Change a Thermostat On a 225 TT
This Technical FAQ (How to...) has been carefully compiled by John Handford
If you are suffering from a low temperature reading, having to wait ages for some warm air, or suffering excess fuel consumption, you may need your thermostat changing - espescially if your TT is an older model. Worn thermostats can stick partly open, behave sluggishly and not regulate the temperature properly, causing your ECU to run a rich mixture. Here's a graph of my engine temperature on the way to work:
You can see the poor regulation with readings as low as 77 deg C. on over-run, to 100 deg C. stuck in traffic. Other people have reported running 75 deg C. and suffering a rich mixture and low mpg. I think mine was verging to that condition.
Diagnosis:
Do the 49C trick to confirm the ECU's reading of the engine temperature via the climate control:
* Press re-circulating air and de-mist up arrow simultaneously.
* Turn temperature control knob until 49C is displayed.
* Press re-circulating air to display reading.
* Press re-circulating air and up arrow again to cancel.
Your needle dash gauge shows a software weighted figure that sits at a re-assuring 90 deg C. normally and only departs when the 49C figure is significantly different (>~10 deg C.). Another thing to bear in mind is that there are two temperature sensors in the same screw in sensor unit on the engine. One sensor feeds the needle gauge and one feeds the ECU (49C figure).
This is useful to know because if the two readings agree and your temperature is low, this generally indicates you need a new thermostat (unless you are unlucky enough to have both sides fail identically!). If one reading is low and the other high (allowing for the software weighting) this either means the temperature sensor needs replacing or your dashpod may be faulty.
For general temperature problems if 49C is fine, a high or low needle reading is going to be caused by the other half of the sensor being at fault or the dashpod. If 49C shows too hot as well as the needle gauge it's more likely to be a stuck thermostat or failed waterpump, collapsed hose, non functioning fan etc.
Presuming you need to change the thermostat:
(Click to enlarge pictures)
These are the parts you'll need:
Some G12+ Audi Antifreeze sold in 1.5L containers. You don't need to replace all the coolant but you may spill some. Total coolant capacity varies but is in the order of 5L (40% mix). It's clear pink/purple in colour. If your current antifreeze is dirty brown you've got a mixture of G11 and G12 and should get the system flushed and replaced with G12+ immediately before you get engine damage. It is unlikely for there to be a mix however.
Thermostat P/No: A050 121 113 C @ £12.41 + VAT from Audi
Sealing ring: P/No: A038 121 119 B @ £1.49 + VAT from Audi
Procedure:
You need to drain the system, so, first get the car up on ramps. The standard ramps don't work too well with modern spoilers. A piece of wooden joist on each ramp, with a couple of bent nails to act as hooks onto one of the ramp rungs, give enough initial lift to ensure the spoiler easily clears the ramps. Still not painted.
Next job is to remove the cover from under the engine. There are quite a few Torx screws to remove. Don't forget these screws that join the cover to the wheel arch liner! Then the cover will pull towards the rear, disengaging the locating tabs at the front of the cover and fall away.
Next, take off the cover in front of the inlet manifold. The screws turn through 90 degrees to disengage. Prise off the hose clip at the end and undo the two socket screws with a 5mm Allen key. Then tuck to one side without disconnecting the pipes and wiring.
You can now catch a glimpse of the thermostat housing behind the alternator. Don't undo it yet until you've drained the system. There's a tap at the nearside bottom corner of the radiator. The tap turns anticlockwise 1/4 turn and then pulls out a 1/2 inch to release. Drain into at least a 5Litre (clean!!!) container - it's a slow trickle.
Now, the design of the cooling system is a little odd I think. The pipe from the thermostat goes down to the bottom of the radiator by the tap. Most cars feed the top of the radiator. Consequently on the TT, the tap does not drain the full system as it can't fully drain the cylinder block. To complete the task you need to loosen off this hose on the oil cooler. If you don't do this, pulling out the thermostat will spill about 1Litre of coolant over the floor! As I found out.
So, now compress the hose clip with some pliars - I used a small Mole wrench and pull the clip back over the hose. Then twist and manipulate until it comes off. Pull the small connector off the alternator for better access. The top bolt can easily be removed with a 1/4" drive universal joint and extensions. It's a 10mm A/F head. The bolts also have a 5mm Allen key socket, so you have options. You can twiddle the last few turns with your fingers - seen through the inlet manifold.
The bottom bolt, however, is more difficult. I fould a ring spanner and a 1/4" drive socket and ratchet was useful here. Wak told me later that removing the main alternator connector and taping it to one side helps but I managed without trying this. Access is difficult and you can hardly see what you are doing with your hands in the way. Finally it's off and you can prize out the O ring around the thermostat and remove it.
Next, presuming you don't have to spend half an hour mopping up, scrape any rust or gunk from the flange face and clean away any debris. The thermostat doesn't seal on this face so you just need it flat. Now lubricate the new O ring with antifreeze and insert the thermostat, spring first, into its recess. Insert the O ring around it, which stops it falling out. Make sure you've got the thermostat's metal bridge (where the actuator bar presses) vertical by rotating the thermostat. Also lubricate the housing elbow face before assembly. The manual says to tighten the bolts to 15 Nm (11 ft/Lb).
Now replace the hose and hose clip and make sure the small tube is in front of the dip stick before swinging the solenoid valve mounting plate back into place, engaging in the dipstick guide tube, and tightening the bolts to 10 Nm. Re-attach the hose clip and replace the cover. I took the opportunity to remove some rust off this plate and paint with black Hammerite before putting the cover back on.
Now, ensuring your drain tap is closed, replace the coolant. I'd taken my covers off to retreive a dropped bolt, but it helps to top up and see what you're doing and achieve an arty lighting shot to see the level:
Recommended protection coolant mixture ratios are:
-25 deg C. = 40% Antifreeze 60% water
-35 deg C. = 50% Antifreeze 50% water
Use distilled water to mix and avoid problems. Keep between these ratios. Too little antifreeze may freeze obviously but too much won't cool as well.
Top up to the full mark and run the engine with the filler cap off. You will probably find the level drops so top it up. As the engine warms up it may drop further. It's best to achieve full working temperature and check for leaks under pressure, so when youre satisfied the level has stopped dropping and the engine has warmed up (thermostat open, coolant circulating), cap it off and get it up to full temperature at a fast idle. You can squeeze the hoses to tell if there's pressure. Don't forget to put the engine undershield back on when you're happy.
Keep some coolant with you on you first run. You may need to top up again. I needed almost 1/2 a litre.
Results:
As you can see, the temperature is now much more stable and the average temperature now sits at 94 deg C. Well away from the point where the mixture richens and your mpg suffers.
