1. The original temperature sender on the VAG TDis is a very unreliable part. However, as it did on my Leon, it usually shows itself by a dodgy gauge i.e. spending most time at 60 degrees, occasionally going up to 90 when it feels like it.
The sender that supplies the gauge also feeds the ECU. If you have had the car from new and not had the sender done, your car must be rare to still have it working as normal after that time & distance!
However, I have a feeling that the warning light is connected to something else (but not 100% on that).
2. My Leon does seem to take a long time to heat up, especially compared to my other car that uses petrol. I believe it is mainly due to the thermal efficiency of the diesel engine.
I have the SE with climate, so the interior fan begins to spin up before the (now fixed) gauge moves off 60 but both take a good few miles.
3. It is too soon to tell, especially since some long runs may have distorted the figure since replacing the sender a few weeks ago, but my mpg seems to have risen from about 48 before to 50/51 after.
Although not as pronounced as the same fault on a petrol that could run lean/rich, the electronically controlled injection pump could be wasting diesel by injecting too much while accelerating.
A sixth gear would be ideal, to keep the revs on the motorway nearer peak torque around 1900 rpm and benefit the mpg.
4. From the two Seat forums I know of in this country, there are plenty of gripes about the dodgy sender but not much posted about how to fix it and definitely no pictures.
I can't guarantee that this will fix your problem, but since the part is highly likely to fail, here's what you would need to do:
Get these parts from your SEAT dealer:
Sender 059 919 501A - £23.72 (as sold in the US for about $4!)
O-ring N90 316 802 - £1.47
U-shaped spring clip 032 121 142 - £0.42
I rang around a few dealers near me, hoping for stock to be able to do it that weekend but none had the sender in. Oddly, the prices of these bits varied between dealers. Any other VAG parts department would also have these parts and probably know the part numbers off their heads due to how common the problem is. The new sender has a green top, the old one will be black, as the part was redesigned.
The clip is not essential but worth buying as the old one is easy to break. Replacing the O-ring is good practice as the old one could be perished.
You almost certainly will lose coolant when doing this so you will need some coolant to top up with. Don't buy any old stuff though! Modern VAG cars are fitted with 'G12' organic acid coolant. Topping up with something incompatible turns it brown. You can pay about £10 for a litre at a dealer, or much less at somewhere like GSF or Euro Car Parts. Alternatively, 5-year coolant sold in places such as Halfords is not too expensive. It should say it meets TL-774-D/F spec for VW / Audi / SEAT / Skoda. I bought a five-litre bottle at Halfords for £15 (reason for that quantity later).
I couldn't find the sender for ages while looking. Standing in front of the car with the bonnet open, follow the coolant hose from the top of the radiator to the right of the engine block. At the T-piece where it meets the side of the block is where the sender is. However, it is under the T-piece and facing the bulkhead so not easy to find. The new sender gives no clues. Best to feel for the wires going to it. I can post a picture to my photobucket account if you can't find it.
It may help to remove the plastic engine cover or air intake pipe but these are not essential on my 110 bhp AHF version of the engine.
I have read that opening the expansion tank then reclosing will relive the system pressure and cause no leak. Squeezing the top hose, putting the cap back on, then letting go is also supposed to help. Neither of these worked for me. I just tried to act fast to minimise the amount glugging out!
The best thing to prepare is to put the new o-ring on the new sender. If possible, try to remove the connector from the old sender while still in place. That is generally difficult due to being so stiff. Mine was easy as it was already broken and held on by a cable tie. Put the connector on the new sender if possible and have the new clip to hand in case the old one breaks.
The old clip is removed directly downwards. A screwdriver may help to pull it down. The sender won't necessarily fall out without the clip holding it in.
Prepare yourself, remove the old sender and panic as the coolant start to pour out. If the old o-ring is not attached, remove it from the hole that is now quickly draining coolant. Plug the hole with the new sender and refit the connector if not done already. I did this in between 5 and 10 seconds.
Now the bit where I went wrong! Refitting the clip is the reverse of removing it, while ensuring the sender is fully pushed in. Due to the poor visibility, you can only see the notch on one side where the 'leg' of the clip goes. I had the clip seated within that notch but not the one on the other side. Consequently, after a test drive to make sure it was okay, some time after reaching 90 degrees, the sender popped out, losing all my coolant, hence the need for so much.
You need to feel around the clip after fitting it to make sure both legs are where they should be.
When I changed the sender the first time, I only had to put about half a litre of fluid in to top it up. When filling it from near-empty, it took five to six litres. I did a 50-50 mix of coolant and distilled water.
The car might benefit from having the battery disconnected for half an hour to reset the ECU. A test drive like mine, long enough for the long warm-up, near enough to home in case of problems, should help confirm the sender has been changed okay.
The reason I think the overheat light may not be part of the sender is that when I lost all the coolant, the gauge dropped to zero (as the sender was just touching air) but the red light flashed with the accompanying beeps.
This is a bit of a long post but the total job time if done correctly is less than the time it has taken me to type this!
Hope this helps you, or anyone else with a Passat, Golf, Bora, Ibiza, Octavia, Polo, A3, A4, Ibiza or any other VAG vehicle built from around the mid 90s to about a couple of years ago.
James
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