2009 LR3 V8 - I started to overheat while at idle when I finally started to use the AC again for the season this past weekend. As soon as I switch it back to hot, the engine temp drops back down. Not sure if this is the "typical" overheating failure, but my symptoms seem to be pretty specific. Seems to be completely linked to using the AC. For the very short time using the AC the air is cold, and it goes back to being hot the moment it's switched back to heat. Almost seems like the coolant flow stops when I switch the AC on.

Anyone run into something similar? Maybe a problem spot to check for a clog? Or is there something else that turns on to flow coolant when using the AC that may not be working?

 

low coolant? stuck thermostat? bad water pump? broken/slipping accessory belt? bad tensioner?
the compressor is always spinning on these, there is just a clutch that engages/disengages

My compressor failed in a free spin state (as designed, pulley breaks off of purpose to avoid breaking belts = no ac)

maybe your compressor is on its way out, and when engaged provides too much resistance (hard spin, slipping belt)
then frees up when you turn the air off (free spin)

the fan is on its own belt, so shouldn't be affected by the compressor belt

 

Sorry guys, but wtf does the coolant level have to do with the air conditioning? Is your electric fan working??? At speed enough air moves across the radiator to cool the condenser and produce cold air. If the fan isn't working there won't be movement at idle and the air will be warm. The engine cooling system has literally nothing to do with the air conditoner

 

A/C compressor and water pump are driven off the same belt - so there is that

However, coolant has everything to do with overheating - which is his main problem

 

I didn't notice the engine working any harder while using the AC. It almost seemed like a coolant flow problem at first. But, thanks @douglastic for the suggestions. Started to check things out closer and found I did find that my fan belt had slipped off a couple grooves. Thinking maybe just a bad belt that was slipping? The thermostat and water pump are both only about 6 months old so I wasn't suspecting them yet.

Plan to check it out after I put the new fan belt on today. Anyone know if there's a way to determine if the fan clutch is bad while it's out of the car? It seems to be spinning nicely and I never noticed any issues with it spinning while the engine was running. Just curious if there's an easy way to check that.

 

Its a gloved belt, for it to slip off is very unusual. Either it was installed wrong or you have a fully failure somewhere. The fan is electronically controlled. So with a cold engine and it off the fan should have resistance. Once the truck is on and warmed up, its easy to stop by hand or with a newspaper, gently... When starting the truck, it should roar for a few seconds and quiet down. If when cold it spins easily then the clutch may be on the way out.

 

yeah slipping belt grooves points to an issue here
- bad or wrong belt, bad tensioner, seized idler pulley

you mention 6mo old water pump - is it OEM LR, or aftermarket?
did you do the work? a shop? were new belts used? pulleys/tensioners checked?

 

Engine load (aka heat production) is higher when AC is on and compressors often share drive belts with water pumps. Also AC condensors are in front of radiators so when AC is running the air the radiator gets cools less effectively because it's been warmed up a bit by the condensor. All these things relate the AC system to engine cooling.
It's a classic sign of a marginal cooling system when the engine runs hotter when AC is on.

 

Just got the new belt on, tensioner and pulley all looked pretty good. I had replaced both belt tensioners when I did the water pump (got those from Lucky8). Water pump was the OEM one from Atlantic British. I had also replaced the radiator as well at the same time because the top connection was breaking away. It was still sealing, but after taking the tube off it clearly needed to be replaced. Anyway right after that work was done I had one additional exhaust leak to fix, had a reliable a shop do that and then flush and fill the coolant. Everything was humming nicely for a few months until I went to use the AC.
My last driving test with it I had kept the heater running on the drivers side and then was using the AC on the passenger side. Both seemed to be working just fine, but if I switched to only AC at idle the overheating came back. I was thinking that the fan wasn't engaged fully and not pulling enough air over the radiator to handle full AC while not loosing any heat to the heater.

Am I way off base thinking that could be causing the problem? Symptoms seem to line up, but if I had a nickel for every time I was wrong....

I'm going to replace the fan clutch next, so waiting on the new one to come. Old one does have resistance spinning against the connection bolt and was roaring on startup as @DakotaTravler mentioned (thanks for your input) but I've replaced just about everything else in that system and given the age it couldn't really hurt anyway.

 

I don't think it is causing the problem.
I think it is exposing a problem that is otherwise hidden.

But I don't run heat and A/C at the same time.