T-stat temp vs. MPG

jayhawkclint · Post by **jayhawkclint** » Sat Feb 07, 2009 9:27 am

Just as an aside to the 0128 codes I see posted:

Early in the winter I noticed my truck struggling to reach 160* on the temp guage. At first I just thought it was cold weather, running the heater, etc. Then we had a couple days get down below 20* and it started throwing an 0128 code. Well, the truck was running well otherwise, and it was too cold to go out and work on it, so I put it off. Finally got around to changing it a few days ago. Much to my delight, MPG has gone up 10%!!

Two weeks ago I made a 350mi interstate trip with the cruise set at 72mph, outside temp 50*, terrain was mostly rolling hills. Got 20.1mpg on the trip.

I noticed I was getting slightly better on my combined city mileage with the new t-stat, but hadn't done the math. Yesterday, measured the mileage on the same exact 350mi trip, cruise set at 72*, outside temp 60*, got 22.2mpg!!!!

It's been over a year since I've got a 22+mpg tank of gas. I'm theorizing now that the t-stat has been going bad for a while, just didn't notice it until the winter months hit. If you've got an old t-stat, or have swapped to a 180* or other non-OEM t-stat, I'd highly recommend installing a new 195*.

killian96ss · Post by **killian96ss** » Sat Feb 07, 2009 11:03 am

I average 19-21 mpg using a Jet 180 t-stat.

Steve

roadrunner · Post by **roadrunner** » Sat Feb 07, 2009 1:49 pm

After my original stock stat started setting P0128 codes I replaced it with a new 195* and also noticed improvement in fuel economy. Not surprised by Steve's mileage. I think probably his ambient outside temps are a little more stable than those in OK or here in KS.

green02crew · Post by **green02crew** » Sat Feb 07, 2009 3:08 pm

I have heard of temperature and fuel economy studies. The warmer the engine, the more efficient it can be. However, now you are going to be on the verge of knocking with the higher temps. Timing has to be retarded and fuel economy and power reduced. There is a happy medium. I think the stock temps are too high while the 180* stat is too low. I would prefer right in the middle however it is not available. I would personally rather go colder than warmer. Sacrifice some economy for power and longevity.

killian96ss · Post by **killian96ss** » Sat Feb 07, 2009 4:44 pm

If you don't have e-fans, which I don't, you really can't take full advantage of the lower t-stat because the factory fan clutch still doesn't turn on until 210 degrees or higher.

The 180 works good in the winter, but I can still overheat my S10 with the same t-stat in the summer.

Guess I'll have to upgrade one of these days.

The 195 is slightly better for emission reasons, but as mentioned above you also lose a little power running that high.

I smogged my ZR2 with the 180 t-stat and all my emission #'s were 0.

Steve

F9K9 · Post by **F9K9** » Sat Feb 07, 2009 5:34 pm

killian96ss wrote:.....................The 180 works good in the winter, but I can still overheat my S10 with the same t-stat in the summer.................

I still threw the P0128 codes in the fall (we do have different climates) and had to change back to the OEM 195° tstat when cooler weather hit me. After relocating the IAT further away from the MAF, I have had zero problems. The codes were thrown with the stock fan and the efans until I relocated the sensor.

killian96ss · Post by **killian96ss** » Sat Feb 07, 2009 7:37 pm

I guess every vehicle behaves differently because I had the 180 t-stat in my CC for 80k+ miles and never set that code.

Even when I stayed at Lake Tahoe for a week in 5-10 degree weather the code never popped up.

I've had the same 180 t-stat in my Z now for 50k+ miles with no codes.

The IAT sensors in both S10's are in the stock location.

Maybe I'm lucky or maybe this problem isn't all that common when running a 180 t-stat.

Steve

jayhawkclint · Post by **jayhawkclint** » Sun Feb 08, 2009 5:06 pm

I should clarify by saying that my t-stat failed to the point I was getting <160* gauge readings on those 20* days. I believe it was probably just running wide open.

Post by **HenryJ** » Sun Feb 08, 2009 5:36 pm

killian96ss wrote:I've had the same 180 t-stat in my Z now for 50k+ miles with no codes...Maybe I'm lucky or maybe this problem isn't all that common when running a 180 t-stat.

I did Ok too until I installed the bigger radiator.
With the stock radiator it was right on the edge. It takes 13 failures (IIRC) to set a code. Over a week of normal driving in cold weather. Sometimes more than a month.

F9K9 · Post by **F9K9** » Sun Feb 08, 2009 5:56 pm

HenryJ wrote:................I did Ok too until I installed the bigger radiator................

OMG, I am forgetting about that mod altogether. I am sure the radiator was after the t-stat but, can't recall when the codes started.