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Discussion Starter #1
Hello MDXers,

I just notice this pattern of my 2004 MDX vehicle overheating when ever I put on my AC and if the AC is turn off the temperature drop for a while and start to overheat again ( I believe due to low level of coolant now). But if the AC is not turn on the vehicle will not overheat.

Whenever this happen I have to top up the coolant both in the radiator and reservoir. I have flush the radiator and the condenser externally and change radiator cap but the issue persist.

Whenever I notice the overheating while the AC is on, I park to see if I will see any coolant dropping from underneath but what I see is just the AC condensed water under the vehicle.

I also check the fans and they seems to be working fan, the right side fan (looking into the engine from front of the car) kicks in at operating temperature and the left fan also start rolling immediately when the AC is turn on.

I try to rule out fan issue because I always have to top up the coolant when the engine start to overheat which means there is coolant leak, but how, when and why does it happen with AC on.
 

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If the cooling fans are both running when the A/C is on, it's likely that your problem isn't really the A/C, but just that the A/C just adds heating load to the cooling system.

Something is apparently preventing your cooling system from working properly - you might have a bad thermostat, a blocked / dirty radiator, or possibly a bad water pump.

It might be interesting to see if turning the heat up full blast will drop your coolant temperature (the heater essentially does the same thing the radiator does, which is to radiate heat from the engine into the air, though in this case, it's the air in the passenger compartment).
 

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Discussion Starter #3
If the cooling fans are both running when the A/C is on, it's likely that your problem isn't really the A/C, but just that the A/C just adds heating load to the cooling system.

Something is apparently preventing your cooling system from working properly - you might have a bad thermostat, a blocked / dirty radiator, or possibly a bad water pump.

It might be interesting to see if turning the heat up full blast will drop your coolant temperature (the heater essentially does the same thing the radiator does, which is to radiate heat from the engine into the air, though in this case, it's the air in the passenger compartment).
If I have a bad thermostat, a blocked / dirty radiator, or possibly a bad water pump as you said is it expected that coolant level to drop or I should be seeing overheating with boiling coolant
 

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That's part of the diagnostic process - figuring out if overheating is causing the coolant level to drop (by blowing the coolant out as it boils), or if coolant loss is causing the overheating (by virtue of the system not having enough coolant to circulate / cool properly).

A blocked radiator or thermostat won't cause a leak per se, but a bad water pump can (it will be pretty obvious though, by following the path of dried coolant). If you're losing lots of coolant without any obvious signs of a leak somewhere, it could be a bad head gasket (or worse, warped head), which causes the coolant to disappear into the combustion chambers and out the exhaust pipe.

If it was my MDX, I'd top off the coolant and let it idle with the A/C on high and see what happens. If the coolant level drops without it dripping onto the ground, it's bad news... if you see coolant dripping out, you've just got to trace it back to the source, whether that's a bad water pump, or whether it's shooting out the overflow due to overheating (which would indicate a problem with the thermostat or radiator).
 

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Usually, yes. The coolant is going somewhere, and if it's not dripping out, it's going into the combustion chamber via a break in one the head gasket. If that's the case, it's always possible that you have a warped or cracked head. That (it goes without saying) can get spendy to fix... This is all pretty unlikely unless you've really overheated the engine, but it sounds like that might have happened.

And - FWIW - there are a few products out there that claim to fix a broken head gasket. I know less than nothing about any of them, but I suppose it might be worth a shot IF you verify that the coolant IS going out the exhaust pipe.
 

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Discussion Starter #8
RE: overheating withe the A/C on: this happened on one one of my old cars - I replaced the radiator and it was fine afterwards
Was there a lost of coolant in your own case too? or you have the coolant boiling in the radiator and reservoir
 
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