Replacing a bad thermostat can save fuel, increase oil life and make your cab warmer.
Early engine builders discovered that an engine would use a lot less fuel and run a lot better if the coolant was kept hot enough.
But when equipped with an adequate radiator for hard work in hot weather, the engine would run too cold almost all the time.
So they devised a gadget called a thermostat, which controls the flow of coolant through the radiator in response to coolant temperature. When the engine is cold, the thermostat blocks access to the upper radiator hose. This allows the coolant to circulate only within the block and head through a small bypass, and through the heater core through the two connecting hoses. When the engine reaches the thermostat setting, the device cracks open and then controls the flow to keep the operating temperature within a narrow, acceptable range.
Many late-model engines run at 180 degrees F and above, and post-2002 EGR engines often run at 185-195 degrees F and above.
Why is hot coolant so important?
Diesels depend on the heat of compression to ignite the fuel. Holding coolant at the required temperature keeps the inner surfaces of the cylinder head and liners warm enough to increase the temperature produced by compression, and this helps the fuel ignite quickly and burn smoothly. The hotter air also helps burn off the soot that forms during the combustion process. In fact, an engine running well below operating temperature will throw a lot of junk, such as soot and even raw fuel, into the oil.
Also, the viscosity of the oil used in your engine is suited to the operating temperature. When an engine runs too cold, the oil will be thicker than it should be, resulting in more friction and higher fuel consumption.
Diagnosis and Repair Basics
We consulted the North American Institute, the school run by Volvo and Mack to train technicians and owners for both brands. Dennis Schantz, service trainer, and Mike Berger, Mack program developer for training manuals, showed us all the steps involved in replacing a heavy-duty engine’s thermostat.
Schantz says truck owners rarely test thermostats after removal any more. Keeping your eye on the temperature gauge will do the job. After a cold start, once the engine starts taking load, the temperature should rise steadily until the thermostat opens. The temperature shown will then level off and remain within a narrow range unless running in extreme conditions (e.g. up steep hills) in hot weather. Carefully note the operating temperature range when the engine is new, so that you can tell if the thermostat has gotten lazy and is allowing the engine to run too cold.
A sudden overheating problem often indicates that a thermostat has failed to shut. But you should also check that there is plenty of coolant in the system, and that the fan and water pump are being driven at normal rpm by belt drives in good condition. You also must have a functional fan clutch, though that is typically not needed during level highway cruise. If the thermostat has failed, the block will be very hot, but the radiator itself will be relatively cool except right at the top. There will also be appropriate codes in the ECM.
Slow warm-up or failure to reach operating temperature when the engine is being worked are always a faulty thermostat problem.
We’ll describe and show replacement of the thermostat on a Mack E-tech engine. While procedures vary a bit from engine to engine, the basics are very similar.
You must remove anything in the way and then disassemble the thermostat housing to remove the thermostat. It is normally necessary to replace two seals on the unit itself, as well as a gasket between the housing and cover. You should pay particular attention to using a new gasket and new seals. These normally come in a kit with the ‘stat, but if they do not, ask for them. Also, carefully prepare all the surfaces both work against by removing not only old gasket or seal material, but also rust or pitting. Coating seals (other than the gasket) with fresh coolant prior to assembly to lubricate them will allow you to install them without marring the surfaces and causing a leak.
Proper thermostat seal performance is “critical,” according to Schantz. Slow warm-up, or a tendency to run too cool, thought to be caused by a lazy thermostat, often turns out to be the result of a seal damaged during installation.
In addition to a basic set of small prybars, sockets and drives, and open-end wrenches, you will need a special mandrel to replace one of the seals. It is ideal to also have a small arbor press.
- Allow the engine to cool until it is no longer hot to the touch, so it will be safe to remove the radiator cap and handle coolant and engine parts.
- Remove the radiator cap, and drain most of the engine coolant into a clean drain pan that will allow it to be reused if it’s in good condition.
- Using the right size strap wrench, unscrew the coolant filter by turning it counterclockwise as viewed from below.
- Loosen the clamp, and disconnect the upper radiator hose from the thermostat housing.
- Loosen both clamps, and then slide the bypass hose, located below the thermostat housing, down onto the pipe leading from the block. The clamp screws can be turned either with a screwdriver or 5/16-inch socket. You can work faster by using a socket with ratchet drive. Inspect the hose for cracks or lack of pliability. A hose that has cracks or is hard and brittle will probably fail soon. Replace the hose in Step 10 if cracked or hard and brittle.
- Using a small open-end wrench, disconnect the two coolant filter tubes. Then remove the tubing fitting on the filter head for clear access to the filter head mounting bolts.
- With a socket wrench and drive, remove the filter head mounting bolts. (Removing the fitting in Step 6 is necessary so the socket can be installed straight onto these bolts and won’t strip the flats.) Then remove the filter head.
- Gently pry out the O-ring in the filter connection. Coat a new O-ring with clean coolant and gently work it into the orifice with your fingers. (The check ball inside prevents coolant loss when you are just changing the filter.)
- Inspect the face of the filter head where it seals. If it has been gouged or pitted from corrosive coolant, use emery cloth to clean it up. If it can’t be made smooth, replace the head or it is likely to leak.
- Now remove the two lower bolts from the housing cover, and remove the cover, gasket and entire housing. Replace the bypass hose at this time if cracked or hard and brittle. Relocate the clamps onto the new hose.
- Slide the thermostat and seal out of the housing. Note the position of the hole in the side of the ‘stat and the jiggle ball. This vents gases from the cooling system and must face the same way when the new thermostat goes in.
- Mack engines using water-cooled EGR have two thermostats, so the engine still won’t overheat even if one of them fails. If you have a problem, it will be very difficult to tell which ‘stat is the faulty one. Replace both thermostats. It does not make sense to replace just one of them even if it is obvious which one has failed, as the other often fails shortly thereafter. Also, the two actually work together to give full flow, so they should be opening at exactly the same time. With this kind of ‘stat, the vent must face forward – toward the housing outlet.
- As mentioned above, make sure the kit contains all the seals and a new gasket.
- The next step is to replace the metallic seal that seats inside the housing and seals the lower end of the ‘stat. Note that the seal has a slight taper, so that the inside diameter is a little greater at the top. This allows the thermostat to slide into it when replaced. Gently pry this seal out using a prybar that has a sharp turn at the bottom, called a “heel bar,” as a lever. Of course, with the engines using two ‘stats, you’ll need to remove both seals.
- Clean up the seal bore just exposed and also the one at the top where the seal that runs around the top edge of the thermostat sits. Use emery cloth if the metal is corroded or pitted.
- The seal must be replaced using a mandrel, which is designed to force it evenly in place and at the right depth. Turn the seal so the taper inside is narrower at the bottom, and then force the mandrel down over it. Then place the mandrel in a small arbor press and position it squarely into the hole in the housing. Force the seal into the housing till the mandrel bottoms out, indicating that the seal is installed at the proper depth, so it will seal the bottom of the thermostat correctly. The arbor press helps to get even pressure on the seal, so it will slide in straight. But Schantz says that if you are skillful and can be gentle with a hammer, and hold the mandrel perfectly straight, you can use the hammer instead of the arbor press and still manage to replace the seal correctly. On the two-thermostat setup, replace the other seal in the same
- Once this seal is in place, make sure the new ‘stat or ‘stats have their own rubber seal(s) properly installed around the upper diameter. The seals come installed from the factory. Coat this seal as well as the bottom of the ‘stat, where it will fit into the lower seal, with clean coolant.
- Position the thermostat’s vent hole toward the upper radiator hose. This means at the top for the older, single thermostat setup and toward the front of the housing for the dual-thermostat setup. This is a critical step because the ‘stat’s coolant flow control function must not interfere with the venting of gases from the cooling system.
- Then gently force the ‘stat straight in till it bottoms out, using your thumbs on both sides. If necessary, repeat for the second thermostat.
- Carefully inspect the gasket surfaces of the housing and cover. Clean all old gasket material off with a screwdriver. Clean up any pitting or rust with emery cloth. The smoother the surface is, the better the gasket can seal. You can check on the position of the vent hole with single thermostat systems at this point, as the venting tube will be visible.
- There are two boltholes and one thermostat hole in the single thermostat setup. The dual setup has two ‘stat holes, of course, and multiple boltholes. The gasket can be installed with either side toward the housing, but be careful to rotate it so that all the holes line up properly. Schantz says the gasket does not need to be coated with sealer.
- Install the thermostat housing, and then screw in the two lower bolts and tighten them finger tight. With the two-thermostat system, install all the bolts at this time.
- On the single ‘stat system, position the filter housing with new O-ring so the bolt holes in the thermostat and filter housings are aligned, and then install these two bolts, making them finger tight, too.
- Using a socket wrench, snug the four bolts up in a criss-cross pattern. Then attach the socket to a torque wrench. Set the torque wrench to 15 pounds-feet if adjustable. Then torque each bolt to 15 pounds-feet, again in a criss-cross pattern. Adjustable torque wrenches must be turned till they click, meaning the torque is correct. With wrenches that have a scale that shows the torque indication as you turn them, turn the wrench slowly just until the pointer shows 15 pounds-feet. Using a torque wrench and the correct torque is great insurance against either a leak or housing damage.
- Reinstall the brass fitting for the filter coolant tubes into the housing and tighten gently with an open-end wrench. Inspect the rubber seal on the filter line, and replace if cracked or otherwise visibly deteriorated. Schantz says a tiny leak from a bad seal ruins the whole job.
- Connect both filter coolant tubes, and tighten the fittings gently till just snug.
- Force the bypass hose up and over the connection on the bottom of the thermostat housing. Reposition the clamps a short margin from the hose ends, and tighten the clamp screws until just snug.
- Spin the coolant filter back on, or replace if appropriate.
Note: Cooling system treatment with SCAs (Supplemental Coolant Additives) or the use of Extended Life Coolant, properly maintained, with antifreeze replacement occurring at recommended intervals, will ease the job of thermostat replacement. This good maintenance minimizes corrosion and pitting of all the sealing surfaces. It can also extend thermostat life. However, you must be careful to change the filter if it contains SCAs only as necessary, after testing the level of protection with a coolant test kit.
If the test shows the level is suitable for replenishing the level of SCAs, replace the filter with a new one. If the vehicle has a filter, but uses Extended Life Coolant, make sure to get a “blank” filter that has no internal supply of SCAs.
- To install the filter, coat the gasket with clean engine oil, and then start it carefully onto the threads. Read the note on the side to determine how far to tighten it, usually about 1/2-3/4 of a turn. Tighten until the gasket just contacts the mating surface, and then continue, watching how far the filter turns. Use a strap wrench if needed.
- Fill the cooling system with fresh coolant, meeting the correct specifications, or the clean coolant you have saved. Make sure to fill the system gradually, repeatedly adding it, until coolant remains at the proper level in the radiator and surge tank.
- Start the engine and operate it till hot, while checking for leaks. After the engine reaches operating temperature and the new thermostat opens, shut the engine down again and allow it to cool until it is safe to recheck the coolant level. Replenish as necessary.
For further information, please contact the following:
North American Institute for Mack and Volvo
Mack Trucks, Inc.
Volvo Trucks North America, Inc.
Truck Engine Manufacturers
Caterpillar Engine Division
(Cat Call Center)
Detroit Diesel Corp.