Is it possible to have a leak in the cooling system when the AC is running, and no leak when the AC is off? (My 98 XK8 with 86k miles..)

About two weeks ago, the dealer fixed a cooling leak on my car (and a lot other things!) and on my way home (25 mile drive), the Low Coolant light came on and went away immediately. The next day and over the weekend I drove the car without any problems, or lights. Last weekend I replaced the struts of the hood (and fixed a lot of things in the car), and when I checked the coolant level it was low. I filled it up, and last night drove to a meeting a few miles aways, and back two hours later. Checked the level this morning, and it was OK. The only differences was that the trips I did first, I used the Auto temp setting that switches the AC on, and not lat night.

I have also noticed when I used the Auto temp, the electric fans seemed to run more frequently, and even continued after I switched off the car in the garage. During last night's trip, I doubt if the fans ever came on.

Any ideas?

 

When the A/C is on, temperature is regulated by balancing the the cooled air against heated air, so unless it is very hot outside, the coolant will be running through the heater circuit (heater valve, heater core , heater pump and hoses).

It is possible there was an air pocket there, or a leak.

 

Update so far.. Dealer replaced the thermostat. It seems to have solved some of my issues, except that I am still loosing coolant.

A few weeks ago when the dealer replaced the tensioners, chains, waterpump, and a few other things, he assured me he checked the complete system and it has now been checked three times in total.

Monday I picked up the car, and drove 30 miles back home. Next morning I checked the coolant level and it was low, but not completely empty. I have seen a bit of fluid on my driveway in two spots after I parked the car in my garage, which tells me that I am losing coolant when I drive as well. I did not fill the coolant, because I wanted to check the next morning after I drive the car to work and back. However, I was 2 miles from my home when the light came on, and disappeared within a few seconds. I turned around and went back home and the light came on two more times and went out as well.

Ruling out the head gasket, any other areas I can look? The spots on my driveway was more towards the right side of the engine. I will take it back to the dealer next week, but I first want to have a look at it myself. Could there be air in the sytem?

 

what happened w/dealer?

 

Nothing happened with the dealer. To make a long story short - it was the head gasket. I took it to another shop to get a second opinion, and they confirmed it. (I had another thread with the possibility that it may have been air locked in the system, but no., it was not.) Now I have two places that can fix the car. Depending on the issue will determine where I take the car, or do the work myself.

 

Wonder if you found the ac leak.
I had a leak as well and it took my mechanic forever to find. It turned out to be the crimp connector on the main hose that snakes from upper engine to the bottom. It seems it only leaked when car was moving/vibrations. When parked, it didn't leak. That's why he couldn't find it. He used a sniffer and searched for over an hour then when he accidentally jarred the hose the sniffer beeped. Was a nightmare to replace!
Just sounds similar to your issue. No leak when not in use and leaking when driving.
Also had a coolant leak and finally found it. The manifold on top of thermostat housing had a leak underneath.