The thermostat is at the bottom of the black tube on the front of the intake manifold. If you have a 3/8 universal socket, it can be done w/out removing the alternator, ac, or the accessory bracket. Basically, drain the radiator, remove the hose clamp, then the hose. Remove the two bolts holding the inlet tube down (1/2 socket, on 3/8 universal on 3/8 extension) then remove it. You will then see the tstat. Remove it. Spray some gasket remover on the manifold fitting and let it sit while you clean up the bottom of the inlet tube. Then go back to the manifold and clean that fitting. dhvaughan made a good suggestion to test the tstat before you install it. After testing put the tstat in, then the gasket on top, then the tube (wich has a tab telling you wich side faces the front of the engine), and tighten the bolts. Put the hose back on, clamp it,tighten the drain plug and refill the radiator.

 

Thanks alotmopowar. Will get it done tomorrow if the rain quits. Great info here....

 

Q. what does it means when the top radiator hose is hot and the bottom one is cool. The temp gauge normally rides at 200* and this is where is was when I checked them.

A. as a general rule, it is normal for the top radiator hose to be hot (about 180-210) and the bottom radiator hose to be less hot (about 110-140). what is happening is the water pump is mounted on the front of the engine, forced to turn by the serpentine belt, pulling cool water from the bottom of the radiator and pushing it through the engine. the water is pushed through the block and up through the heads and into the top of engine where the thermostat sits. the thermostat is a thermally controlled valve that is preset to a given temperature, usually 180* or 195*. when the water in the top of the manifold is less than 180, the thermostat is closed, and the water is force to recirculate through the engine again, where it will get hotter. when the water in the top of the manifold is greater than 180, then the thermostat will pull open, against the pressure of the spring, and allow the water to escape out the top radiator hose. now the thermostat opens up slowly, so it can be fully closed, partially open, or fully open. when fully open its about a 1 inch gap for the water to flow through.

so what you will see happen is you when you crank up the engine cold, the thermostat will be closed and the coolant trapped inside the motor will recirculate until it reaches a temperature hot enough to open the t-stat. during this time both the top and bottom rad hoses are cold, and if you have the rad cap off you will see no water movement. after about 3 or 4 minutes the engine will warm up and the coolant trapped inside the engine should get to 180-195. at this point the t-stat will open up a little and allow hot water to flow through the top rad hose to the rad. at this point you will feel the top rad hose start getting pretty warm, maybe even hot. you will see some water movement inside the rad. so now the water pump is pulling cold water out the bottom of the rad and that bottom hose is still cold. so now you go through about a 5-10 min period where the cool coolant is sucking the heat out of the engine very well, and the thermostat is only about 1/2 open. pretty soon all the coolant has made several trips through the engine and all the coolant is pretty hot, and the rad is pretty hot. at this point the top rad hose is way to hot to hold (180-195) and the bottom rad hose is about 120 and just about too hot to hold onto. so assuming everything is working right, this is where everything reaches a point of stability, where the radiator and fan has enough cooling capacity and the t-stat cycles between various degrees of partially open to maintain that magical 180* or 195* temp.

so what can go wrong.
1. if the t-stat is sticky, or rusted up, or scaled up and cannot open fully, then too much coolant recirculates through the engine instead of going to the radiator for cooling.
2. if the radiator water passages are internally stopped up with rust, scale, stop leak, or mineral deposits, then the coolant cannot flow though the rad in enough volume to cool the truck.
3. if the radiator air passages are externally stopped up with mud, grass, leaves, weeds, seeds, then not enough air can flow through the rad for proper cooling. fix this by squirting a water hose through rad from inside to outside, backflushing it.
4. low on water. improper mix - should be about 50/50. belt off or slipping, water pump impeller broke or fell off. water pump pully slipping on shaft and not turning the impeller.
5. sitting in traffic with AC on, or very slow trail riding. the fan is turning and forcing some air movement, but there is not as much air going across the rad as when you are driving along at 40 mph. so its normal to see some temp increase when sitting in traffic. should not be much increase, only about 10* if any.

 

A quality post if ever I've seen one. Good job, DHV. [8D]

 

Well, I finally got around to replacing the stat with a 180* from NAPA. Now it just takes longer to reach the same high temp. I thoughtI had it fixeduntil I drove it for about an hour the other day when it was about 85* with the A/C onand it still worked it's way over the 200 mark. Granted it took longer to get there, but it finallydid.So back to square one. Any ideas where to go from here? As I said, the fan clutch has been replaced and supposedly the radiator was replaced by the previous ownernot long before I bought it. Don't know about the water pump...

 

Bump...

 

Mine wasn't getting that hot but certainly abnormal. I was told the pump is fine but needed a new t-stat and fan clutch. I've been hearing that a weak or bad fan clutch will prevent it from cooling properly like this. I've now got a new t-stat and new intake manifold gasket and will have to wait a bit on the fan clutch. Not sure how to check the fan clutch to be sure though.

 

Some people have experienced air lock after replacing the tstat. One thing you can try:
Fill everything up. Then disconnect the top radiator hose, lift it up, and fill it with coolant. Keep it lifted and set the coolant down. Then quickly slam the hose back on the radiator fitting loosing as little coolant as possible. The other fix is to drill larger bleed holes in the tstat, but that requires removing it again.

 

For anyone having problems with overheating in this make, model, and generation. mine is due to the Fan Clutch. right now as of June 20, 2008 I have found out the replacement is on National Back-Order. The part is currently $194 and is Part # 52029876

2002 Dodge Ram 1500 5.9L(360) V8
(Canada)