Thermostat Effects on MPG

minis2003 · Mar 15, 2015

How much does the Thermostat have on MPG?

I finally connected a scan gauge to my TDI and found out that it was a constant 170F. If I am not mistaken it should be atleast 180F to 190F.
My question do the BEW engines run cooler or is my thermostat going bad

I am leaning on the side of going bad and will replace it next weekend but I thought I would ask first since I am still new to TDIs

Drivbiwire · Mar 15, 2015

Big effect on a BEW engine.

Napa sells a 195F thermostat, give that a shot. It's easy to replace.

minis2003 · Mar 15, 2015

Just finished changing out the thermostat and now he temp is at 195F to 199F
Almost a 30 degree difference so I hope it will increase my mpg
Have to wait and see until next fill-up

Sleet · Mar 15, 2015

please update us after you fill the tank. I'd be curious how much your mileage improves.

-J

joep1234 · Mar 15, 2015

Diesels need heat. If your temps are low the ecm thinks the engine isn't up to operating temp so it is running rich trying to get up to temp.

minis2003 · Mar 16, 2015

I did a fill-up today but it only had 130 miles on it and half of that was with the old thermostat but it does look promising so far. Will report back next week once I am able to drive a few hundred miles on the new thermostat.

stennyboy · Mar 18, 2015

Also interested to see how this will effect the fuel mileage.

RacerTodd · Mar 18, 2015

My thermostat was running about 10 deg low - 80C instead of 90C. Replacing it increased fuel mileage by about 2 MPG or so.

minis2003 · Mar 19, 2015

i am trying to use most of the tank full before i fill-up so i will get a good idea of how it if any effected my MPG. it is a good mix of highway and city but that is my normal route.
the only good think is that i been recording my mpg since i got the car last may so i have a really good idea of my avg.

stennyboy · Mar 19, 2015

What year is your VW? 06 beetle? I just picked up an 06 jetta with a couple hundred thousand on her and i'm wondering if I should look into this.

minis2003 · Mar 20, 2015

Will I saw a 5 mpg increase during this fill-up but I will continue to keep track if that improves once I try to keep my speed below 70mph

I would recommend. Everyone to get a OBD-2 sensor then pair it to your smart phone some thin like this one
http://www.amazon.com/Version-Inter...im_auto_7?ie=UTF8&refRID=0GTFR8KTP5XAXPCJ6R4Z
This why you can see what you engine is doing engine temp, boost, intake temp and RPM
Every program different on android and ISO on what they will displayed so read them carefully. Torq lite worked well for me.
Hope this helps

fruitcakesa · Mar 24, 2015

I believe my 04 Jetta has a bad tstat.
First tank yielded 38+ mpg and that was all highway miles though speeds were in the 70-75 mph range for much of the tank.
New parts on the way.

stennyboy · Mar 26, 2015

I just replaced my thermostat on my 06 Jetta brm engine, with a 195 degree fail safe brand thermostat and after driving 20 mins my temp is 169, 170. I do not have the underbody shield but I didn't think it would have that much effect with it 50 degrees outside

minis2003 · Mar 26, 2015

The aftermarket thermostat have a larger by-pass valve to allow more water to flow around the thermostat before it opens then the stock ones.

The stock thermostat I took out had a larger valve then the aftermarket one I put in.
If my temp did not reach 190+F I would have replaced it with an OEM one

This is why most people here would only recommend the OEM thermostat

94cobra2615 · Mar 28, 2015

I'm running a hybrid thermostat and consistently see 197 to 204 degrees on my alh. Just another option

ottomatic · Mar 28, 2015

stennyboy said:
Also interested to see how this will effect the fuel mileage.

X2. My Dmax runs around 190 and I've thought about changing my thermostat in the JSW.

poormanq45 · Apr 24, 2015

I just replaced my thermostat a week ago on my 2005 BEW Jetta 5 speed.

I'm just over 1/4 tank at this point. I'll report the results in a few days.

The biggest thing that got my attention was when turning on the heat dropped the temp gauge to the 1/4 mark. VCDS showed 160 degrees.

I put in a Stant 45379 SuperStat which is 195 degrees. VCDS is now showing 195-205 degrees steady. I changed the coolant sensor at the same time.

So far it doesn't look like I'll be getting much of a change in MPG. Previously I was getting 42mpg. I put cruise control on at ~58mph and drive 50 miles round trip a day.

scurvy · Apr 24, 2015

joep1234 said:
If your temps are low the ecm thinks the engine isn't up to operating temp so it is running rich trying to get up to temp.

Do what now?

For the rest of the posters here, I am going on 2 years with a 195ºF Stant Superstat. Coolant temps maintain warmer than stock by about 5 - 12ºC as displayed on my SGII. No statistically significant difference in fuel economy or warm up times.

john.jackson9213 · Apr 24, 2015

A properly function thermostat has a big effect on your daily MPG. Several years ago I was getting consistant 37/38 mpg. Then after a long trip during which I saw coolant tempertures never getting above 160 on my ScanGuageII, I changed the thermostat to a new 195 degree version from a local chain auto supply store. Since that change, mpg has not been lower than 40/42, frequently and consistantly 44-45. Car still runs at 197 coolant temperature.

vwman2 · Apr 24, 2015

Does anyone know what temp a common rail TDI should be running at heat wise?

poormanq45 · Apr 24, 2015

scurvy said:
Do what now?

For the rest of the posters here, I am going on 2 years with a 195ºF Stant Superstat. Coolant temps maintain warmer than stock by about 5 - 12ºC as displayed on my SGII. No statistically significant difference in fuel economy or warm up times.

Was your old thermostat bad?

With the bad t-stat mine would only reach 190 if it was a hot day and I was sitting in stop and go traffic.

I can't comment on the MPG yet. It looks like I'll get within margin of error or my normal mileage.

j_martell · Apr 24, 2015

I'm gonna order parts and do mine(2006 Jetta wagon BEW 09A)....since installing my scan gauge i only see 165-170* with the grill blocked and no belly pan. even when the ambient temps were mid to upper teens (*C).....but i have noticed the buffer in the dash gauge...it will read 90*C when the scan gauge reports 165*F or more

minis2003 · Apr 24, 2015

On my new beetle bew used to get 34-36 mpg until I changed the thermostat and ran a can of BG244 and now I get 40mpg
The last 4 fill-up I got 40.46, 42.21, 40.66 and 40.70 mpg
I would recommend after you changed your thermostat run a can of BG244 in the fuel tank or a can od diesel purge just to make sure you injectors are clean

joep1234 · Apr 25, 2015

scurvy said:
Do what now?

For the rest of the posters here, I am going on 2 years with a 195ºF Stant Superstat. Coolant temps maintain warmer than stock by about 5 - 12ºC as displayed on my SGII. No statistically significant difference in fuel economy or warm up times.

An engine that doesn't reach the operating temp that it is programed to do, the ecm will see the low coolant temps as the engine is not up proper temp and will throw extra fuel in to help it warm up thus lower mpg's when not up to operating temp. Just saying...

scurvy · Apr 27, 2015

poormanq45 said:
Was your old thermostat bad?

Most definitely. I would barely break into the 70ºC range, even in stop & go traffic.

meerschm · Apr 27, 2015

vwman2 said:
Does anyone know what temp a common rail TDI should be running at heat wise?

this is an '09 over a ten mile drive, showing initial warm up. degrees C as this is what the car operates in, and VCDS records. start is ambient temperature after parking overnight.

87 degree c thermostat (starts to open temp) leads to a coolant temp reading that bounces around 200 degrees f, sometimes less, sometimes more, depending on load and conditions.

AVTechMan · Apr 28, 2015

I was wondering, on my wagon using Torque I noticed that at highway speed the coolant temp was reading 172F. Should I replace the thermostat with a 195F one and would that affect the MPG?

compu_85 · Apr 28, 2015

vwman2 said:
Does anyone know what temp a common rail TDI should be running at heat wise?

The Passats run quite warm... normal under load is about 213*F and it will go up to ~225*F on long hills.

At idle it will sit around 207*F

-J

poormanq45 · Apr 28, 2015

AVTechMan said:
I was wondering, on my wagon using Torque I noticed that at highway speed the coolant temp was reading 172F. Should I replace the thermostat with a 195F one and would that affect the MPG?

What is your ambient temperature?

I'd replace the coolant temp sensor and the thermostat. A 195 would be my choice.

My first tank I am seeing a ~1.5mpg increase.

nicklockard · Apr 28, 2015

minis2003 said:
How much does the Thermostat have on MPG?
I finally connected a scan gauge to my TDI and found out that it was a constant 170F. If I am not mistaken it should be atleast 180F to 190F.
My question do the BEW engines run cooler or is my thermostat going bad
I am leaning on the side of going bad and will replace it next weekend but I thought I would ask first since I am still new to TDIs

Massive. Probably the single largest source of bad FE when driver error is ruled out.

Thermostat Effects on MPG

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