VW TDI fuel mileage "not good"

[Alcohol]

I recently purchased a 2006 Beetle w/1.9 tdi engine. Is it common to get 10 mpg less in winter than warmer weather? Struggeling to get 38 MPG now in 20 degree weather,,Mother is not happy!!

[doc32469]

Originally Posted by Alcohol

I recently purchased a 2006 Beetle w/1.9 tdi engine. Is it common to get 10 mpg less in winter than warmer weather? Struggeling to get 38 MPG now in 20 degree weather,,Mother is not happy!!

Hmm that's not good. It sounds like needs tune ups. Mine gets 34-36. When the last time maintenance done on to it?

sent from DROID. 2001 NB 2.0 manual Cyber Green

[Alcohol]

Originally Posted by doc32469

Hmm that's not good. It sounds like needs tune ups. Mine gets 34-36. When the last time maintenance done on to it?

sent from DROID. 2001 NB 2.0 manual Cyber Green

I am getting 38 now but got 48 when it was warmer,,,mtnc has always been a priority to me.

[Smileybug]

10 less? Sounds like too much. 3-5 less in the winter though isn't uncommon.

Is this an auto or manual? When was the fuel filter done last?

[janitz]

How are you measuring your fuel economy?

Things to watch out for in winter;

- In colder climates the diesel fuel is replaced with winterized fuel, this has a much lower Cetane value and results in liter for liter loss of fuel economy. You can use a Cetane boost to offset this.
- In the winter the fuel seems to gas/foam up more. Often I've been at the pump and filled up to where it automatically stops (and then I get in the car and go because it's freakin' cold) and after driving for a few minutes the gasses vent out and the full tank is now only 7/8 full.

So in winter a fillup of the tank may not be as much fuel and it is of lesser quality. So a winter full tank gets me about 800 to 850km while a summer full tank gets me 950 to 1000km.

[05yellowbeetle]

My '06 gets 42 mpg, i run the silver bottle of diesel kleen/cetane boost in every tank. Any changes in your driving habits? I dropped to 38 mpg after a snow/ice storm when my 30 mile commute was done at 20-30 mph instead of highway speeds, next 2 tanks were 44 mpg with a 150 mile commute.

Check on tdiclub.org, i think thermostats have been mentioned for causing a drop in mileage.

[WardB]

My mpg dropped to 39 and change once this winter. I chalked it up to winter diesel.

[jfoj]

Anyone ever look at the coolant temperature to determine if the thermostat is soft and the engine is running too cold?

I do not work on the Diesels, but the engine temp would be the VERY first thing I would confirm.

[h_2_o]

funny you would say that i just helped a buddy with his bad mileage because of this. finally figured it out after we logged the engine temp and it was wicked cold for the tdi.

Originally Posted by jfoj

Anyone ever look at the coolant temperature to determine if the thermostat is soft and the engine is running too cold?

I do not work on the Diesels, but the engine temp would be the VERY first thing I would confirm.

[OmahaTDI]

Man, you guys are getting pretty good mpgs. My TDI is only averaging 34-36 mpg in town. I don't drive the highways much.

[jfoj]

I have been working on cars/trucks a long time and I am finding about 30%+ of the vehicles on the road have engines that are running too cold due to soft failed thermostats and most of the drivers have no idea.

This is due to the thermostat spring getting weak because of constant thermal cycles and the main spring being constantly under compression.

I typically suggest replacing the thermostat every 3 years.

What also complicates the entire situation is a total lack of understanding of how a cooling system functions and what the proper engine operating temperature should be.

Again, I do not know the VW Diesels well, but it appears that many of these engines use a 195F thermostat. Assuming this many may thing the engine temp should run close to 195F, but this is not the case with most engines.

It depends on where the temperature sensor is located on the engine. Most have the engine temperature sensor located in the cylinder head at the end of the coolant circulation path before the coolant is sent back to the radiator.

If this is where the coolant temp sensor is located, then the proper engine operating temperature will likely be 15-20F above the thermostat operating point.

Again, I have not had a chance to baseline a group of these Diesel engines, but I would expect that the baseline engine operating temperature should be 215-220F once fully warmed up.

This temperature should hold very close as both idle and highway cruise.

Most fuel economy and oil related problems with cars and trucks these days can be directly associated with cooler than normal engine operating temperatures.

Unfortunately you cannot rely on the cars temp gauge most of time for an accurate value of the engine temp, you have to use a scan tool that can display real time data to verify engine coolant temperature.

You can only verify soft thermostats when ambient temps are usually below 70F. I have found that once ambient temps are above 70F you can have a shift in engine coolant temps that may cause a soft thermostat to be overlooked.

Would be interested to hear what eveyone's Diesel baseline engine temps are running. This data would help all Diesel owners to determine what their engine temp should typically operate at.

[boertje]

We are averaging about 40mpg with winter fuel with our 06 DSG trans. About 45 summer.

[janitz]

Did a freeze my balls off fillup two weeks ago and pumped until there was no room left in the tank (52.8 liter fill, 55 liter tank) with power service additive.

I was able to get to 950km on the odometer, 590 miles, I filled up again to 52.5 liters.

So my approximate fuel efficiency was 42.5mpg