Crazy A/C problem on my 2003 Subaru Forester

My Forester blows freezing cold air sitting in the driveway idling or at a stop light. As soon as you get out on the highway, it blows about 10-15 seconds of cold air and then 10-15 second of warm air. On a really hot day it is borderline uncomfortable. Already had the freon checked and it is fine. Any ideas what this could be? I’ve searched all over the internet for people having a similar problem, but everyone else who has intermittent A/C has like ten minutes of cold and then it blows warm until they stop. Just my luck, nobody else seems to have my problem.

Just a diy’er here with no actual AC-fixin’ experience, but what comes to mind is something is preventing the heat produced by the AC from being dissipated when you are driving down the road for some reason, but at idle it is being dissipated. AC doesn’t really cool anything, it just moves heat from one place to another. And for that to happen it has to put the heat somewhere. The way it does this is by air movement though a radiator-like contraption in the engine compartment. I think that part is called the “evaporator”. Whatever it is called, that heat has to be removed. It may be the fans in the engine compartment are not working correctly, not turning on when they should, or not spinning as fast as they should. That’s what I’d check first if I had this problem.

Maybe it could be as simple as that. It’s really hard to diagnose when you have to be on the road for the problem to present itself :neutral:

Does the coolant temperature gauge seem to be reading the same as when the AC worked correctly? Especially when driving down the road?

What coolant temperature guage?

Oh, nevermind. Blonde moment. The guage reads the same.

This is probably a lot more than you want to know about auto AC systems, but you may find it informative, might help when you discuss this problem with your mechanic.

Edit: I just read the first part, decided it is the “condenser” that is responsible for dissipating the heat, not the evaporator. So check your condenser fan. That it is working, turning on when it should, and running at the right speed.

How do you check that when the problem only shows up when driving? I guess get it on a rack somewhere so that the wheels can turn at highway speed?

As a DIY’er, about all you could do is check those functions are working at idle. You won’t have a way to test them while driving on the road. You might spot something amiss while doing the idle checks. If that doesn’t discover the problem, your best bet is to head to a shop that specializes in Auto AC. AC is a very common problem reported here, so I expect you won’t have much trouble finding an AC specialty shop in your area. They’ll have the proper tools and equipment to diagnose the problem. After they diagnose it, you can decide if you want them to fix it, or fix it yourself.

One thing you can do is rev the engine while standing still out of gear.
Hold it at 2000 rpm and have a helper look under the hood for anything unusual.
Check that the compressor clutch stays engaged (all parts of the pulley spinning) and the belt’s not slipping.

From what you say about the issue I would check for an problem with the air blend door. Especially if it is vacuum actuated. The door separates the heater and the AC ducting. There may be a vacuum leak causing the problem.

Good idea by @Cougar . That could be a simple explanation for this symptom. Applies if OP’s vehicle has vacuum operated blend doors. At idle you have a big vacuum in the intake manifold, so “iffy” blend door actuators might still work. But when you are driving down the road the intake manifold vacuum is less, so the blend door actuator might fail to work properly then. Hope for this, as it would mean the AC system may be working ok, just a blend door problem.

If the actuators are vacuum operated, OP should check all the vacuum lines for breaks, leaks, measure the vacuum and compare to spec, etc. On some vehicles with vacuum actuators there’s a sphere-like gadget which stores vacuum for when it is needed, and sometimes those sphere’s leak or otherwise go berserk.