willysmjeeps.com

I have a cooling problem with my M38A1. The temperature reads 200-220 degrees when up to normal speed. If I stop for a few minutes with the engine running, it cools down to 180 degrees. As soon as I get back up to 30 MPH or so, the temperature rises back up to 220. The radiator never boils over. I have removed the thermostat, the radiator has been cleaned and tested, all new hoses, water pump rebuilt, new temperature gauge. Has enyone else experienced this problem?
Thanks,
Randall

The first step when troubleshooting an indicated problem (one displayed on an instrument) is to check the accuracy of the instrument. Replacement does not guarantee it is correct. For the coolant temp gauge you remove the sender from the block. With it's wire connected immerse it in a pan of water sitting on the fender or head. Keep it from touching the pan itself. Place a jumper wire on the body of the sender and ground it to the body. Suspend a thermometer in the water keeping it away from the pan. Now with the master switch on, monitor the gauge as you heat the water with a propane torch. The temps shown on the thermometer and the gauge should remain within 2 or 3 degrees of each other all the way to the boiling point of the water.

Next thing to check while you still have the pan out is the thermostat. Again replacement does not mean it opens fully or at the correct temp. Suspend the thermostat in the pan. Heat the water and watch the thermostat and the thermometer. The thermostat should start to open at it's specified temp and be full open at it's operating temp and as it cools down it should close fully. Do this test on any new thermostat before you install it in any engine.

All that Wes said is good, on my WW2 Jeep I had the same issue. I tested the thermostat as Wes stated and installed it. Found my engine not running at 180 as the thermostat had proved to open at. Borrowed a infared shoot and read thermometer and got a head temp that read the same as the gauge of 210 and no boiling. The gauge would sometimes go to 220. I thought since the hose necks showed 185 top and 170 bottom, that the head temp was being transfered to the gauge bulb or sensor and I was reading metal heat temp, not water temp. WRONG! My best reading was a candy thermometer placed in the filler neck of the radiator, engine running and flow of hot water at running conditions batheing the candy thermometer. It also read 210. New radiator, new water pump, and I though a good original 180 T-stat. I put a new 160 T-stat in the engine, and it cooled it down 20 degrees accross the board. I'm living with that, but the 180 old stat tested good???????Less flow??? Don't know why. John

Hi there!

One thing to note when removeing a thermostate you should always replace with the washer part of the thermostate!

Why... is that the thermostate provides some resistance to the water flow and with out this resistance the water can flow too quickly through the radiator and not have time to properly cool down.

Phil...

Are you talking about the retaining metal riser under the Tstat, or the skinny gasket that goes between the neck and the top of the tstat? Another thing: you need a small hole through the tstat to equalise the water on both sides and eliminate any air pockets while the tstat is closed. Otherwise the temp. goes very high before opening. I've also heard that if you rebuild your water pump, pay close attention to the clearance between the impeller and the body. It won't pump as well with too much clearance. My MB now runs at 185-195 with the 160 Tstat with both the gauge and candy thermometer having the same reading. Long hard hot weather climbs and it will touch 200, but drop as soon as you level out of the climb. Higher pressure radiator caps will keep it from boiling sooner, but you are not supposed to go over 4 pounds. I don't think I'm alone in my findings from talking with other Jeep owners. John

Many folks confuse the thermostat's start to open temp as the system operating temp.

The original low temp CJ2A and CJ3A thermostat was designed to start to open at 150 and be fully open at 170.

The later standard low temp thermostat for the F134 and DJ3A was designed to start to open at 165 and be fully open at 188.

The optional high temp thermostat was designed to start to open at 180 and be fully open at 202

The CJ2A and early CJ3A used a 4 1/2 PSI cap and the rest of the CJ's use a 7 PSI cap. (M38/M38A1/M170 use 4 1/2 PSI cap.)

FYI

2 PSI Saturated Steam is about 220F in temperature.

7 PSI Saturated Steam is about 233F.

Phil4280 wrote:Hi there!

One thing to note when removeing a thermostate you should always replace with the washer part of the thermostate!

Why... is that the thermostate provides some resistance to the water flow and with out this resistance the water can flow too quickly through the radiator and not have time to properly cool down.

Phil...

That's an old myth that I figured would pop up in this discussion. By that logic, blocking the flow to almost nothing would result in better cooling. The goal of cooling system design isn't to create a big temperature drop across the radiator - it is to keep the engine at proper temperature. High flow rate does not reduce the ability of the cooling system to extract heat from the engine and reject it via the radiator. In fact, it does just the opposite - it improves it. Here is a website that has a good explanation: http://tinyurl.com/3uc3ed3 Coolant flow rate is discussed about halfway down the page. Here's another site that has good "thought experiments" without the equations: http://yarchive.net/car/radiator_diagnosis.html

Wes...are you saying that on my m38, I should be using a 7 PSI cap, instead of the 4 PSI one I now have?...I have replaced both the T-stat(180 degree), and the sending unit, gauge bench tests good...gauge reads 160 degrees when hot...Wilf

Without the think tank web sites one need only read the Willys description of the thermostat's function. It is not there to prevent rapid flow of coolant across the radiator. It is there to ensure a rapid warming up of the engine to improve driveability and extend engine component life. It does this by retaining the coolant in the engine until the 4 cylinders warm it up to operating temperature then it begins to open and allow the coolant to flow through the radiator. From then on it helps to maintain a constant average coolant temp. During this phase of operation it does very little in the form of restricting flow across the radiator.

Hello Wilf,
No I am not. I didn't even mention the M38/M38A1 in the sentence on caps. However I went back and edited it by entering the M38/M38A1 specifically in the sentence.

I have a recently restored M38A1 with a funtioning thermostat. It has not been on any trips of over 15 minutes total driving time yet. It has a new water pump. It suddenly began overflowing coolant after running for a short time. The temp read 160-180 the first time it happened. I went through several thoughts, the radiator is not blocked, the thermostat functions fine. there are no bubles in the coolant. when it overflowed during a test today it was at about 120 degrees. The lower hose felt hot and the upper hose was cool.

Question is ... It sounds like a blockage overpressurizing the coolant in the radiator, but where? In the engine? Why did it start suddenly?

Thanks.

You said the LOWER hose was hot and the UPPER was cool? That is opposite of what it should be. The hot water should come out of the thermostat and through the upper hose to the radiator. It sounds like the thermostat is completely blocked and hot water is being forced back through the water pump when steam forms in the engine. I'd check the thermostat again and while you are at it, drill a small hole, 1/8", through the flange of the thermostat to allow any air trapped under the thermostat to leak through.

Jim...

That's an old myth that I figured would pop up in this discussion. By that logic, blocking the flow to almost nothing would result in better cooling. The goal of cooling system design isn't to create a big temperature drop across the radiator - it is to keep the engine at proper temperature. High flow rate does not reduce the ability of the cooling system to extract heat from the engine and reject it via the radiator. In fact, it does just the opposite - it improves it. Here is a website that has a good explanation: http://tinyurl.com/3uc3ed3 Coolant flow rate is discussed about halfway down the page. Here's another site that has good "thought experiments" without the equations: http://yarchive.net/car/radiator_diagnosis.html

Firstly in general it is not a myth @ all & secondly in general it does not do the opposite.

We can read all those links primarily pointed toward hot rodders & folk re-powering etc. but if we do not understand basics we will get nowhere.

Cherry picking incomplete points [like the second sentence below which is possibly correct as far as it goes] then adding your first sentence take, without foundation to support fallacious arguments, will get one & others into trouble

By that logic, blocking the flow to almost nothing would result in better cooling.
The goal of cooling system design isn't to create a big temperature drop across the radiator - it is to keep the engine at proper temperature

1. The cooling system was designed with a thermostat in it using all the calculations & formulas.

The thermostat needs to be there & it fluctuates between open & closed to maintain a constant engine temperature.

It is the thermostat that REGULATES FLOW through the radiator to maintain that constant temperature.

2. It is correct is that in the JEEP [which has a relatively big radiator] removing the thermostat from the cooling system will generally cause the Jeep to run colder.

3. Other vehicles, especially modern ones, will generally run hotter as the flow through the radiator is faster than heat can be dissipated.

So let's run with this information instead of misinformation?

I have not read all of this thread but has the questioner checked the point dwell & ignition timing....in that order?

I tested the thermostat again and it funtions fine, it has a small hole in the flange already. refilled and started it up and the lower hose again begain to get hot right near the pump. I stopped it before it overflowed again. could a complete pump failure cause this?

Did you bleed the air out by filling to the top of the head then installing the thermostat?

You could simply remove the top hose from the radiator and point it to a bucket and start her up. With the radiator full, If coolant does not flow from the top hose then remove the thermostat. Run again. If fluid does not flow the pump is inop. If fluid does flow the thermostat is bad or incorrectly installed. Thermostat goes up in the elbow first with temp sensing bulb down. Then the support sleeve.