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Discussion Starter · #1 ·
Need to ask here as its bothering me in some way - I had thermostat replaced over 2 weeks ago - old one was just a bit iffy, ocasionally dropping the temp a little bit but in general it was still working well.
Since replacement, I am getting ridiculous (for me) MPG averaging 26+, recently even 26.5MPG. For all 3 years of having car I rarely could get over 24MPG, usually it was 220miles until full tank refill with my way of driving.
Nothing else changed - I still drive the same way I always do, same road I always do (60miles daily to work) , same petrol station etc etc, only difference - I can easily do 250+miles now on full tank ever since thermostat was replaced.
Im on my 3rd full tank now since, still averaging way over 26mpg, resetting the count after every refill - is it possible thermostat really affects MPG so much? I am sceptical, also somehow little wary that it might be sign of some power-starvation problem but car doesnt feel sluggish or less powerfull it used to be, so what gives?...
 

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Cars and their tunes are designed to run at a given temperature.

For example, in my NA 2.2L Ecotec from my 2006 Chevy Cobalt - that was 194*F. These cars are slightly higher at 205*F iirc. When the thermostat was stuck open and needed to be replaced - even cruising around the 90-110*F Las Vegas, NV air, the car would regularly be at ~140*F coolant temp. It is realistic that if your thermostat was frequently stuck open, you were running at much cooler temperatures. Ergo just like a car warming up in the cold the fact that you gained about 10-15% of your fuel efficiency back from being back at the proper operating temp, it's not suprising.

In theory it should have triggered some emissions based CEL's like it did on the Cobalt, but that's all EPA based and per car manufacturer (i.e. domestic vs import, and how they applied the standards, etc), but that's country based, so your country may be different irt emissions and not as strict.
 

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The 24 mpg sounds a bit low. I wonder if you had a slight vacuum leak that was throwing off the fuel trim that was corrected when things were reassembled.
 

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Discussion Starter · #4 ·
Avenger - old thermostat was operating correctly for atleast 2.5 years, it started to go weird only within last 6 months. I did some Vida checks etc and it was behaving OK for like 99% of time, just sometimes was not closing fully and it was visible by needle slightly off the vertical. All those times, almost always, I had 24ish MPG. I know proper operating temperature will affect MPG, just Im surprised its by THAT much given all circumstances.

Autogeek - my first though also, but my mech assured me he assembled it the same way as it was disassembled, no extra vacuum pipes "found and connected" etc. I also did dismantled intake and OTE few times so not like it was never removed or checked. I was also considering its because battery was disconnected and car needs to calibrate itself but it wouldnt last for so long. Today I filled up 44 liters with 251Miles on clock, averaging 27MPG - literelly NEVER happened before. 24MPG may be low, but I wasnt really bothered about it knowing my driving style - I am not very economical, drive pretty aggresively and in general do not save the car much.
 

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The ECT is definitely a big factor in the mixture calculation, and a bad reading will throw things way off. The gauge in the dash has a really wide "flat spot" in the center and won't tell you a thing. But the value read from the OBD port is giving the actual ECT value. A thermometer can give you the actual coolant temperature. If you're truly concerned, compare them! Personally I'd just enjoy the newly found economy. Mine is nowhere near that.
 

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Discussion Starter · #6 ·
The ECT is definitely a big factor in the mixture calculation, and a bad reading will throw things way off. The gauge in the dash has a really wide "flat spot" in the center and won't tell you a thing. But the value read from the OBD port is giving the actual ECT value. A thermometer can give you the actual coolant temperature. If you're truly concerned, compare them! Personally I'd just enjoy the newly found economy. Mine is nowhere near that.
Vida reports the same operating temperature as before (90deg C if i remember correctly), I noticed needle starts go left when temp drops by 4 deg. I am gladly accepting better MPG but I just wish my guts were not telling me its "not normal, something is wrong"... :p You know the feeling when everything you planned goes nice and smoothly and at some point you are starting to expect an earthquake? ;)
will keep on monitoring anyway, just wanted to hear out some thoughts on that aspect.
 

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I had the opposite after changing my oil filter (PCV) housing. I used to get 30-33 MPG highway and now I'm down to about 28 MPG. Not sure if it is one of the PCV hoses or the intake plenum fom manhandling the air filter box, but I think I have a small leak. My long term fuel trim is running about +7%. I had replaced the longer hose that goes up to the top of the engine when I did the oil filter housing but no the small right angle hose. I got a new hose and intake gaskets to try an solve this. Just need time...
 
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