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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 October 23rd, 2019, 08:10   #1
Join Date: May 2015
Location: Nashville TN
Fuel Economy: 44.361
Default Mileage increased after running engine hot

I noticed something peculiar lately with my car. Since DieselGate, my mileage went down to about 48 miles/gallon average. Recently i installed a Curtis hitch and bought a 5x8 trailer. I used it to drive down from TN several time. Due to the poor aerodynamic the mileage went down to about 22 MPG. During those trips, I was monitoring the Oil temperature, from 230 to 260, reaching the max of 278 degree F for long period of time. Water temp gauge barely registered above normal . So I kept going. I made the trip back to TN twice since without the trailer and noticed my mileage went back to before DieselGate , around 54 MPG.
Question: did my "overheating" oil cleaned up the engine and made it run more efficiently ? I recall on my old 2009 GTI, the intake would coke up with carbon deposit and after cleaning those deposits , the engine would respond better. Something similar here or I am just lucky (and delusional)?
2015 VW Sportwagen S TDI
1999 BMW M Coupe
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Old October 26th, 2019, 16:40   #2
Join Date: Apr 2005
Location: Mersyside, UK

Heat always plays a good part in the combustion, there's been loads of people playing with run hotter thermostats with good results
Another theory that may also comes into play is the fumes from crack breather getting drawn into the intake even if it separates oil out
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Old October 26th, 2019, 20:12   #3
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Join Date: Jan 2004
Location: planet Earth
Fuel Economy: 52/47/39

I have been noticing better mileage as well since running the car really hard when possible. I am regularly driving 585 miles to/from San Diego and driving really spirited. For going 20mph over what I usually drive it is only costing me 2mph. The car is also not smoking as much at WOT.
'03 MK IV Sedan 339k miles and counting
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Old October 28th, 2019, 05:37   #4
Powder Hound
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Join Date: Oct 1999
Location: Conkud, New Hampshiyuh, USA
Fuel Economy: pretty freakin' good.

Originally Posted by Crispy_8V View Post
Another theory that may also comes into play is the fumes from crack breather getting drawn into the intake...
Huh. I have my doubts that crack would really increase the mileage on my car, and trying it would be illegal possession. Maybe you could post results from a controlled experiment?


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Old October 28th, 2019, 15:02   #5
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Join Date: Jul 2006
Location: Charlotte, NC

You were likely able to do a strong passive regeneration with that trailer run, it cleaned out the DPF and likely the vanes on the turbo if they were starting to stick at all. High EGT is your friend with these DPF equipped engines.
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Old October 30th, 2019, 13:25   #6
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Towing is your friend, it gets a diesel hot, which they like. Burns off all the carbon etc, rings get some heat, seal better etc.
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