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I have a 2011 Grand Cherokee Overland Summit with ~192k miles on it. The water pump went out once, then that replacement went out under warranty and have had that one about a year. Doesn't appear to have any leaks in water pump or anywhere in the radiator. Last week, I'm on the highway and I get the high coolant temp warning and pull over. I don't see any leaks, and so I wait for it to cool. Eventually I get it towed when I see there's no cooling it. My mechanic replaces the thermostat and the problem over ENTIRELY overheating is fixed, but my temps still climb well past the halfway point of the analog gauge, digital gauge indicating temps getting as high as 230-240. This really only happens when I'm at lower speeds or at idle entirely. Occasionally it'll cycle on while idling and cool me back down to 190-204 degrees. But I can cruise at 50+ with no overheating at all, hot air blowing. The second I come to a stop I can feel the hot air start to fade from the HVAC and the temp start the climb, eventually putting the radiator fans at full throttle. I had a check engine light on during this for a misfire in cylinder 5 that was solved with a new coil pack. Another code showing wrong engine oil also came up but hasn't come back since codes were cleared.

What is most wild is that I will occasionally have a drive (or a string of drives) where this doesn't happen AT ALL and the car is as usual. I am absolutely baffled by this entire situation. Trying to avoid paying $138/hr for labor at the dealer to diagnose this issue until it's my last resort. Can't find anything online about this, so figured now was as good a time as any to post on here. Thanks in advance.
 

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What engine do you have?

The engine temp will climb up when you are stationary or moving very slowly when the AC is off. In summer when you have the AC on, the fan is on to cool the condenser, so the engine temp doesn't climb up as much or as fast as in winter when you have the AC off and the fan is off until the engine temp climbs to 225°F.

The temp shouldn't go much over 225°F though, and you should never lose heat from the heater core.... ...and you have those symptoms.....

Have you checked your coolant level, the proper way, the O.M. tells you to check coolant level the wrong way because of liability. Only when the engine is COLD, you open the pressure cap and look into the filler neck of the radiator, the coolant should be within an inch of where the pressure cap seals.....

Your symptoms sound like low coolant level..... ....keep in mind, you can have a small leak in the cooling system that isn't easy to find, that leak can allow coolant to leak out and air to be drawn back in instead of coolant from the overflow jug, so the overflow jug stays at the same level as the coolant level continues to drop inside the system.....

It also sounds like a HeadGasket Leak, if your misfires come back on cylinder #5 even though you changed the coil pack, I'd start suspecting a HG Leak....

If you've mixed incompatible coolant you could have clogging in the radiator or heater core.....
 

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What engine do you have?

The engine temp will climb up when you are stationary or moving very slowly when the AC is off. In summer when you have the AC on, the fan is on to cool the condenser, so the engine temp doesn't climb up as much or as fast as in winter when you have the AC off and the fan is off until the engine temp climbs to 225°F.

The temp shouldn't go much over 225°F though, and you should never lose heat from the heater core.... ...and you have those symptoms.....

Have you checked your coolant level, the proper way, the O.M. tells you to check coolant level the wrong way because of liability. Only when the engine is COLD, you open the pressure cap and look into the filler neck of the radiator, the coolant should be within an inch of where the pressure cap seals.....

Your symptoms sound like low coolant level..... ....keep in mind, you can have a small leak in the cooling system that isn't easy to find, that leak can allow coolant to leak out and air to be drawn back in instead of coolant from the overflow jug, so the overflow jug stays at the same level as the coolant level continues to drop inside the system.....

It also sounds like a HeadGasket Leak, if your misfires come back on cylinder #5 even though you changed the coil pack, I'd start suspecting a HG Leak....

If you've mixed incompatible coolant you could have clogging in the radiator or heater core.....
Engine is the 5.7L Hemi. Also, I just checked the coolant the way you described and the filler neck (as far down it as I can see before it angles) is dry and no coolant visible. But my mechanic insists my coolant level is OK. My check engine light DID come back on showing a misfire (I'm unclear on which cylinder as I didn't check the codes myself) but the code was cleared and I've been driving it for a couple days now with no light coming back on. Can't say much about clogging either the radiator or heater core. Would need a thing that measures different parts of the radiator for that. Thank you for your answer though! Probably going back to the mechanic to further assess things once I can get another car to use in the meantime.
 

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You need to fill the cooling system at the Radiator, use the HOAT anti-freeze, Zerex G-05 is the exact same thing as Chrysler HOAT, its just dyed the Ford Amber, since both Ford and Chrysler use the same HOAT anti-freeze..... ....except Zerex G-05 is a lot cheaper than HOAT at the Dealer and available at most auto stores....

No heat from the heater core when engine temp is normal or hot is a classic sign of low coolant, if it gets to low it stops circulating through the heater core....
If coolant level gets too low, 1st the heater core doesn't make heat and then the engine starts running hot....

If your mechanic said coolant level was normal, either of three things....
*Your mechanic check the coolant level incorrectly, checking the level at the overflow jug...
-OR-
*You have an intermittent leak of the cooling system
*Some headgasket leaks are know for sucking a gallon of coolant out of the cooling system, causing misfires at the cylinder it sucking the coolant out, for short periods intermittently......
*It is possible to have intermittent leak at a gasket or seal, that lets a lot of coolant out, but doesn't leak while you're looking at the coolant system....
 
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