Replace a Thermostat

A car's thermostat controls the flow of coolant to keep the engine in its prime operating temperature. Thermostats can become stuck in either the open or closed position. In either case, it will need to be replaced.

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The following instructions are based off a thermostat I replaced on my 1996 4.0L Ford Ranger. Other vehicles will be similar but these instructions may have to be slightly altered for them.

Too Hot, Too Cold, or Just Right?
I noticed the temperature gauge needle never got off the very bottom mark unless I drove on the interstate for over 20 minutes. Even then, it barely got above the cold mark. I knew either the thermostat or the temperature sending unit were bad. Since I was flushing my cooling system, I decided to go ahead and replace the thermostat to see if it would fix my problem. Thermostats are inexpensive (around $5 for most domestic vehicles), so well worth the price to "throw parts" at a problem to see if the problem can be fixed.

Troubleshooting the Thermostat
A working thermostat will let the engine get into its normal operating temperature within a few minutes of running and keep it there. You will also be able to get heat from the HVAC controls relatively quickly. If your car has a problem with any of these, the thermostat may need to be changed.

The thermostat may be stuck open if...

• The engine takes a long time to heat up - indicated by either the temperature gauge not wanting to leave the "C" or "Cold" mark. (Check to make sure the Temperature Sending Unit is working properly, as this may cause an incorrect temperature gauge reading).
• It takes a long time to get heat from the HVAC.
• The upper radiator hose never gets hot, even after the vehicle has been driven for several miles.

The thermostat may be stuck closed if...

• The engine overheats after a consistent amount of time driving, shortly after starting the engine and no leakage can be found from the radiator, hoses, head gasket, or heater core.

Tools and Materials

• 10mm socket (1/4" drive)
  
• 1/4" ratchet (there is not enough clearance for a 3/8" ratchet)
• Small 1/4" extension
• 10mm combination box-end and open-end wrench
• New thermostat with gasket
• 50/50 mix of coolant and water to top off the radiator if any spills out during operation

Changing the Thermostat
A good time to change the thermostat is when the cooling system is empty, like during a coolant system flush. It is possible to change the thermostat with a full cooling system but some coolant will leak out of the disconnected hoses.

Follow these photos and captions for the thermostat changing instructions on a 1996 4.0L Ford Ranger:

Locate the thermostat housing by following the upper radiator hose to where it connects with the engine block.


Here you can see the thermostat housing tucked away.


You will have an easier time accessing the thermostat by removing the throttle cable splash shield and disconnecting the electrical connector that goes into the air intake. You can also completely remove the air intake hose, but I opted not to.


You will need to remove these two 10mm bolts.


And there is one more underneath.


Here is the thermostat (red arrow). The blue arrow shows the Temperature Sending Unit which controls the temperature gauge. The TSU will always have one wire coming out of it. The green arrow shows the Temperature Sensor, this sensor tells the car's computer how hot the coolant is. This sensor will always have two or three wires coming out of it.


Remove any old gasket material from the hose side and engine side of the thermostat housing.


Here is the new thermostat with rubber gasket installed. Note how it is installed. The spring side should go towards the engine.

To finish: follow the steps in reverse.

Before it took a lot of driving to even move the temperature gauge needle on my truck, now the truck warms up quickly and stays in the normal temperature range like it should. This should help keep the engine running well, improve gas mileage, and keep emissions low.

Summary

1. Locate the thermostat housing, it will always be at the end of the upper radiator hose where it connects to the engine block.
   
2. Remove anything that is in the way, such as electrical connectors, splash shields, etc.
3. Remove the thermostat housing bolts (3 10mm bolts on this particular engine).
4. Pull the hose side of the thermostat housing away from the engine block and remove the old thermostat.
5. Install the new thermostat.
6. Reinstall the thermostat housing, bolts, and any other components you removed to access the thermostat.
   

An Easy Fix
Thermostats are usually very inexpensive and easy to change so I would almost consider them a routine maintenance item if the original one did not last almost 200,000 miles. Nevertheless, I would still recommend making a thermostat replacement part of you preventative maintenance schedule by changing the thermostat about every other time you flush your cooling system.

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Relevant Sites
Here are some external links to sites with information relevant to this article.

How to Test a Thermostat

Automotive Home Outdoors Tool Talk Miscellany