Thermostat Change for Better MPG???

shizzler

Veteran Member
Joined
Jun 25, 2005
Location
Ann Arbor MI
TDI
05 BEW Wagon
Nice! 34 mpg and 205 whp = awesome.

Yeah residual heat is definitely a nice benefit in the winter time. Hotter coolant/oil can heat soak the block and engine bay pretty quickly. Combined with some grill blocks and an intact skid plate + side shrouds sealing off the engine bay, and I find I can retain noticeable warmth even over 6+ hours out in sub-freezing temps.
 

corrado tdi

TDIClub Enthusiast, Vendor , w/Business number
Joined
Oct 3, 2000
Location
San Antonio, Texas, U.S.A.
TDI
2016 Golf TDI
Yesterday's 34mpg tank was also with the A/C running and 95-100 degree ambient temps. I wonder if the cooler weather we eventually will see will net better mpg since the A/C will be off or will the cooler temps cancel out the non A/C operation. I think the engine will run closer to 100C when it cools off. It used to run at 88C with the stock tstat. Interstingly, the Evans coolant allows a much greater variation in running temps than EG/water.

I'm thinking that warmer engine oil and tranny oil are important for reducing fuel use.
 

shizzler

Veteran Member
Joined
Jun 25, 2005
Location
Ann Arbor MI
TDI
05 BEW Wagon
Definitely. Warmer oil and coolant = warmer cylinder liner, cylinder head and piston = less heat transfer through them during combustion = better thermal efficiency. I.e. inching closer towards adiabatic operation. The downside is less thermal safety and stability of components, but thankfully OEMs engineer us a nice huge safety factor there. The only thing I fear with hot coolant is piston safety during extended WOT runs.... so far so good.

Of course warmer engine and tranny oil means less friction loss due to lower viscosity, also.

If your operating temps cool off as the weather turns to fall, just block your grills off accordingly! You can control operating temp as desired by altering allowed airflow through the radiator and engine bay.
 

BakoTDI

Veteran Member
Joined
May 5, 2005
Location
Bakersfield, CA
TDI
Jetta, 99 MK4
Sorry to resurrect this thread but is it possible to change through VAGCOM when the cooling fans come on? Instead of messing with t-stats and losing coolant, wouldn't it be much easier to change the programming? just a thought.
 

adjat84th

Veteran Member
Joined
Dec 13, 2008
Location
Virginia Beach, VA
TDI
'01 Jetta TDI/'15 Golf TDI
I can't remember the last time my cooling fans came on in either of my TDIs, these things just run cool. It's the initial warmup that kills fuel economy so the thermostat is the best solution to replace.
 

BakoTDI

Veteran Member
Joined
May 5, 2005
Location
Bakersfield, CA
TDI
Jetta, 99 MK4
I can't remember the last time my cooling fans came on in either of my TDIs, these things just run cool. It's the initial warmup that kills fuel economy so the thermostat is the best solution to replace.


I see. That’s what’s been killing me. My commute is short and with lower temps, engine barely gets to halfway to operating temp. I didn’t have an insulating pad on my hood (I just installed insulating matting) and just swapped the thermostat yesterday. Hoping to see a change. Just wondering if it was possible to assist engine by changing any programming. Anyway, hope my changes help some. Thanks!


Sent from my iPhone using Tapatalk
 

adjat84th

Veteran Member
Joined
Dec 13, 2008
Location
Virginia Beach, VA
TDI
'01 Jetta TDI/'15 Golf TDI
I changed the thermostat in my '01 after a winter when I noticed my gauge wasn't getting to 190 on the highway. After I replaced, not only did it stay hot, but it warmed up way faster. You should definitely see an improvement if it was an old thermostat. They just get lazy and don't stay closed when they need to.
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
A two year followup on this.

I never saw any improvement in fuel economy with the 91 degree thermostat. The only change was that on hotter days when shutting down, the engine would be hot enough to keep the cooling fans on for an additional minute or 2 after shut down.

I'm going to be swapping an 87 degree thermostat back in.
I finally got around to swapping out my lazy thermostat - 2003 Jetta was running at 80C on the highway, now runs at 92C.

Average fuel economy since purchase (May 2009) = 50.66mpg
Average fuel economy for last 10 tanks at 80C = 52.39mpg
Average fuel economy for first 10 tanks at 92C = 50.34mpg

These results may be counterintuitive, but there it is. YMMV.

Simon
 

amit9

Veteran Member
Joined
Oct 7, 2004
Location
Toronto/Scarborough
TDI
2003 Silver Jetta TDI GLS 5MT
I finally got around to swapping out my lazy thermostat - 2003 Jetta was running at 80C on the highway, now runs at 92C.

Average fuel economy since purchase (May 2009) = 50.66mpg
Average fuel economy for last 10 tanks at 80C = 52.39mpg
Average fuel economy for first 10 tanks at 92C = 50.34mpg

These results may be counterintuitive, but there it is. YMMV.

Simon
Simon, those are impressive fuel economy numbers, even with winter fuel on the tanks at 92C! Did the PP520 injectors and/or RC2/3 tune (in your signature) improve your fuel economy?
Any tips to improve my overall 39-40mpg fuel economy? I have tried everything by the book and still can't get any improvement. :(
 

P2B

Top Post Dawg
Joined
Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
Most of my mileage is on the highway in 6th with cruise engaged - very little city or rush hour driving, very few short trips.
Simon
 
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