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Discussion Starter · #1 ·
Hey guys, having some overheating issues in my 2008 ranger 500 that I rebuilt the engine in recently. What’s weird is it only overheats at idle. I have replaced the thermostat, washed and blown out the radiator. I have taken apart the water pump several times to try and verify that the impeller is not slipping on the shaft. What’s weird is I’ve put probably 50 miles driving it, and it will run all day long without overheating. In fact, it runs too low of temp. On a non hot day (less than 80° ambient) it won’t get over 160° driving down the road. But then if you stop and let it idle for more than 5-10 minutes, it will all of a sudden start climbing in temperature upwards of 225° until it starts squirting out of the reservoir. The fact that it stays too cool while driving then overheats while idling leads me to believe maybe my new thermostat is faulty, but I’m open to any other suggestions. Thanks in advance
 

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2021 Ranger Nortstar Ultimate
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Hey guys, having some overheating issues in my 2008 ranger 500 that I rebuilt the engine in recently. What’s weird is it only overheats at idle. I have replaced the thermostat, washed and blown out the radiator. I have taken apart the water pump several times to try and verify that the impeller is not slipping on the shaft. What’s weird is I’ve put probably 50 miles driving it, and it will run all day long without overheating. In fact, it runs too low of temp. On a non hot day (less than 80° ambient) it won’t get over 160° driving down the road. But then if you stop and let it idle for more than 5-10 minutes, it will all of a sudden start climbing in temperature upwards of 225° until it starts squirting out of the reservoir. The fact that it stays too cool while driving then overheats while idling leads me to believe maybe my new thermostat is faulty, but I’m open to any other suggestions. Thanks in advance
Have you made sure you have no air in the coolant system. Make sure it is bleed, lift the front end up as far as you can. Pop radiator cap off and bleed the water pump.
 

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Discussion Starter · #3 ·
Have you made sure you have no air in the coolant system. Make sure it is bleed, lift the front end up as far as you can. Pop radiator cap off and bleed the water pump.
I have not tried physically lifting the vehicle. I believed the system was bled, because I had removed the water pump bleed screw and thermostst bleed screw and didn’t seem to have air. But I could be wrong. I also thought about using my vacuum refiller
 

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Discussion Starter · #4 ·
I have not tried physically lifting the vehicle. I believed the system was bled, because I had removed the water pump bleed screw and thermostst bleed screw and didn’t seem to have air. But I could be wrong. I also thought about using my vacuum refiller
I just tried bleeding. Lifted the front end up probably a good foot or 2 off the ground, the. Opened the water pump screw while keeping water coming in the radiator. Then did the same thing with the thermostat screw. Same result, let it idle 10-15 minutes it’ll start climbing to 225 or so and blow coolant out. If i rev up the engine and hold it revved up, it cools back down. Fan is functioning correctly, radiator is clean
 

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Maybe verify the flow by cranking up with the rad cap off. Coolent comes in from motor right at the neck. Watch for thermostat open and difference in rates with rpm after thermostat open.
 

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You have never mentioned if the fan comes on. Idiling it will raise the temp until the fan comes on at about 205 and off about 185. If the fan isn't coming on the temp will continue to rise because there is no air going thru the radiator.
 
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