Re-printed with permission from Bob Riley

Detailed Description


Here’s a little background on the Ford Powerstroke thermostat…


Until 1996, all 7.3L Powerstrokes that were manufactured used an International water pump and an International short stem 203° thermostat.  In 1996, Ford changed to a lower degree 195° thermostat.  They changed water pump designs at this time as well.   The new pumps required the new long stem 195° thermostat in place of the short stem to properly seat the bypass disk.  All 1996 – 2003 Ford PDSs now come equipped with a long stem 195° thermostat.  Recently the factory replacement thermostat available at the parts counter changed to 192°.




Diesels run most efficiently at approximately 200° or above.  A factory equipped 192° / 195° thermostat will maintain an operating temperature at approximately 165° - 170°.  The Powerstroke has such an efficient cooling system that this low flow, when the thermostat is slightly open, is normally enough to keep the engine cool.  The downfall to this is that the engine - - when our trucks are not under load - - never begins to reach its optimal operating temperature.  Our 203° thermostat maintains a minimum operating temperature of 190°.  This brings the operating temps up from as low as 170° to as high as 203° depending on the operating conditions of the truck.  This is closer to the operating temperature range that is preferable in our trucks for optimal efficiency.


When our trucks are under load and are producing higher combustion temperatures, the 203° will allow the engine to run even closer to its optimal operating temperature.  If the truck is under load and reaches the 203° setting, the thermostat is then fully open.  This means you are at the maximum coolant temperature that the truck will reach.  Your maximum temperatures are only a few degrees higher than in stock configuration, but these are degrees that you want for better performance.  The best benefits come when the same truck is under a light load and would have a hard time reaching this optimal temperature.  As stated above, you will never reach this optimum temperature with the stock 195° thermostat.  The end result of installing the 203° thermostat should be slightly better fuel economy, less smoke, and maybe a few extra “ponies” to go along with it.




Although International still uses a 203° thermostat in some applications and that thermostat is available at most auto parts stores, it is the wrong part for the 1996 – 2003 Powerstroke.  DO NOT USE THIS!  It has a shorter shaft and will not shut off the internal pump bypass.  This will cause inaccurate coolant flow direction through the engine.


The thermostat in the Powerstroke not only controls operating temperatures, but properly directs the flow of coolant through the engine.  As the thermostat opens, it proportionally closes the bypass.  With the thermostat fully open, the bypass is mostly closed, and vice versa.  With the International (Pre-96 Ford) thermostat, this cannot be controlled properly because of the shorter bypass stem.  The shorter stem of the IH (Pre-96 Ford) thermostat allows the thermostat to be fully open while the bypass is fully open.  This means the coolant can travel in any direction available, whatever direction it chooses.  It can either travel through the radiator or simply make the shorter, less restrictive path back through the front of the engine.  This will cause uncontrolled overheating in the back cylinders of the block, with absolutely no signs of danger showing on the water temperature gauge in the cab.  It is possible that the back of the engine can have no coolant flow at all yet the thermometer in the outlet of the water pump shows all to be normal.  Multiple things can happen at this point.  Cylinders can seize, freeze plugs can fall out and other normal symptoms of an overheating engine may occur.  Worst case is a blown motor and it won’t be covered under warranty.


We know the warranty departments and engineers at Ford have verified engine loss due to the short stem thermostats in previous cases where people were sticking into their trucks to increase their operating temperatures.  As soon as they see the short shaft thermostat, it’s your problem.  Our 203° thermostat will not encounter these problems!  It’s designed to work as the OEM unit did WITH the long stem for bypass protection.




Higher optimum operating temperatures can be reached safely by using our thermostat with the long stem for bypass protection.  It’s that simple.  We’ve combined the two units to bring you a safe alternative.  Now you can have the optimal operating temperatures without risking the chance of uncontrolled overheating in the back cylinders of the block due to an uncontrolled bypass.




  • 90 days on manufacturing defects and workmanship only.
  • There are no other warranties, written or implied.
  • Improperly or poorly maintained systems can lead to failures in thermostats.
  • As with all cooling system modifications, a water temperature gauge is a highly important tool in monitoring the system.


As with the OEM thermostat requirements, we recommend that you change your thermostat out once a year to insure proper functioning of the unit.



Frequently Asked Questions:

(Answered by Bob Riley of DIESELSITE)


Question:  Do you carry an IH version 203° for the 1994 – 1995?


Answer:  Yes, call us for this item.



Question:  Does your thermostat have a way to bleed air from behind the valve?


Answer:  Yes.  There is a small calibrated “V” notch cut into the side of the disc for air bleeding.  Do not drill a hole in the disc as this can change the opening temperature of the thermostat.  Our thermostats went through extensive durability and flow testing to make sure they were perfect.



Question:  Why do I want my truck to run warmer?


Answer:  You want the engine oil / water temperatures to be warmer so you get a cleaner, more complete combustion.  This can actually bring exhaust gas temperatures DOWN.  Diesel ignites from heat of compression.  This idea is to compress the air charge to build enough heat to light the fuel, but if it’s fighting against a low engine heat, the fuel will ignite, but not as complete as if the cylinders were just a bit warmer.  He more complete the combustion, the cooler the exhaust temperatures.  Diesel fuel combustion tends to work best at temps above 210° oil temps.  Water temps are usually about 10 – 30 degrees cooler than oil.  An added benefit from using the 203° thermostat is better working cab heaters in the cold months.  Another added benefit is better fuel economy.  We have seen everywhere from 1 – 4 mpg increases.  This depends on climates, driving habits, etc.



Question:  Will I need any other parts or O-rings to install the Diesel site 203° thermostat?


Answer:  Each DIESELSITE.COM 203° thermostat comes with a new O-ring.  We recommend new thermostat housings when installing a new thermostat if your vehicle is over a year old.  Many are rushed too badly to properly seal.  We sell the OEM housings for 1994 – 1997 as well as the 1999 – 2003.  We also offer a billet aluminum thermostat housing in several colors.  You can purchase your OEM housing at your local dealer as well.






Question:  What can I do to insure no leaks during the installation?


Answer:  Be sure to start with a clean, very dry surface.  Most installation errors are caused by those who did not wait for the surface to dry or they did not clean and dry it well before installing the thermostat.  Although it is not needed if you start with a clean, dry surface you can use regular RTV sealant during the install.



Question:  What temperature should I be seeing on the factory gauge in the cab after installation?


Answer:  The factory temperature gauge is not a true temperature gauge in that it does not show actual temperature.  It has three settings:  cold, normal and overheating.  Cold is when you first start it up.  Normal is anything that the computer thinks is normal operating temperatures.  This can range from 90° to 220°.  Overheating is just that.  The gauge is not designed for measuring your actual water temperature; it is designed to tell you whether you are in the normal operating temperature or overheating.  There is no in-between.  With that in mind, the gauge on your dash may stay the same, may go up or may go down slightly.  It is NOT an indication of what the thermostat is doing.  If you want to know your actual water temperature, you must put in an aftermarket water temperature gauge.



Question:  I have a 1995 Powerstroke and it looks like I have a long stem style thermostat in my water pump.  Is this possible?


Answer:  Yes.  If you’ve had your water pump replaced, chances are the installer replaced it with a newer style pump which uses a longer stem 195° thermostat.  If that is the case, our 203° thermostat is the correct replacement for that style.



Question:  Does the thermostat affect my engine oil temperatures as well as the coolant temperatures?


Answer:  Yes it does.  In fact, that is the whole point.  It raises engine oil temperatures proportionate to the coolant temperatures.  The engine monitors oil temperatures and our thermostat does not raise the oil temperature out of the normal acceptable range.  It only raises it to a more desirable range for the diesel engine.






Question:  Can’t I just block off part of my radiator to achieve higher coolant temperatures?


Answer:  Yes, but this is an uncontrolled situation.  You have effectively reduced the size of the radiator and if more cooling were needed for higher loads, the engine might overheat.  A higher rated thermostat is the only safe way to raise operating temperatures.


Question:  Does my thermostat really need to be replaced every year?


Answer:  This is like any thermostat.  There is no magic.  Its design is identical to any OEM thermostat with only the opening temperature rating as its difference.  A thermostat is a heat reactive spring that changes shape every cycle.  The more hot / cold cycles the thermostat sees, the faster it wears out.  So, depending on the driving conditions a truck is under, a truck may see years of dependable operation, or may see as little as a year.  This is common.  We decided to run a test and wait for it to fail to see how long we got out of our personal company truck.  The thermostat has been in service for almost three years now.  The thermostat finally reduced its max opening size not allowing as much coolant to pass, so temperatures ran a bit higher than normal.  It didn’t actually “fail”, but merely failed to operate with design specs.  We tested the thermostat in a pot of boiling water to confirm the problem.  We install a new thermostat and all is fine again.  So, in closing, follow your Ford recommendations to check and replace your thermostat when needed.