John Deere 40 Cooling System

(This following information is also applicable to the M and the 320.)

Do you have problems with your tractor running hot? Does the coolant simply not circulate very well? Here are some suggestions that may help.

The whole idea of "thermo-siphon" is that this is a gravity-driven system. Heat (hot water) rises and cold (cooler water) sinks.

The water gets heated in the block and rises to the top of the block and into the radiator. The water cooled in the radiator sinks to the bottom and is drawn into the engine as the hot water rises out of the top.

If anything interferes with the gravity flow, then you have a problem. Possible options include the following:

If the coolant is too low, hot water can't travel from the block back to the radiator tank.
If a hose is used at the bottom from the radiator to the block, if the hose sags or is too small it could restrict the gravity flow.
If too much antifreeze is used, it can thicken the coolant to the point where it won't flow well.
If antifreeze and water is not mixed well before filling the system (i.e., coolant was dumped in, then water added rather than mixing first), the thicker coolant can become trapped and interfere with flow.

Some people suggest that if the lower pipe was replaced with a hose then the coolant won't circulate. I have even seen suggestions that the pipe somehow aids in the cooling of the coolant. I don't see how a pipe or hose (on the lower end) would make any difference, provided the hose is the same size as the pipe and does not sag. The cooling effect takes place in the radiator, not in the pipe.

You might try running it with water only, just to see that everything works as it should. If it works with water only, but not with coolant added, it may be a problem with the water/coolant mixture. If it doesn't work with water only, then there may be some sort of blockage somewhere. Try a VERY thorough flushing.

A suggestion from another person: Seems like I read in Green Magazine something about some of the smaller 2 cylinder tractors showing higher than normal temparture because of the location of the temperature sending unit and was corrected later when they relocated the sending unit. Is it possible that your temperature gauge is reading the hotter water? My 40 crawler has the temp unit on the lower part of the block and my 420 has the unit above the top of the head.

Quentin writes: My 40 T would run REALLY HOT to the point that it almost pegged the guage. I checked my coolant ratio with a tester (the kind you look through like a scope).

Problem #1 - Coolant was at -45 degrees F.
Problem #2 - Drained the coolant and further inspection revealed about 70% of my radiator fins were clogged.
Problem #3 - Turned out to be about 1 inch of sludge in the lower radiator hose (the pipe is long gone).

I gently scrubbed the fins with a very soft bristle (nylon) brush and full strength dawn dishwashing liquid (much to my wife's intense displeasure). After much soaking and many scrubbings the radiator came clean. I hosed the block and radiator out and was appalled at the amount of crud that came out. After replacing the hoses ($10 per foot), cleaning the radiator, flushing the block, and pouring in the proper 50/50 mixture, she runs like a champ. The needle goes to the "o" on normal on the guage and stays.

I should mention that the manual says 3 1/2 gallons of coolant/water. If you take a paint stir stick (a clean one), place it in the tank and place a mark on it even with the top of the radiator (not the raised area where the cap and overflow tube is) you will have a full radiator. You will also have about a quart left over. Save it as if you put in the whole 3.5 gls, when the system pressurizes, the excess will run out the overflow and give you a false indication of a leak and will lead to depaneling to doublecheck (not fun!). My sending unit is on the top of the block in the head. Hope this helps some people.

Click here to see pictures of Quentin's tractor.

If you have any other good ideas, please send them to me and I'll add them to this page.