Does bad thermostat, lower operating temperature, significantly decrease MPG?

[ksing44]

The operating temperature on my car has decreased. The last 2 days, on a 4-hour drive each way, the temperature was lower on the OEM gauge and measured ~155 degrees F on my supplemental gauge. My MPG for the trip was at least 5 MPG less than expected. I wonder if maybe the lower MPG is because my thermostat is bad.

It's also been cold, I suppose we still have winter fuel, my car has the FIX, and I have new tires that are not advertised as having low rolling resistance, but the new lower MPG on the longer trip makes me wonder if it's because of the thermostat. In general I've been getting lower MPG lately, but I thought it was the short trips because Covid changed my travel habits by eliminating my long commute.

Thank you for your help!

 

[03TDICommuter]

It will definitely reduce your MPG

 

[DivineChaos]

5 mi to the gallon these cars takes so little to change. It could be tire pressure, it could be your new tires have a little higher roller resistance. It could have been a slight stronger headwind. A very slight dragging break. A wheel bearing that's starting to drag. But being your thermostat isn't working properly and is stuck partially open. I would change it. It is a pain, but it needs to be done.

 

[MrCypherr]

Wondering the same thing but Ive noticed it on cars that have DPF/EGR parts gone. Since there is no EGR, I know the temps drop alot easier but usually only when im stopped/idle. How could you tell the thermostat is bad in other ways?

 

[ksing44]

How could you tell the thermostat is bad in other ways?

I don't know my thermostat is bad. I just think it must be bad because my car never gets to temperature anymore. I have a secondary way to monitor temperature with my P3Cars gauge, so I know it's not the OEM gauge that's wrong.

5 mi to the gallon these cars takes so little to change. It could be tire pressure, it could be your new tires have a little higher roller resistance. It could have been a slight stronger headwind. A very slight dragging break. A wheel bearing that's starting to drag. But being your thermostat isn't working properly and is stuck partially open. I would change it. It is a pain, but it needs to be done.

It's true the car is sensitive to change in MPG, but this is a significant change and quite obvious on my 4-hour highway trip that I've done many times over the years. It's actually more than 5 MPG, probably more like 10 MPG less than in the past. Even after the fix, I could always get there and back with one tank of fuel. Now I have to refuel on the way home during the trip.

And the fuel was $5.95/gallon, the most ever in the 12 years I've owned the car. I also wonder if it could lead to issues with the DPF and/or EGR, if the car never gets to the proper operating temperature. I guess I'll have to get it fixed!

 

[DivineChaos]

A weak tstat will kill mileage, let your car never totally warm up, cause frequent regens. I noticed an immediate change after it was replaced. I'd have the heat cranked and it'd take 45 min if driving to get okay heat. Now with lt on the same settings. It's too hot in 20 miles.

 

[ksing44]

^^My heat seems to work okay so far, but I am concerned about maybe experiencing more frequent regen activity. I'm not a DIY mechanic, so I wonder how much it will cost to get the thermostat changed. I also wonder if there are any other things to get done at the same time, e.g., maybe a new water pump even though it was changed with the timing belt at about 130K miles. I'll hit 190K miles this week.

 

[DivineChaos]

^^My heat seems to work okay so far, but I am concerned about maybe experiencing more frequent regen activity. I'm not a DIY mechanic, so I wonder how much it will cost to get the thermostat changed. I also wonder if there are any other things to get done at the same time, e.g., maybe a new water pump even though it was changed with the timing belt at about 130K miles. I'll hit 190K miles this week.

tb kit should be done every 120k. Thermostat is 4 hours labour. So 600 bucks to replace a $40 part. Plus new coolant.

 

[MTB_TDI]

My t-stat was bad when I bought my '11 Golf. Gauge would read up to temp most of the time but drop when I went downhill. Seemed to never close so it would never get to full operating temperature. Mileage was about 37 mpg. It had a bad dpf too. I did the TB, WP, all maintenance and fixed some restrictions in the exhaust and replaced the T-stat. Now I routinely get 47 mpg. So, yes, the T-stat will affect mpgs.

 

[ksing44]

I don't have any fault codes so I asked about the thermostat at my dealer where I go for service. The service writer said they can test the thermostat to know if it's not working properly before doing the work. Does that sound reasonable and correct?

Of course I already know the car doesn't get up to the same operating temperature as it did in the past, but it would be nice to have a definitive measurement before doing the work.

Thank you again to everyone for their comments!

 

[MrCypherr]

Im curious to monitor it with cars that have a full EGR delete since there is no warm air going back into the engine to heat it up quicker. I know cars that have a full EGR delete tend to take a bit longer to warm up and the coolant temp needle drops when coasting but goes back up when on some load. I guess it would be harder to determine if its a t-stat or just a no EGR condition.

 

[DivineChaos]

Im curious to monitor it with cars that have a full EGR delete since there is no warm air going back into the engine to heat it up quicker. I know cars that have a full EGR delete tend to take a bit longer to warm up and the coolant temp needle drops when coasting but goes back up when on some load. I guess it would be harder to determine if its a t-stat or just a no EGR condition.

My car was very hard to get to full operating temperature and very good heat. After I replace the thermostat I would get really good heat pretty quick.

 

[Mongler98]

Absolutely.
I use a real time MPG and gph display with a scangauge II and the mpg and more importantly the gph level drops significantly once at or near operating temp.
Let's put it this way.
These cars have a dummy coolant temp gauge. Wide range from below thermostat temp to well above will indicate as normal operating holding stead on the gauge. VW thinks that it would be bad for a driver to see a wild swinging needle. Diesels and especially small diesel engines fluctuate widly in coolant temp.
That being said... having a hotter tstat isnt a bad idea.
On a modified engine..... bad idea!
Usually tstats only fail open but sometimes fail shut. Usually never fall in any other way. They do have very hot or miss QC though. So having one open colder from the factory would not surprise me.

 

[jesus_man]

Mine wasn't coming up to temp either. Changing out the T-stat is probably the single most difficult job I've performed on ANY vehicle and I've done Timing belts, clutches, etc. If you are hiring it out, make sure you find someone who knows what they are doing! Best to remove (if not replace) the cross-over tube that runs behind the oil filter housing and towards the firewall. I only replaced the o-ring, but there is just no room in there to properly seat the thermostat without destroying the o-ring seal to that pipe. This resulted in a leak at that juncture and I got to do it all over again! Removing this cross-over pipe requires new gaskets for the oil filter housing as it has to come out too. So maybe coincide with an oil change too.

I've got a thread on here you might want to find.

 

[ksing44]

I had the thermostat replaced at the dealer. As indicated previously, my car ran at 155 to 165 degrees but I had no malfunfunction indicator lights and no faults with VCDS. The dealer said, "measured values from the thermostat were off, but no faults in the system". They had most of the parts, but had to order a washer and a gasket. They charged $1,154.95 to replace the thermostat, including a full coolant exchange. That was much less then the cost of a new 2022 Golf R 6-speed manual, if I could even find one!

 

[Mrrogers1]

I'm one of the, lucky (maybe) ones that had tstat replaced at the dealership as part of emissions warranty before they could fix/replace DPF. I have Scangauge II and i know the water temp was where it should have been @ 191deg once operating temp. I didn't complain but it's tied to emissions very closely so definitely should be replaced when you can get to it and really before cold weather.