Increasing engine operating temp to increase MPG

[Taylor95]

I have seen a small improvement from doing a partial grill block. This has helped my engine reach operating temperature quicker in the cold weather and it runs slightly warmer.

I agree with a hotter thermostat being more efficient. Here is someone else's experience:

On an LJ I used to have, it gained 1 to 2 mpg (depending on temp/weather/season) from installing a Stant 205F thermostat (Stant part # 13352). That's a good example of a Stant 205F thermostat in a Jeep 4L engine improving gas mileage in a moderate to cold climate.

A 205F thermostat is helpful in moderate climates to cold climates because it allows engine to run at (or near) full 210F op temp year round (instead of under-heating at 185F to 195F). IME, this is a big help in moderate to cold climates and improves gas mileage, especially during Winter. Over the last 20 years, I've done this on 3 Jeeps and 2 cars in a climate that varies from moderate to cold. It also works fine in Summer where we usually have 75F to 85F days, and sometimes mid 90s. Occasionally it gets up to 105F here in Summer. I find a 205F thermostat helps a little on cooler Summer days, more help in Spring and Fall, and a great help in Winter. On our occasional hot Summer days it's no help, but doesn't hurt anything. I'm using a Stant 205F thermostat (part 13352), which is same as Napa 205F and Gates 205F (Stant makes those brands).

If you live in a frequently hot climate your engine is already at, near, or above full op temp (210F) with stock 195F thermostat. If you live in a hot climate, a 205F thermostat offers no benefit and would probably be a detriment. If you're in a hot climate, I recommend upgrading to Stant 195F Superstat (part 45359), which is same as Napa 195F Superstat (Stant makes Napa). It opens at stated 195F temp and maintains a more stable, consistent temp than most thermostats. A stable, consistent temp helps the engine's computer fine tune combustion for better gas mileage. Another great thermostat is Motorad 195F Highflow (part 2000-195).

If your thermostat is to hot for your climate, your fans will work harder, which increases drag on engine and reduces gas mileage. It takes more power to turn mechanical fan when fan clutch engaged, which increases drag on engine. It takes electricity to turn the electric fan, which makes alternator work harder, which increases drag on engine. Increased drag on engine reduces gas mileage and performance.

XJs in very hot climates will have their fans turning a lot no matter what thermostat they use. For that situation l recommend a Stant 195F Superstat or Motorad 195F Highflow thermostat (high performance thermostats) and passive cooling mods: such as a larger radiator, oil cooler, transmission cooler, and/or hoodvent(s). Passive cooling mods don't use energy and therefore don't decrease gas mileage. In fact, in a hot climate, passive cooling mods can increase gas mileage by reducing the amount of work the engine fans do.

Gas burns more efficiently at hotter temperatures, but you want engine cool enough so your fans aren't coming on. The ideal situation is engine running as hot as it can without engaging mechanical fan clutch or turning on electric fan. The fans engage at around 214F to 216F. Gas burns really efficiently at engine temp of 208F+, and especially 210F+ (IME). The ideal engine temp is anywhere between 208F and 213F (IME). That's hot enough for an efficient fuel burn, but cool enough so fans don't come on.

https://www.jeepforum.com/forum/f11/...5-ats-1551782/
I have also read experiences where people have resolved their bad gas mileage by replacing their 165 degree thermostat with an OE 195 thermostat.

[squirrl22]

Nice find, Taylor! Actual hands on experience! So, if the LJ averaged 16mpg, going up by 1-2MPG translates into 6-12% improvement in MPG, switching to a 205F thermostat!
Thanks for the data!

[JRMichler]

Originally Posted by squirrl22

I remember being thrilled that I could now get hot air blowing from the heater less than 1 mile from home, instead of 5-6 miles.

This has nothing to do with a hotter thermostat. It means that the old thermostat was not fully closing, so the engine warmed up slowly. If a thermostat is fully closed, the engine will warm up at the same rate regardless of the thermostat opening temperature.

[ksa8907]

Running hotter does typically increase efficiency, 20% seems a bit high to put squarely on operating temperature change. Perhaps hotter under hood temps also caused a warm air intake effect.

The reason mfg's don't go much hotter than 200 is thermal expansion and oil degradation. The metal parts push on eachother more at the same time the oil life is being reduced. Perfect recipe for destroying an engine for 80% of vehicle owners who dont maintain ther vehicle properly.

[RedDevil]

My Insight uses double spark plugs, has EGR and a thermostatic valve that will open partially at 81 degrees Celsius (177.8F), and fully at 92.

Usually the temperatures will stay in the low (1)80's even with my partial grill block. But on longer trips the intake temp will gradually build and at some point that reflects in the coolant temp reaching to the 90s.
All is fine up to 94 degrees (200 F, indeed). Above that MPG suffers noticeably. I then set the blower to max and the cabin temp up; coolant temp drops quickly and good MPG returns.

So it depends.
You need to monitor what's going on, and don't expect miracles when you already have EGR.
I don't see anything more than a small gain (2-3%) and taking it too far does hurt. But a different car may react differently.

[squirrl22]

Originally Posted by JRMichler

This has nothing to do with a hotter thermostat. It means that the old thermostat was not fully closing, so the engine warmed up slowly. If a thermostat is fully closed, the engine will warm up at the same rate regardless of the thermostat opening temperature.

Could be, I don't remember if the old mk 2 Jetta heater core was on a separate line from the thermostat.

The big question, is:
Have you had any experience raising the coolant temp to increase MPG? If so, this is the place to share that experience.

[RedDevil]

Originally Posted by squirrl22

Could be, I don't remember if the old mk 2 Jetta heater core was on a separate line from the thermostat.

The big question, is:
Have you had any experience raising the coolant temp to increase MPG? If so, this is the place to share that experience.

He refers to the thermostat that closes off the radiator loop when the engine is cold, not that of the heater core.

If the radiator stat is bad it will even when the engine is not yet warm allow coolant to the radiator which feeds back cold coolant to the engine, preventing it from heating up quickly.
This is indeed very bad for economy. I had this happen in my '85 Civic. I had no economy readout back then, but it made itself felt at the pump.

[Piotrsko]

You do kinda want to keep the internal parts temperatures below 250 because at that temp the oil starts to break down and lose lubrication factors.

Typically at the rod to piston junction, but I don't have current technology information and have no way to measure that. Historically rod bearings fail from heat/lube failure.

Additionally, since there seems to be a dearth of information, some experimentation needs to be performed because typically most ICE favor different operational temperature efficiency.

I am 100% in favor of grill block, test, modify, test rinse, repeat until trends show you went too far or oil temps get to be about 225f, or the smoke point of whatever oil you use. Smoke point should be available on the mfr website.

[oldtamiyaphile]

Originally Posted by Taylor95

I agree with a hotter thermostat being more efficient. Here is someone else's experience:

I have also read experiences where people have resolved their bad gas mileage by replacing their 165 degree thermostat with an OE 195 thermostat.

My 05 TJ has a 210* thermostat stock. I don't like running so close to boiling point. Remember this is an ancient engine. It's ridiculously fast to warm up considering there's 200+lbs of iron in it! Lots of excess heat.

All 'new' cars I've had run 190 degrees and in normal driving the thermostat doesn't actually open fully since their normal operating temp is 176 degrees. Its been this way on my last three Euros so it's not just coincidence. Fans kick on at 200 and it seems the only way for this to happen is with a really aggressive grill block.

Could also be that this is done for NOx control. Two of them have 20k mile oil change intervals so if lower temps reduce oil deg then it could be done for that reason too.

But it seems to me new cars run cool.

[samwichse]

Honda put a hotter thermostat in the first gen Insight than any other car of their lineup at the time, which tells me there is SOME efficiency to be gained. Although the claimed in this thread doesn't seem realistic to me. Seat of the pants driving my Insight and watching the temps tells me it's probably <5%.