: I prefer slidingpillar's idea, that's got zero risk, it's dead easy to do and it's accurate.

[anonymous-user]

EVen easier, how hot does the heater blow? if not very then either 1) the heater was crap in french cars anyway, or thermostat is jammed open.

Another easy test is start car from cold with bonnet up, leave idling, feel temp of top hose/rad entry. it should not get warm immediately (as with thermostat shut, coolant should not be flowing to the rad straight away).

[GroundEffect]

If your thermostat is stuck open and the vehicle isn't warming up properly try touching your radiator when your temp sensor is reading 60C or so. If the rad is hot then you have a stuck open stat.

[mikeyr]

Great advice guys - will do the heater/hose thing. Much appreciated!

[Searider]

Temp sensor will likely be a dual sensor. One signal to the ecu, one to the gauge. Obd reader may help here.

Simpler test is to let the car idle after driving. If its the thermostat the temperature WILL rise to approx 90 or more before the fans kick in.
If its the sensor then the temp will show the same even when the fans have kicked in.

[Matt_N]

Most definitely the thermostat.

They tend to fail by sticking open, meaning that the coolant will be flowing through the radiator from the moment you start the engine meaning it takes much longer to reach full temp if at all.

In normal circumstances the thermostat will remain closed until the temperature sensor reads that the coolant is reaching a certain temperature, normally around 85-90 degrees, up to this point the coolant is only circulating the engine block, cylinder head and heater matrix. The thermostat will then open allowing the coolant to flow through the radiator and maintain optimum engine temperature.

[ShaunTheSheep]

Searider said:

Temp sensor will likely be a dual sensor

Do you know the reason behind this? I've seen this (e.g. mk4 golf) but never figured out why you'd want 1 signal going to the dashboard and a different signal to the ECU. There's just scope for differing readings.

Maybe i've misunderstood and both signals go to the ECU and allow the ECU to accurately detect a stuck thermostat or blocked rad, e.g. to set a trouble code. No idea, out of my depth but would like to know.

[mikeyr]

UPDATE

Hmmm, just had to pop out in car and guess what - temp gauge slowly rose to about 88 degrees and then stayed there for the rest of the journey. No issues, no drama, nowt.

Too wet to check under the hood afterwards for the hose but the blowers were working nice and toastily. Guessing if the car has been sat for a while before I purchased it there might be a chance that the stat had got stuck open?

I'll see how it runs over the next couple of days...

[anonymous-user]

ShaunTheSheep said:

Searider said:

Temp sensor will likely be a dual sensor

Do you know the reason behind this? I've seen this

Before the days of CAN being the std coms method in cars, in a platform that had multiple powertrain options, and when you want to share parts between models (ie same dash in different cars) it was impossibly to ensure that the dash could be driven by every single engine controller type. Hence it was easier to have a sender for the engine, that could be changed for any specific engine/ecu type and a sender for the dash, that mean't you could use that dash in everything.

Since the introduction of CAN with it's standardised protocols, sending information between devices is now easy, and any module can talk to any other ecu etc