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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old April 12th, 2018, 14:40   #1
jshwhite
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Default Fluctuating fuel economy - I'm stumped

I have a 2010 Jetta wagon, 2.0 TDI, 6sp manual. It's been having completely random fluctuations in fuel economy. I'm at a loss and hoping for some ideas. Here's the story:

- I got it (salvage) in October 2016. First trip was in November with a 12 hr drive one way and I averaged 45 mpg without trying. I was happy
- December I took a few short trips and found I was only getting 38 mpg. I wasn't so happy But I live in Michigan and figured it must just be the winterized diesel or the sub-zero temps.
- First of January I drove to Texas and back, tried Lucas injector cleaner, got diesel down south, enjoyed 70+ temps, was careful to avoid lead-foot - made no difference. Averaged 37 mpg.
- Couple weeks later I took a two hour trip in northern Michigan, snow and ice, cold temps, got 42 mpg.
- A week later I took a similar trip and got 38 mpg.
- A week later I drove to Florida. Got 42 on the way down, 36 ish on the way back. Absolutely no difference between these except I was going south one way and north the other.
- I was out of town for a few months and got back to driving it mid-2017. Getting about 38 mpg, and also noticed front and rear control arms were bad. I decided to go all-out and threw about $1k in suspension at it - struts/shocks, mounts, springs, bushings, arms, etc. etc. New tires. Absolutely no difference in mpg. One day it gets 38, the next 45.
- October I drove it 5 hrs and got 46 mpg. Sat over the weekend and then drove it 16 hrs and got 38 mpg. Again, no difference at all between the trips except the day of the week!
- December I drove to Phoenix, AZ (from Michigan). On the way out the clutch started slipping and the DMF started rattling. Figured I didn't want to be stranded on the way back so I had the clutch/flywheel replaced in Phoenix. I wondered if possibly the slipping clutch or DMF had anything to do with the MPG. Nope. Got the same coming as going - fluctuating day-to-day by as much as 6 mpg.
- Lately it's been short (1-2 hr) trips. One day I might get 36-38, the next day, on the same tank of fuel, same road, I'll get 42-44.
I've pretty well ruled out breaks and suspension, so now I'm wondering if it's something engine related.

Are these fluctuations in MPG normal? I haven't come across anyone else saying anything about this. My mom has a 06 1.9 TDI 5sp and she gets very consistent mpg, no matter the load, season, or driving conditions.
Obviously I'd like to get rid of the fluctuation, but I'd also like to get back to the original 45 mpg goodness!

Sorry for rattling on. Hope someone has some ideas. Thanks!
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Old April 12th, 2018, 15:22   #2
Tdijarhead
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Are you using the unreliable, useless gizmo on your dash that attempts to calculate fuel mileage or are you using the tried and true old school pen and paper method?
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Old April 13th, 2018, 09:13   #3
jshwhite
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Haha, that was actually my first thought that maybe the "dash gizmo" is giving me bad readings. So I checked it a few times the old fashioned way and found it was the same. So I haven't measured every tank of fuel with pencil and paper, but I've done enough to know it's definitely not consistent fuel economy.
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Old April 13th, 2018, 10:11   #4
oilhammer
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Track every tank, with pen and paper, for 100,000 miles, and get back with us and see what the average is. I bet it will be within normalcy for the car. Remember, one trip may experience a regen, another not. You can try and keep track of those, too, if you like but it probably isn't worth the effort. Drive it. Enjoy it. Know that it gets "very good" fuel economy no matter what, and keep it in good order.
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Old April 15th, 2018, 17:22   #5
jerrymander
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I had a 2011 Golf with an extremely similar engine in Minnesota over the winter. I never saw a tank above 35 mpg during the winter. I typically would get 42-50 mpg in the summer. And yes, oilhammer, this was with me keeping an autistic-tier spreadsheet of tank calculations.

On my 2015 I make a trip up the coast and down the coast pretty regularly. If I perform this trip in the daytime when it is 70-90 F outside and I don't draft cars, I get about 45 mpg. If I take the trip in the night when it is 40-60 F and I don't draft cars, I get about 39 mpg. I noticed this with my 2011 as well: the cars like warmer temperatures.
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Old April 16th, 2018, 07:11   #6
jshwhite
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Thanks guys. I'll get out the old pen and paper and keep track for a while and see what happens. I do think there might be something to the temperature, but I'm not sure that's the only factor.

oilhammer, how does one know when it does a regen?
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Old April 16th, 2018, 07:22   #7
oilhammer
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Quote:
Originally Posted by jshwhite View Post
Thanks guys. I'll get out the old pen and paper and keep track for a while and see what happens. I do think there might be something to the temperature, but I'm not sure that's the only factor.

oilhammer, how does one know when it does a regen?
The engine will idle higher, the fans will be running at full speed, and the engine will seem a little bit less powerful and a little more noisy. Others can chime in with a more informed experience though, as I do not currently own one and have only experienced a few in customers' cars.

There are two types of regen, too. The "basic" version is less noticeable and is often all that many drivers ever need to make the DPF pressures happy. The more aggressive version is for sure noticeable.
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Old April 16th, 2018, 08:18   #8
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I got my Passat at the end of May 2016. Since then I religiously record how much fuel I fill the tank with and the odometer readings.
My Diesel usage is 5.57 litre per 100 km or 40 Mpg US. I do not have a lead foot and neither do I drive a constant 80 Mph as many in these threads claim to do and get in the mid to high 50 Mpg.
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Old April 17th, 2018, 07:47   #9
jshwhite
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Quote:
Originally Posted by oilhammer View Post
The engine will idle higher, the fans will be running at full speed, and the engine will seem a little bit less powerful and a little more noisy. Others can chime in with a more informed experience though, as I do not currently own one and have only experienced a few in customers' cars.
There are two types of regen, too. The "basic" version is less noticeable and is often all that many drivers ever need to make the DPF pressures happy. The more aggressive version is for sure noticeable.
So THAT'S what it's doing. I've noticed it and wondered what was going on. Thanks for the info.
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