VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat

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Fri 2 Jan 2015 10:31
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Bonebaker

In my 2005 1.9 TDi PD 100 passat, 95k, the thermostat failed open about 6 months after buying it

The car ran cooler than normal (of course), and after between 500 and 1000 mile later, I replaced the thermostat. The car runs well now as it did before with no notable running issues, no smoke, good starting, idle OK and pretty steady

So whilst I was initially really impressed with mpg in this car when we bought it, it seems markedly worse now. I don't have the figures to back this up as it was the wife's car and she seems to think that MPG is for nerds

When I drive, what used to be 50+ mpg around town and easily 60+ on a highway, it is now 40 around town if I am lucky, and low 50s on the highway (at say 60-65mph).

So my question is whether running at low temperature (say 70C) instead of 90C would do something that would affect MPG like that, and if so, what would you look at?

thanks

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Fri 2 Jan 2015 12:21
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Railroad.
Are you actually measuring your fuel consumption or reading the trip computer which is not completely accurate anyway? I would've said the figures you've quoted are about right for this car and the type of use you're describing, and for winter conditions. The trip computer parameters can be changed using VCDS if you think the displayed MPG differs from the actual MPG.
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Fri 2 Jan 2015 19:36
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Bonebaker

Thanks for reply. I had assumed that the in-dash computer was pretty accurate, thanks for the info re VCDS, I had though that the data was fixed and was based on fuel pressure, injection duration etc.

I haven't had a look, but wondered if the EGR might have got more clogged than normal due to lower temperature and perhaps not fully burned up fuel being sucked acround through the EGR system.

I have had 3 TDi passats, the first was pre-PD and I did maybe 20-25k pa, and it was always economical and I didn't really notice a summer vs winter difference, though it was usually doing 100 mile each way journeys down the M4 at a steady 60-ish.

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Sat 3 Jan 2015 17:45
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Railroad.
And the fuel level in the tank. The tank gauge unit is a variable resistor connected to a float which sits on the fuel. There is no way this can ever be 100% accurate. In VCDS select the 'Instrument' module and go to 'Adaptions'. From there you can alter the trip computer parameters expressed as a percentage. Just make a note of where you start so you can always go back. Alternatively don't take the trip computer as gospel. It's only a guide, and your car is ten years old and to be fair it's best days will be behind it. It's never going to be completely accurate.
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Fri 2 Jan 2015 12:34
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - craig-pd130

Your mpg during winter months will be a LOT worse than in the summer, especially for town driving, so it's hard to make valid comparisons of consumption unless you've got accurate figures from previous winters.

I ran a 2003 Passat for over 5 years from new and the mpg always decreased dramatically on short trips in winter.

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Fri 2 Jan 2015 19:41
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Bonebaker

thanks for the benefit of your experience, Previously (in a pre-PD passat) I did maybe 20-25k pa, and it was always economical and I didn't really notice a marked summer vs winter difference. Though I fairly quickly got onto the M4 and drove100 miles each way, which I am sure affects the overall figure as the warmup time is small compared with the overall journey time.

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Fri 2 Jan 2015 13:06
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - FP

As other posters have said, MPG is much worse in winter.

It's unlikely that a spell running the engine with a faulty thermostat would affect it in a permanent way.

It is just possible that the new thermostat is also faulty - perhaps it does close, but is opening too early. If you can compare the temperature reading before the failure of the old thermostat and now, that might be informative.

Edited by FP on 02/01/2015 at 13:07

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Fri 2 Jan 2015 19:45
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Bonebaker

Thanks for your reply. I have tried out a new OBDII interface and Torque Pro this week and it fortunately gives me access to the sensor, it says that I get to 88C, and on a hill up to 90C before falling back to 88C. So I think that the new stat is working fine, and I assume that the coolant temperature sensor also works too.

You can't get a lot of information from the ODBII interface, but you get some, and I will do another post to ask whether my boost and MAF readings seem fine.

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Fri 2 Jan 2015 23:16
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Andrew-T

As other posters have said, MPG is much worse in winter.

I'm not sure. I have driven Pug diesels for many years and have looked for seasonal variations in fuel consumption without success. The numbers fluctuate, perhaps ± 5%, but not apparently connected to any yearly cycle. Admittedly the roads I drive on may not be typical, but there it is. And I don't rely on the car's computer.

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Fri 2 Jan 2015 23:58
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - Big John

For a 1.9 TDi PD 100 Passat 40 mpg around town and low 50s on a run sounds normal.

I have a Skoda Superb PD 100 and it has always done this sort of mpg and I've had the car for many years. PS my on board computer now reads about 4mpg under e.g. 50mpg shows as about 46mpg on the display. For some reason this gap has got worse with time.

Brim to brim on a long run I've always got about 600-650 miles. Your Passat should have the same size tank.

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Sat 3 Jan 2015 19:44
VW Passat TDi PD 2005 100 - Poorer MPG after stuck open thermostat - madf

I keep detailed records and can show mpg improving summer/deteriorating in winter..Been like that since I started driving ....

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