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Over heats while idling/sitting still


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I'm about to buy my 3rd MJ next week from my parents. Their are afew problems with it but its a 1988 Pioneer LWB "rust free"!

The problem is that when the MJ is idling in traffic or stoplight, it tends to start over heating. If your driving down the road, it cools down and won't over  heat until you stop moving again. I havent seen this Jeep in 2 years so idk if its the fan, water pump, radiator..

Anybody have an idea? 

Eventually, i plan on installing the 2nd electric fan in parallel with a switch.

 

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Mine did this once before.  Turned out to be broken water pump.  Driving was fine because the water would circulate itself while moving.  Once I stopped, the water stopped circulating.  

 

Next possibility would be e-fan.  Check the temp switch and the relay.

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Mine did this once before.  Turned out to be broken water pump.  Driving was fine because the water would circulate itself while moving.  Once I stopped, the water stopped circulating.  

 

Next possibility would be e-fan.  Check the temp switch and the relay.

:agree:  

The fact that it cools and stays at a steady temp in motion tells me that you either have a circulation problem at low engine speed (pump) or your electric fan, the switch/sender in the rad, or wiring between the two is not functioning.   

 

 

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Sooo if the fans are working, I should replace water pump, thermostat, surf. belt, and new fluids?

No.

 

Replace the fan clutch on the mechanical fan. (That's the viscous hub that bolts to the water pump, and to which the fan blade is bolted.

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Sooo if the fans are working, I should replace water pump, thermostat, surf. belt, and new fluids?

No.

 

Replace the fan clutch on the mechanical fan. (That's the viscous hub that bolts to the water pump, and to which the fan blade is bolted.

 

This^^^ first.

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yes sounds like the fan clutch is not turning the fan as it should. When truck is still or idling the fan clutch should be pulling air thru when it is warm. If it is electric fan only then you need to check if temp sensor for e-fan is turning it on. That's the places to start before you move on to other things!

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The fan clutch turns the fan at 60% of engine speed, to test you need to measure temperature in the flow.

It's cheap and if suspect, replace it. The first sign of a failing fan clutch is over heating at a stop ... It is the primary cooling fan, the electric fan only comes on if the engine gets hot, not going to cause an overheating issue if one isn't already present.

My XJ is on it's 5th fan clutch and it's third waterpump. Although the second water pump is currently still in service (on the MJ - because I replaced the pump when the issue was actually the fan clutch) ... I just recently replaced the last failing clutch with a Hayden 2737. First symptoms ... Higher Temps on the trail, and in traffic. Afterwards, golden. Some don't like the higher noise levels with the stronger fan clutches, but I come from the days of direct drive fans so I could care less about fan noise.

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Both fans are working though. But ill keep this in mind next week when i can see this in person.

But the mechanical fan doesn't have a direct, positive, mechanical connection to the pulley. The fan mounts to a housing that contains a viscous clutch. Think of it like a small torque converter, with NO lock-up. The fluid inside is formulated to get stiffer as the temperature increases, yet still allow enough slippage so that at higher engine speeds the fan isn't being driven by the engine, thus saving horsepower and increasing fuel economy.

 

Over time, the fluid breaks down and isn't stiff enough to drive the fan fast enough at idle to pull enough air to cool the radiator. The classic symptom is overheating at stopped idle, and the temperature going back to normal as soon as the vehicle is moving. There is a way to test this, IIRC, but I don't remember what it is. Yes, the fan is turning -- it just isn't doing much work. Appearances can be deceiving.

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  • 2 weeks later...

Okay, I been working on this Comanche all day. I took out the interior "carpet and seats" and threw them out. I checked the fan clutch, it looks new actually and the bolts look new too. Must have been changed recently I guess. I let it warm up and I tried stopping the mechanical fan with a board, that didn't work so the fan clutch must be good. I watched the temps rise past 210 and the electric fan never came on. Tomorrow I'm going to get tags, registration, and some other stuff for the trip. I gotta check if it's the rad temp sensor??? or just put in a switch and relay to turn the electric fan on myself, I'd prefer the switch.

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