I've recently replaced the coolant temperature sensor (CTS)on my 2004 1.9 AWX engined B5.5 Highline Estate and have been astonished on how much the MPG has improved as a result. I only replaced the sensor as a result of the engine management light coming on with an "Emissions Workshop" warning flashing up. When scanned with VCDS an intermittent short code presented.
I did a how-to on the CTS swap out (link below) for anyone interested.
viewtopic.php?f=33&t=75857
The previous sensor was a cheapy and lasted two years. I thought all was well during that time but it wasn't. Turns out the cheapy CTS was telling the ECU lies and the ECU was over fuelling as a result.
I urge everyone who has VCDS to get logging and take a good and close look at there temperatures to ensure their cars are running to their optimum.
Below are four logs graphically represented to show what is what. The critical thing is getting the coolant up to 90c as a lot of TDI's seemingly are running well below that. This is due primarily to a dodgy thermostat and or CTS. Secondary are the air intake and fuel temps.
Although having read up on Martins TDI the fuel temp could be even more important as it is the primary input that the ECU uses to adjust fuelling. I'm now obsessed with temps as can be seen by the creation of this thread and the graphs I've produced.
Below the graphs, which hopefully are self explanatory. The X axis is time in minutes, the Y axis is multi purpose as I've scaled everything to fit it! The graphs are of two sets of to and from work.
The most interesting thing for me is the variablity of the time taken for the coolant to get to 90c.
Also of great interest is the seeming error or discrepancy between the fuel and coolant temps at the start of the log, as far as I'm concerned, they should be very close to each other on a cold engine. I'm investigating possibly changing the fuel temp sensor due to this but it'd be nice to get some comparable data from other cars.
If you have VCDS, do some logging, graph it and post it up so as to add to the knowledge base. If you are not sure about how to produce a graph, either e-mail the log to me or I can send you a template where you can paste your own values in.
All are welcome, new and old / 1.6 /1.9 / 2.5 / 2.7 / 3.0. The more data we get the better this will be, that is if anyone else is as sad as I am in such matters!