Hi All,

Got an overheating problem with my 9N3 1.2 petrol.
Driving around day to day, its all good, temperature gauge sitting at 90. Sit in traffic for 10 - 15 minutes and its alright. However sit in stationary/stop start traffic in hot weather and the temperature creeps up before eventually giving the overheat warning at 130. Get back to a more steady driving speed and the temperature goes back down and sits at 90 as if nothing happened.
If I switch the AC on, fan kicks in and holds the temperature down a bit longer. But on hotter days it only holds it below 110 and at 90 with the help of the heating on full blast.
Suspecting a dud fan switch, I had it replaced, but nope, still overheats.
Also checked all the fuses. Doubting the thermostat as it gets to running temperature and holds fine as long as moving.
Now I am fresh out of ideas.

Any help would be greatly appreciated.
Iain

If you take the plug off the fan switch and bridge terminals (in the plug), does the fan run?

First Welcome to the forum.

 

With the engine cold, remove the cap on the expansion bottle, start the engine and see if you get water returning to the bottle via the small pipe, if there is nothing at idle rpm, try increasing the rpm to about 2500 and see if there is a flow now, as the symptoms sound like a failing water pump

Thanks For the help so far guys.

So results from a bit of fault finding today.

So starting with the pump and expansion tank, both seem fine. Coolant coming in and flowing through extension tank. Both inlet and outlet pipe hot and warm.

Now the intersting bit

Fan switch. Unplugged the 3pin connector. First went to test if getting power using a illuminating circuit tester. Upon checking first two pins as suspected nothing. Upon checking the live, tester illuminated AND the fan came on....? (Cue a puzzled look between me and my more tech savvy mate)

Is this a computer fault then?

As you have AC the fans are controlled by the fan control unit, it mounted on the passenger side chassis leg under the battery near the horn

Ok - not quite what I asked - there is a difference - it sounds like you've tested for power reaching the plug and maybe connected the control line via your tester?

Put the tester down and pick up a big paper clip. Straighten the clip.

 

Put the paper clip ends into the plug so you join one end connector with the middle connector.

If the fan comes on - Remove the paper clip - let us know - Don't go any further.

If the fan doesn't come on, - Remove the paper clip and use it to join the OTHER end connector with the middle connector.

Note if the fan comes on - Remove the paper clip.

 

Let us know if the fan came on under either of these conditions.

Edited 11 April, 20179 yr by kbs

Sorry, should have clarified a bit more. Tester was connected to earth point on the car,  then touched to all 3 lines individually. This was following a Youtube video which showed the same then went on to do bridge the terminals. 
The first part was to make sure there was power coming to the plug (in the video the fan did not come on for any of the lines, but the tester illuminated for the live), then when bridging the terminals the fan came on. 
Just found it odd when testing for power the fan came on.
Will skip that step tomorrow and go straight to bridging as suggested and report findings.

Thanks again.

12 minutes ago, IainC14 said:

... when bridging the terminals the fan came on...

 

That's great - the wiring from the fan switch is good. This also tells you the downstream control unit (AC) is working.

 

So the problem is either water circulation (its hot at the guage sender but its not hot enough around the fan switch)

OR

mickey mouse fan switch - there is some shocking s**t being sold, even (especially) by regular names that advertise very heavily on the web.

 

If you have/can borrow one of the cheapy IR point&click thermometers, check the temperature near the fan switch and compare that with the thermostat housing (where you temperature guage sender is). If the temperature is quite different then its time to look at thermostat, blocked radiator and water pump (impellers can rot meaning the driven wheel rotates but no water is moved).

 

You could hook up wires to the fan switch and heat it in a pan of water to check if it switches uses a continuinty tester, but if you can use a IR thermometer to confirm the temperature at the switch is close to the temperature at the guage sender - its time to curse the retailers of nasty far east copies of VW parts.

 

Good luck

Edited 11 April, 20179 yr by kbs

And the saga continues.
So, as stated before, testing the power of the fan switch with a illuminating tester, turns the fan on.
Bridging the live with the top connector, turns the fan on low speed. Bridging it to the other terminal doesn't turn the fan on (nor with the ignition on)
Now the fan doesn't come on when the AC is switched on.

Checked the terminals again, still all the same.

Edited 13 April, 20179 yr by IainC14

Here's the wiring diagram

As you can see the temp switch in the radiator F18 switches an earth to the fan control unit J293, bridging pin 1 Brown wire, to pin 2 Brown/Blue wire should bring the fans on at the low speed, bridging pin 1 to pin 3 Brown/Green wire should make it run at full speed, don't use your power probe just a simple bit of wire to bridge the pins.

 

As fuse #164 ( 3rd in from left) 40amp is one of the flat metal strip fuses on the battery, remove it and inspect closely as these are known to fracture over time and might look OK when in position but fall apart when removed, the others on the battery are the normal blade fuses, but again, remove them and check for corrosion/burning on the contacts

 

Could be a case of the radiator needing a flush through, top to bottom and bottom to top. Even with good antifreeze containing corrosion inhibitors you still get a build up of sludge, partially blocking the core and not causing any problems until the circumstances you have said. Done lots with success including my brothers old jeep that took two good sessions. Same time it's worth diconnecting the hoses that feed the heater matrix and give that a flush if main radiator is sludgy.