Cooling System Tech Tips

Why Do You Need a Thermostat?

James Pye wrote on the CJ-3B Bulletin Board, "I have always been told that a thermostat was to maintain a good operating temp and to heat your heater up quickly. But I have run several vehicles without it and they did just fine. I was told the other day that if you do not have a thermostat you run the risk of overheating. That sounded crazy to me. Is it? What exactly does the thermostat do? I understand how it works but what is its purpose?"

ThermostatWes Knettle: "Its purpose is to maintain the engine operating temperature within its design parameters. This engineered temperature range will give the best balance of power (horsepower), economy (fuel mileage) and longevity (engine life between overhauls). The thermostat does this by trapping the coolant in the block until it reaches a specified temperature. Then it will slowly open and allow the coolant to pass through the radiator where is is cooled. It will cycle toward close each time the temp of the coolant drops below a specified temp allowing the engine to warm up.

"Usually when you run an engine without a thermostat it will run cooler. People usually do this because the engine had been overheating. They would have spent their money more intelligently by determining what was causing the overheating and fixing the real problem. This will also caused heater output to be too cool. In some engines, under the right conditions, the removal of the thermostat will cause the temps to run higher than normal."

Sergio Lwoff: "The technical explanation from Wes is 100% correct. The ONLY reason for which a motor with the correct thermostat could overheat would be when the thermostat is spoiled and remains closed. I experienced this problem with my old Fiat 127 some 25 years ago. Fortunately I had a temp gauge (which I consider to be essential if you have a thermostat) and was able to "save" the engine.... The factors which influence the need for one or other thermostat are, the climate (temperature range) in the region in which you use your 3B. Also, the need to have an efficient heater, and whether it is dense city traffic. Moreover, the lack of a thermostat will increase the pressure of the water pump on the upper hose of the radiator, which will, in my view, shorten the life of that part."

Vern: "To build on what the others have said, the thermostat is very important. I've heard dozens of tales of engines that ran just fine without the thermostat, but this creates new problems that are not evident at first.

"When an engine is run below the normal operating temp range, the internal clearances are way out of whack. Pistons and rings are too loose for the cylinder walls, for example. This is not a major problem for the brief time when a cold engine (with thermostat) is warming up. The richer fuel mixture effected by the choke compensates for this. But if an engine is continually run cold there will be excessive clearances on a constant basis. Performance will drop and wear will be increased. There are other factors, too, like moisture in the crankcase not evaporating as quickly if the engine is kept cold.

"In short, I fully agree with the need to determine why an engine overheats if the thermostat is in place. I know it is commonplace to do so, but running without a thermostat should be considered only in emergencies or for troubleshooting."

Ed Wilson: "I just went through this with my '54. Overheating. I replaced the 180 degree thermostat with another one. Some improvement but still not right. So I cut the center out of the old thermostat to restrict the flow and re-installed this. I then ran it till it held a constant temperature, 140 degrees in this case. I decided to try a 160 degree unit and Bingo! a constant 180 degree operating temp. This just proves that auto thermostats are not perfect; in fact it is common to get a 'bad' one right out of the box."

Jyotin: "The only thing that I'd add is that an engine that is running too cool will tend to build up sludge and over time cause problems.

"Based on all the posts I've read here, I would imagine that you will be adding a thermostat soon. Don't forget the gasket!"

Engine Running Hot

Movie frameSteve Harcharick: "I just completed rebuilding my engine and got her started. Got a problem: as the coolant temperature rises, it appears the thermosat does not open at the correct temp. At approx 210 degrees F it opens. I think what is happening is a gas bubble forms and insulates the thermostat from the coolant. I think a good solution will be to put some small holes in the thermostat to allow hot coolant to contact it. I do not have the small bypass hose, but do have a factory heater of unknown type. Also I do not have a shutoff valve in the heater hose. Fresh engine I rebuilt myself; I do not want to damage it by overheating."

Wes Knettle: "If the block is clean, the head gasket seals correctly, all trapped air is removed, timing is correct, radiator is clean, anti-freeze is mixed correctly 50/50, belt tension is correct, exhaust system is NOT restricted, piston blowby is not excessive, bores do not have excessive carbon buildup, fan belt tension is correct, radiator shroud is installed, correct pressure cap is installed and system was serviced correctly (fill, run at least 1 minute at moderate speed with cap installed or feel radiator hoses and when both get warm stop engine and check fluid level, add as necessary and repeat as necessary until you are sure all air is removed and system is full; if you have a heater, disconnect the heater return hose at the engine as you initially fill the radiator until liquid runs from return hose, then re-install hose and continue filling) -- then you should be able to run with a thermostat without overheating."

Glenn Smith: "This is amazing that this is such a common problem. It prompted me to go out to the garage and look at mine wondering why, what could be the cause. Personally when I do a coolant drain, replacement or such, I leave the radiator cap off until the thermostat opens up, that way I can watch the coolant level to see what happens; it should drop a bit, and trapped air has a place to escape after it's up to operating temp. Also, you have to leave a couple inches head space in the radiator; is it possible anyone is not doing this? No expansion space will definitely cause a boilover. With a coolant recovery system this not the case obviously, but that's a different story. Then there's the radiator that may not be as clean inside as one thinks; having one rodded out definitely could make a difference. You have to experience this to fully realize it. As Wes related, all these things have to be correct, but you should be able to use a thermostat, and you really should use a thermostat."

Jeff Frick: "I rebuilt a '64 F-head and I'm having a similar problem. When these engines are going up to 210 or so, is this after the first 1-1/2 miles, then they run cool again? I retorqued the head bolts. I believe I'm having trouble with a crack in the head. Before I retorqued you could see a little exhaust in the radiator, but not now."

Ed Wilson: "My new overhaul did exactly the same thing, wanted to run hot. It is quite unnerving to see the temp gauge crawl up on that much money! I've related my story elsewhere on this site, but it is very interesting that this seems to be a common thing. Wes K may have the common cure; sounds good to me.

"My F-head has always wanted to spit up coolant and required topping off now and again. I didn't like that so I installed an aftermarket coolant recovery tank and a 5-7 pound pressure cap. The tank was around $13 and I hooked the line right to the original overflow tube. This solved the overheating and coolant loss problems."

Bob Van Deusen: "I had a similar problem, and after reading tips from this page, I drilled a small 1/8-inch hole in the thermostat to let the water fill all of the water jacket; after that, no more problems. I think I have a 180-degree themostat. Also on a rebuilt engine, all parts have a much higher friction level and until run-in will tend to run hotter. Also check head gasket, compression test; you may have a slight leak or hot spot into water jacket, but should see overpressure also. "

Tom Fredericks: "Every few months, I get back to this problem and I still ain't got it solved yet. I just put in a new water pump as the old one had a leak at the shaft. Last year, I had the radiator pressure flushed. I have also replaced the thermostat with one I tested in a bowl of hot water to verify that it opened. So now, I put everything back together, fill the radiator with coolant, make sure the thermostat is pointing the right way, with the spring mechanism facing the block. I fire it up and maybe 10 minutes later, the temp gauge starts to climb. As it approaches 'H', still no circulation to the radiator. Then, the gauge is pinned, and all of a sudden, the hot stuff comes out to the radiator. So, I add more coolant to fill the system. And, the thing is, the temp gauge is staying at or just above or just below 'H'. The engine seems to be running very hot without the radiator getting anything. This has been a problem I've never really solved. I ran it last year without a thermostat but I sure miss having good heat on a cold day. Perhaps the temp gauge and/or sending unit isn't working right? But, my impression is that the engine really is running hot. Anybody know the answer?"

Steve: "You might try a cooler thermostat, such as a 160-degree unit. Also, be sure that the little sheet metal doodad that holds thethermostat against the head is present. If not, the thermostat will just kind of flop around in the housing, giving unpredictable results.... The gasket was not meant to hold the thermostat in place."

Ed Wilson: "It sounds like possible air in the cooling system to me. With all components in good shape as you indicate, air trapped in the system will do this. As the engine heats up to the boiling point, it creates steam in the trapped air, which is hotter than the coolant itself, causing the high temp readings. It also builds up pressure until the thermostat opens. When it does open this steam pushes the coolant out the radiator cap. I have had this happen. My new rebuild including thermostat and gauge will reach near 230 degrees indicated before the 185-degree thermostat opens.

"When filling the cooling system, be sure the vehicle is level or even elevate the front end somewhat. This insures that the radiator tank and fill hole are the highest points in the system, allowing any trapped air a chance to escape as you fill the system."

Tim: "When you ran it last year without the thermostat in place, aside from taking forever to heat up, did the pump circulate, and everything else work? If so, it seems most likely that the thermostat is not opening, or letting antifreeze past. Check, like Steve suggests, for the metal tube in the thermostat housing. Also make sure that when you fill the coolant, that you run the engine up to operating temp and top it right up before you put the cap on. This way, as the coolant expands, some will drain out the overflow tube but the engine will not have an air pocket."

Joel Kamunen: "Mine had a similar problem. When I started it up cold, it would get hot, puke out some anti-freeze, and then the temp would come down to normal operating temp. Rather than beat my head against the wall trying to figure out what was wrong, I just drilled a little hole in my thermostat, to relieve the pressure. It doesn't get hot or puke out the anti-freeze anymore."

The Bypass Hose

1953 CJ-3BSteve Lane: "Been a while since I worked on an F-head, but isn't there a little bypass hose on the top of the water pump on an F-head, that is the size of a heater hose and is about 4 inches long? Some engines have this to relieve the pressure built up before the thermostat opens up. On other engines I have seen this port on the water pump blocked off and forgotten, especially if the engine has seen parts swapped out, or new components such as a water pump replaced. A new water pump may have the port, but a threaded plug in it from the rebuilder, as opposed to a pipe nipple. It is easy to do, but not all water pumps have a bypass provision built in to recirculate a portion of its flow. All that pressure has to go somewhere!"

Jim Sammons: "O.K. guys, here goes some controversy: F-heads with heaters don't have bypass hoses. But there is a plugged hole in the top of my water pump with a plugged hole the same size right above it on the head. I would assume that without a heater this would be a place for the bypass hose since there would be no way for the water pump to circulate when the thermostat is closed."

Eric Lawson: "I've always installed the thermostat bypass line that is described by Jim (from the top of the water pump to the obvious port on the front of the cylinder head) anytime I've seen it missing. This has always taken care of the problems described above."

Don Nicholson: "I replaced the water pump on my CJ-3B and had trouble finding the bypass hose since the usual online suppliers were out and on back order. I found at NAPA a part number 9801 that is of excellent quality and fit with just a little trimming of length needed."

Glenn Smith: "If there is not the little bypass hose as mentioned, then a closed valve in the heater line could cause problems, at least the way I read it, and experienced it. I don't have the bypass but have always had the heater so didn't experience this until last fall. The core started leaking in the old heater and I wanted a defroster so I bought a J. C. Whitney heater/defroster combo, and installed it with an inline valve, and sure enough I had temperature problems with the valve closed. When I put it in I left the valve open while I got it up to temperature and proper coolant level. Then the next time I drove it, the weather was warm so the valve was closed and the temperature got way up before the thermostat finally opened. I suspected something to do with the valve so I took it out to eliminate it getting closed and it was OK.

"You don't need a valve and are really better off without one. Coolant free-flowing through the core gives just a little more cooling effect for the system, and you shouldn't notice a difference inside the Jeep. Besides, free-flowing helps in reducing corrosion problems."

Note: A photo of Larry Ford's 1964 3B shows a valve controlled by a cable (50K JPEG) activated by one of two knobs on the heater.

Water pump

Mike: "I'm in the process of putting my '53 3B back together and have run into a situation with the cooling system; I can't remember where some of the coolant hoses go and haven't been able to figure it out based on pictures of other engines. There are four roughly 1/2-inch hose fittings and I'm not sure how they connect. The first comes out of the water pump and points toward the right side of the engine. The second and third are found coming out of the top rear of the head. On other engines there seems to be only one hose fitting but mine comes out of the head, into an elbow, and then splits into a Y of two hose fittings. Do I get rid of the Y and reduce it to one fitting? Does this connect to the water pump?

"The fourth fitting in question comes out low in the middle of the block on the right side between the starter and generator. I think other engines have a radiator-style drain cock in this location. Why do I have a hose fitting here and should I just put in a drain cock? The Jeep used to have a heater but I'm not putting it back in.

"One more thing; there is a short piece of hose that connects from the water pump to the front of the head immediately above it. Is there a special fitted hose for this application because I can't seem to get a piece of standard heater hose to bend the fairly harsh 90 degree turn without it flattening and kinking itself closed."

Glenn Smith: "You don't need the little bypass hose if you run a hose from the right side of the water pump to the top of the rear of the head. You don't need the Y fitting you mentioned and just put a radiator drain (petcock) in the side of the block."

Jyotin: "If you have a heater, the top rear of the block goes to the heater. The other side of the heater goes to the water pump. The side of the head has an access hole and that is for the temperature sender."

"If you are not using a heater you will need a bypass. You can do this two ways. Remove the plug on the front of the head and the plug on the water pump directly below it. Install a hose between the two. Generally the hose is preformed, and if you don't have fittings you can make some by using pipe nipples from Home Depot and cutting them off so that you have a stub for the hose to clamp to. Put a plug in the opening at the rear of the head.

"Alternatively you could run a hose from the fitting on the top rear of the head to the water pump and put plugs in the bypass holes on the front of the head and the water pump.

"I recently did one without a heater and what I did with the opening in the top rear of the head was to install a petcock in that opening and use the bypass on the front of the engine and water pump. The rear petcock allowed me to bleed out any air that might have been captured in the head when I filled it with coolant.

"Either way, you need a bypass hose in one of the two positions. Put a plug in any of the unused ports."

Water Pump Leak

Question: "I noticed I had a leak and was losing water from the water pump. At first I thought it was cracked and then I realized it had a hole on the bottom of the pump. The hole looks like it can be plugged with a screw, bolt or JB weld. What happened? Did a bolt come loose? Or should I just replace the water pump? It does not look hard to do. Thanks for any help"

Al Satterfield: "It has been many years since I replaced my water pump but I believe I am correct in saying that the hole you are seeing is correct and should not be plugged. If you plug it and it fills with water the water pump bearings would be operating in a water environment and would not last long. Since you have lost your seal on the impeller the solution is to replace your water pump with a new one. Hope this helps."

Bob Van Deusen: "Your water pump needs replacing, the leak is an indicator that the pump seal or bearings are about to go. Replace the pump and the leak will stop."

Wes Knettle: "Buy a water pump overhaul kit and go to a factory manual illustration (paragraph 9, figure 17 in TM 9-1803A, or page 167 of SM-1046) and rebuild your pump. The kits are available. Beachwood: 2A0502 Pump $55, B0503-R Rebuild kit $11,B503-O O/H Kit $33."

Thanks to all the contributors, and to Ed Wilson for editing the material. Thanks to Don Kostanski for the engine photo from his 1953 CJ-3B. -- Derek Redmond

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Last updated 8 November 2005 by Derek Redmond redmond@cj3b.info
All content not credited and previously copyright, is copyright Derek Redmond