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Over heating situation!!


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Another overheating MJ! I have a 1986 MJ with a 2.5L motor, 4 speed manual tranny, 2wd etc. Last weekend after traveling 25 miles at 60 mph and towing a trailer with 3 motocross bikes the temp light began to flicker on and off. I turned the heater on and the flickering stopped. With the heater on low the temp light stayed off the remainder of the trip. Same thing happened on return trip except temp light came on and stayed on for 30 seconds after I turned heater on, and then temp light went out.

 

I have installed new water pump, thermostat, radiator hoses, serpentine belt, and fan clutch. There are no leaks in the cooling system and there is always the proper amount of coolant in the radiator and overflow tank when I check. This overheating doesn't happen in the winter when I am running the heater. Is my temp gauge bad? Does my radiator have a blockage? Can a temp sensor be tested? Am I overlooking something?

By the way the thermostat is a 190 degree unit. Radiator is stock and I have not tried to flush it. I am puzzled by the amount of time/miles that pass by before the temp light comes on! This happens with the truck empty or loaded. If I travel at 55mph the light doesn't come on. At 60 plus it will come on after 15 to 20 miles. Sorry for the long post. Thanks in advance for all advice.

Craig

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Its virtually impossible to accurately diagnose this on the Internet.. BUT, I would start with a flush and fresh fluids.

 

Al-tho I have never overheated mine, I do not like the temps in the summer on the freeway above 65MPH. I have a new three core rad, new heater core, new hoses, clutch fan and all fluids. STILL, I have the temps climb to almost 240 on my fact gauge. That's another point of contention... I don't trust my fact gauge. Normally, I don't travel even that fast. Remember, we have old trucks, they where never made to be go fast vehicles.

 

Good luck,

CW

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For the cost of a temp sensor, I would do that too, just to be sure, in addition to the other things mentioned above. I have the same setup you have...2.5 4 speed, and my light has never come while driving at highway speeds. Being that it is just for an idiot light I would change the sensor too to be sure. Also... change to a 180 thermostat. That should help as well. I know the 190-195 is original equipment, but with wear on an engine as it ages the 190 may be too late to open for it to cool properly anymore. A 180 or even a 160 tstat is available, but a 180 should help you by allowing the coolant to get through the radiator to start cooling just a little sooner.

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Engines work more efficiently as the temp goes up. But the parts inside the engine can't get too hot so there's a compromise point in the middle. For the renix 4.0, that number is about 210-220*. A thermostat lower than 190* will not prevent overheating if the rest of the system is not up to snuff. If it's too low, it can slow the engine getting to the open loop computer cycle.

 

For cooling problems, I recommend borrowing or buying an inferred thermometer gun to track the heat around the system. I picked mine up for about $40. It'll show you if the coolant isn't getting through the radiator. It'll show you if the lower passages of the radiator are plugged. It'll show you if your dash gauge is just plain wrong. With that knowledge you can then decide what part of the system is bad. And if the radiator is original equipment, it is probably the first thing that should get replaced. :thumbsup:

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Make sure that you have clear flow of air through the grill and to the radiator. Those big offroad light can and will block air flow if you have them. Also make sure the radiator is clean in by the fins. A muddy or bug filled radiator will cause it to overheat. Also make sure that you do not have any engine condition that may make it run hot like a lean condition or off timing also.

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  • 2 weeks later...

Okay I decided to back flush radiator, heater core and engine block AND replace the temp coolant sensor. While trying to remove the sensor it snapped like a potato chip leaving the top part in my socket ant the bottom part still threaded into the block. Wonderful! I sure could use a plan B on how/where to put another temp sensor into the cooling system. Is there a way to remove that part still stuck in the block? Do I have to remove it? Sure could use a plan B on how/where to put a temp sensor in the cooling system at another location. Thanks everyone. Woe is me.

Craig

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An '86?

 

The original radiator, if that's what's in it, is 23 years old. It doesn't owe you anything -- replace it. It was done ten years ago. My '88 Cherokee is on its third radiator right now, and my '88 MJ is on its second.

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What CW said. My 97 recently decided it wanted a new water pump the block and head and inside the rad are 100% corrosion free, being all relatively new, so was the pump, not a spec of rust or coolant scale on anything. The thermostat was a 195 which I replaced with a 185 at the same time as the new pump because as CW observed highway cruises can get things heated up above the T-stat temp... In both cases the T-stat temp is maintained at idle and low speed driving. No matter what is done it wants to run well over the T-stat temp at 65 or over... usually 210-215* but as much as 220-230 (this is confirmed by a Scangauge II which reads the engine sensor) fighting the wind takes work and the stock pump just doesn't keep up, especially in summer. It probably doesn't help that a stock 97 radiator is a single core design, but I feel the stock pump is the real weak link. This is with no mods, bone stock 97 XJ, even has stock wheels and tires with a fresh and strong engine.

 

My stroker XJ has a Flo-kooler water pump and it never goes above the temp of the T-stat. Even at 75mph which is as fast as I would want to maintain with 33/12.5's and 4.56 gears. due to the RPM's being fairly high at that speed. So IMO a higher flow water pump is a good upgrade, at least from my experience.

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try an easy out to get sensor out

i put a manual gauge on mine but it was a pain in the #$%

i will try to snap a pic later

but it took some pluming pipe and a T were the heater hose goes into the intake

i ran a pipe off the intake strait up and put a T on top of it sensor in one side and heater hose in the other

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Engines work more efficiently as the temp goes up. But the parts inside the engine can't get too hot so there's a compromise point in the middle. For the renix 4.0, that number is about 210-220*. A thermostat lower than 190* will not prevent overheating if the rest of the system is not up to snuff. If it's too low, it can slow the engine getting to the open loop computer cycle.

 

For cooling problems, I recommend borrowing or buying an inferred thermometer gun to track the heat around the system. I picked mine up for about $40. It'll show you if the coolant isn't getting through the radiator. It'll show you if the lower passages of the radiator are plugged. It'll show you if your dash gauge is just plain wrong. With that knowledge you can then decide what part of the system is bad. And if the radiator is original equipment, it is probably the first thing that should get replaced. :thumbsup:

 

So does that mean the best temp is about 210?? :???:

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