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Discussion Starter · #1 · (Edited)
This was copied from another thread and edited for use as a STICKY.
The thread was about the 203* thermostat and questions of the validity of running higher temps. Fuel Mileage was also discussed here...

A brief search brought up many positive comments about the DieselSite 203*thermostat, but it is important that people know why higher temps are needed. Thanks for bringing this into the discussion.

Although most of what I am going to say has been said over and over, it never hurts to educate the new site members that the International engine was BUILT with a 203* stat. It got changed to the 195* for emissions reasons with the 1996 model year. It seems some folks would like some information on why running hotter is a good thing. In assuming that it is common knowledge when we say “diesels run better warmer”, I am not providing the technical information that some may want. Sorry about that. I don’t believe I have ever gone into the technical depth that I would like to.

About dyno numbers: I’m not sure if you looking for HUGE horsepower gains? You can dyno the same truck three times in a row and get different numbers – dynos aren’t usually that precise. Maybe you are looking for 1 HP or 5 HP maybe? That could never be attributed to only one variable. There would be too much variance on other things to attribute a change that small to any one thing about your truck. So, I’ll scrap the idea of a dyno run or a thermostat HP number. But, before closing that discussion, I will say this though –the 203* tstat has been purchased many times by guys wanting it for their new race engine rebuild.

What I have to say is not that technical, so let’s look at the TECHNICAL INFORMATION ABOUT HIGHER TEMPS SPECIFIC TO THE 7.3L NAVISTAR ENGINE first. Click that link for a great explanation from an engineer who specifically wrote an article for us about higher engine temps in our engines. Luckily for everyone he did write this because as you’ll see later, I’m a lot less “technical” about my thermostat.
After you’ve read that let’s move on to the biggest benefit that people want right now - Fuel Mileage gains. I never come out and give specific increase promises on how much anyone will gain. If fuel mileage interests you, then you already know that every truck is different, every driver is different, fuel all over the country is different. You’re going to have different results from truck to truck, driver to driver. If I were to guess the most common MPG gain that I hear about from customers, it would be about .5 MPG (point 5 miles per gallon) or ½ mpg. Not bad for a mod that costs less than $40 and makes your truck run more efficiently.

I started thinking about customer testimonials and laughed that I sometimes forget my BIGGEST customer testimonial – ME! I made this because I WANTED IT. I researched it and beyond a shadow of a doubt knew that this is what I wanted in my truck. I was lucky enough to get a hand from a thermostat engineer that knew the benefits of keeping the 203* in this engine. And then even more help from the engineer you just read from who was versed in measuring the effects of the stat on frictional horsepower losses in an engine based on engine temperatures.

This is the less technical end of things… Before we manufactured the DieselSite 203* stat people used the short stem International version of the 203*. WHY? Because it made a difference. A noticeable difference that many many people could “feel”. Running a 203* instead of the 195* MADE A DIFFERENCE.

We didn’t start the idea of using a 203* thermostat, we just made it possible to run it without overheating the back of the block and losing an engine. Here’s my testimonial: When I put the first 203* in my truck, I definitely noticed a difference in fuel economy and a difference in how the truck ran “empty”. My truck always runs quieter with better response when I hook a trailer up and let it do what it was meant to do. Now it just does it all the time. It runs the same ALL THE TIME. These engines weren’t designed to run cold. For anyone looking for a quantifiable number to stick to it – sorry – here’s the bottom line…it feels better. I don’t think that anyone who uses the stat and likes it cares if others are convinced of it’s supernatural magical powers (kidding).

Yes, the change can be subtle and sometimes completely unnoticeable if a guy loads his truck every day. In fact, that guy is NEVER going to reap the benefits of the 203* because he DOESN’T NEED IT! His truck already runs like it should. Hotter because it is under load. While some notice subtle, others notice dramatic results. The phone calls from guys picking up 2+ mpg when they didn’t expect it are enough for me to not care if there are people not convinced that the 203* could make a difference. Will it work for everyone? Yes, it works in every truck. In power, efficiency and economy. What you measure or feel is different in every truck. But in NO CASE has anyone ever lost fuel economy and it never hurt a vehicle.

As someone has already mentioned, there is post after post talking about all of those benefits. We try to be conservative with numbers at DieselSite as we don’t want to be promising customers the moon when they can’t really have it. But the fact is, we have thousands of thermostats (new sales, referrals and some are repeat sales for replacement stats for maintenance) out there with all kinds of feedback and it’s just one of those products that works. If the results are less lackluster than they were two years ago, I have to wonder if there may be fuel differences that would affect the results. Who knows? Something to check into? I haven’t had any lower percentage of “happy” calls though so I will assume that the results are still the same.

My site has tons of information on the thermostat, no need to dig it all up again. It seems like there is a post a day on this topic. No wonder very few people post on them anymore. It HAS been beaten to death. BUT – if you’ve missed the feedback over the last 6 years, here’s a few samples: (THANKS TO EVERYONE LISTED)

DHTD I do a lot of around town short trips and it is probably the best mod I ever did and the billet housing got rid of my slow leak. Engine runs nice and smooth, noticable especially around town because it will warm up faster. Runs great on the highway as well, I havent calculated but it does feel like it gets better mileage.

I went with international's prescription on this one, ford must have been smoking something, and seeing the large cooling capacity of our trucks with the diesels natural urge to run warmer for more power, I bought it and love it.

I have less smoke too, Im telling you I love this mod!!

tjsulli790 I have consistently seen a 1 mpg increase since installing that stat. And I've been keeping track for several months

Tony_ONeil I noticed a 1-1.5mpg increase, the heat itself feels good, can't say if it blows any much hotter for sure though.
mainly did it cause that's the t-stat the engine was intended to have in the cooling system.

Snowman269 Well I installed the 203 stat and billet housing from diesel site & I am absolutely amazed with the increase in mileage.

I & my wife have been driving the Excursion & my F250 between Mich & Minn the past few months. I installed the 203 stat in both trucks. The X has always gotten (at best) 2 miles per gallon less than the F250. Well not now.

The Excursion has a 4 trip average of 21.8 mpg. up from 18.6. The X has NEVER gotten this kinda mileage driving 65-70mph constantly.

Now the F250 only improved a little. Maybe a .5 mpg improvement (maybe). But the truck is getting 22 - 22.5 on the same run.

I have been neglecting for a long time on buying the 203's because of mixed responses. Some have had improvements & some haven't. Well these 2 trucks show that results may vary. But I'm a believer, Your truck may like the hotter temps, one of mine sure did!

Thanks Bob Riley!


Tailgate I installed Bob's 203 T-stat and housing a month ago. I then went on a 500 mile road trip for the weekend. I DEFINITELY got better mileage. My results were similar to yours...about a 1.5 to 2mpg increase on the highway.

LoveLearn NChornet said, "But some will claim 25 extra HP and 2 more mpg."

I've searched this site's active and archived files about higher temperature operation, most commonly enabled by restoring the 1995 203 degree specification thermostat value to newer engines which were fitted with 195 degree thermostats ONLY to improve nitrous oxide emission values at a clear disadvantage to their thermal efficiency. I've also read a lot about this issue as it relates to non NaviStar diesels. Your statement is the only example I recall claiming even close to "25 extra HP and 2 more mpg."

Please provide a citation by which we can verify that anyone ever made such a claim. I've seen consistent references to small incremental gains, typically translated into about 0.5 mpg maximum gains on these trucks, and then only when they are fully warmed. Frankly, that's a LOT! We can easily estimate miles traveled and fuel burned before 203 degree conversion cost is fully recovered by fuel expense saving. After that, fuel expense savings would be a permanently flowing gain.

This change produces small but 100% consistent MPG improvements while marginally worsening nitrous oxide emissions. Power gain at any fully-warmed throttle setting does exist. But those incremental changes are very small so they can only be measured through sensitive tests. That power gain is much too small to be noticed through "seat of the pants" acceleration comparisons.

As a certifiable MPG contest participant for about 4 decades, I will install an engine temperature changing system into my recently acquired 2000 truck. By far the easiest way to do that well would be to buy one of these aftermarket 203 degree adapters. A thermostatically-controlled electric-motor-driven plunge valve with dash-mounted adjustable temperature threshold control would allow exploring the full range of performance differences thermostatic controls can produce. Creating and enjoying devices like that is only well suited for people who love exploring maximum efficiency just as drag strip contestants enjoy exploring ways to minimum their elapsed times. I've done both and the fun from testing limits in both performance arenas feels similar.

You're wrong to attack 203 degree thermostat replacements. Your claim that others claim "25 extra HP and 2 more mpg" sounds exactly like "putting up straw man to knock down" because that claim does not square with any credible previous reference I've read about this issue.

Dave Whitmer 0.6 MPG for the 203 degree stat is what I told him.

CSIPSD ……………………….I never see above 190 on the water temp, and will be changing to the 203 stat soon. I had one in my 99 and notice about .5-1 mpg better, but it just seemed to run better...

That took me five minutes to round up and these are posts on THIS SITE ONLY. You can find similar ones on every forum on the net for a Ford Powerstroke. If it didn’t work or it sucked, you would surely know about it. Even the negative posting by a board member here has been confirmed to be the short stem 203* stat – not the DieselSite 203*. His apology not long ago is greatly appreciated and we look forward to his deletion of the negative comments that may be confusing site members. We would like everyone to have the correct information.

Have you ever thought about what .5 (or even .1 or .2) mpg means? Adding a 1/2 mpg to trucks averaging 15 mpg before the thermostat install produces an average savings of $105 a year if you fill up once every two weeks. It saves you $205 per year if you are filling up every week. THAT’S A LOT for a thermostat changeout.

Many have posted they didn’t notice “much” if any of a fuel mileage gain, but “not much” can be considerable, even moreso with today’s fuel costs. The higher the fuel costs rise, the more you save by running the thermostat. It’s all simple numbers. I’ve never personally installed a thermstat that got NO mpg. You just have to be aware that it may be a small increment and be looking for that. No matter how small though, you are saving $$$ with ANY increase or gain. Let’s use the MINIMUM possible gain (other than 0):A .1 (point 1) mpg increase in a 15 mpg truck would save you $21 if filling up every two weeks, $42 if once per week. That may seem like you’re not even paying for the stat since it is recommended by Ford to change stats once per year, but they last much longer than that. AND, that isn’t even discussing the benefits of running your truck in it’s optimal efficiency range. Other benefits are priceless to some owners. Ask the northern guys how much they love their heat in the cab after a t-stat change. I think one fellow already mentioned that here. Lastly, we hear over and over from customers that they see about ½ mpg. So .1 is very conservative, but also very unlikely to be all that you see.

Yet another example is the guy that calls me with a 2 mpg increase!!! Say his truck gets 14 mpg. At $4 per gallon (I used $4 for the other examples as well). He saves $390 with a fill up every other week, and $780 if he fills up every week!!!!!!!!!!!!!!!!!! Now you can see why some are willing to spend the $35 a for product that has been proven to give something – even if less than ½ mpg - back.

Dave Whitmer was titled the “MPG KING” and has gotten a 7.3L PSD to run at 26.9 mpg. What he did with his truck was incredible!!! His truck was featured in Diesel Power magazine and one of the mods listed was the 203* thermostat with a .6 mpg gain. That was impressive to me since he was already way way way way over the average.

Anyway, it doesn’t surprise me that the thermostat sells consistently and I never hear anything but good feedback. It’s just a great product. When you consider I get calls from customers wanting to spend hundreds of dollars on an exhaust because they were told they could get ½ mpg, I don’t feel guilty at all pointing them toward the 203* stat. Freak of nature that my wife is with numbers she made me this chart for all of you with examples of possible $$$ savings based on fuel mileage gains. Click here to see $$$ SAVINGS POSSIBLE Chart.

The website write-up on our thermostat is a little easier to read than the technical paper above but it is right on point for someone perusing my site and needing basic information on the differences in tstats. Aything on my site written about something that is manufactured by us is fact, not something I made up. People who read these forums have come to know that they can trust what I have to say. There's a respect that I have earned in not giving people a line of bullcrap over the years. I intend to keep that trust for as long as I am in business.

This poor little 203* really doesn’t get the respect it deserves. It’s simply one of the best mods there is for the 7.3L PSD. (Not just because it’s mine LOL). I have NEVER promised anyone specific fuel mileage increase numbers. I won’t start now. But I will promise that this mod is loved by THOUSANDS of Powerstroke owners.

Maybe a “sticky” in the DieselSite forum would be a good idea to avoid the multiple threads started on the 203* thermostat?

Sorry for the long post, hope it was informative, thanks for reading!


Satisfied DieselSite 203* Thermostat User :thumbsup:
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