OVERHEATING 2f 77 FJ-40

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Mar 9, 2009
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Everytime I get on the highway it starts to overheat, I was thinking it was the thermo-stat, so just out of curiousity I removed the t-stat and took it for a test drive.. It made it worse. It was running hot before I even hit the interstate. I'm wondering if I may have a air lock in the system somewhere, is this possible,if so whats the best way to relive the air, or am I way of base. I previously flushed the cooling system, as-well and also no luck. I am beginning to wonder if its the water pump,its not leaking, but wondering if it is just not keeping up at highway speed's at most 60 mph, Its a stock 2f with a header stock trans and rear, running 33's. I can trail ride all day as well as city drive and never gets passsed the first line on the gauge.. I spray the radiator down with the hose thru the fins and it was real dirty but I have'nt taken it for a spin yet, so maybe it was just not breathing enough thru the radiator. Is an airlock possible? Is it my water-pump? When a water pump stops working does'nt it usually leak, squeal, shaft fall out or all of the above? Any help would be awesome, I've never had a situation like this in the past so I'm asking the guru's... Thank's in advance... Ryan
 
I am not a guru but I do have a 77 that exhibited signs like yours. Mine turned out to be the fan clutch. There are many things it COULD be and a bad fan clutch is just one of the things that will make it overheat.

I explain how to check your fan clutch in this thread...
https://forum.ih8mud.com/40-55-series-tech/243016-78-fj40-overheating.html

Again, there are probably other ways to diagnose it but this is the way I use. It works for me. Good Luck.

Copenhagen1
 
The fan clutch is a good guess. It could be something as simple as a clogged radiator(although you said you did the flush) and air in the system. A couple of quick things to check, would be your timing, and make sure that your rig isn't running lean. Timing that's advanced too far, or detonation, from a lean running engine can cause you to overheat as well.
 
are you sure it's hot? did you try a hand held thermo gun? don't always trust the stock guage.

yes, plug fins either inside or on the outside of the rad will cause your problem, just because you flushed it doesn't mean it's clean, known from my experience. It needs to be rodded out or buy a new one. could also be an air pocket, try burping it.
 
Mine did that as well. I cost me $100+- to get it re-cored and solve the problem. A good investment.
 
I will try the clutch fan diagnosis, I have not ran it since I cleaned the radiator fins... If it is an air lock how did/would you burp it on this system? Also would a person rule out the water-pump if it is'nt lleaking or squealing?
 
The fan clutch really is only for drawing air through the radiator at low speeds. Unless it overheats at slow speeds the fan clutch should be perfectly fine. Overheating over 40 MPH and the fact that you made it worse by removing the thermostat points to a bad radiator. If the radiator can't sufficiently cool the coolant as it goes through the radiator removing the thermostat and making it flow faster will only make matters worse. If it's clogged with mud and bugs try cleaning them out, but you may very well need a new radiator.
 
I know it makes no sense, but my cruiser ran hotter with no thermostat than with one. My 2F was heating up, and when I checked the system I found that the thermostat was installed without the do-nut gasket -- the equivalent of running w/o a thermostat. I installed the do-nut and it consistently ran at the correct temperature afterward.

The only explanation I could think of was that with the system in full flow the coolant spent too little time in the radiator to lose enough heat to effectively cool the engine. But that doesn't really make sense because the cooling system (as I understand its function) is in full-flow when the engine is hottest . . .
 
The fan clutch really is only for drawing air through the radiator at low speeds. Unless it overheats at slow speeds the fan clutch should be perfectly fine. Overheating over 40 MPH and the fact that you made it worse by removing the thermostat points to a bad radiator. If the radiator can't sufficiently cool the coolant as it goes through the radiator removing the thermostat and making it flow faster will only make matters worse. If it's clogged with mud and bugs try cleaning them out, but you may very well need a new radiator.

Depends if he has stuff in front of it. (Winch, lights, etc...)
 
I have 2 t-stats gonna check them on the stove tonight, one has 170 stamped on the bottom, not sure about the other, I am assuming thats the temp of operation, am I correct? That seems high or is it? Or does the number stand for something else? I have an open grill area no winch, or lights. Thank's for all the input, I will keep you guys posted on my outcome... ryan
 
No loss of fluid, anywhere, I drove it with no t-stat, and after a major radiator douche'n thru the fins it still was getting warm.I'm heading out to try the clutch fan test.....
 
I have 2 t-stats gonna check them on the stove tonight, one has 170 stamped on the bottom, not sure about the other, I am assuming thats the temp of operation, am I correct? That seems high or is it? Or does the number stand for something else? I have an open grill area no winch, or lights. Thank's for all the input, I will keep you guys posted on my outcome... ryan

170 is not high. Just right.
 
I tightened the belt, flushed the whole engine AGAIN, and instaled a spring out of anothet hose i had in the bottom hose, and I drove it the other night on the expressway and it did'nt overheat, but it was raining so I don't know if the rain coming in thru the radiator was keeping it cool ( it was a major down pour) or if I fixed the problem.. So there might be light at the end of this HOT tunnel...
 
If the engine runs hot when on the freeway it is usually a radiator capasity problem. If the problem is a low speeds, it is usually the fan. Running without a thermostat makes the water move so fast it doesn't have time to cool in the radiator (at least that is the theory I've heard)

If you flushed the radiator out and everything looks good then you might want to consider a new radiator or a re-core.
 

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