As you probably know, the coolant level in the overflow bottle fluxuates; It rises as the engine heats up and should fall as the engine cools down. The coolant level after the engine is cold should be the same as at the beginning of the engine start. If the coolant system has been opened, it may take 2 to 3 heating-cooling cycles to arrive at a stable level, and it may need extra fluid added to the reservior. Howver, if you have to contine to add fluid several times to maintain a level in the bottle, then fluid is being lost somewhere.

Check the condition of the hose that goes to the overflow bottle. Lift it and look for splits on its underside. If it leaks air on engine cooling, air is admitted into the system and the engine will not be kept filled - a bad situation.

Other places where slow leaks may occur:

1. Leaky rediator -has a slow leak, slow enough that the evaporation rate of the fluid nearl matches the leak rate. sometimes can be detected by small wisps of steam while stopped with engine running. Odor of hot coolant may be present.

2. Leaking water pump. If the leak is small, it may be hard to see. The leaked fluid exits a drain hole in the bottom of the water pump hosuing.

3. Any hose in the system. Look at heater hoses. Lift up when inspecting.

4. Head gasket leak.

5. If the intake manifold has coolant passages, its gaskets may be leaking.

6. Leaky heater core, and this may make an oder in the passenger cabin.

If you are having to add fluid on a regular basis, it is leaking out somewhere (unless it is ovefilled in which case, the excess fluid spills out the overflow tube when hot).

Could be leaking from the reservoir - small crack, the hose to the radiator, or from the radiator tank, which is plastic and separate on newer cars.

Either take it back to the shop you had it too already, or, if the first shop was no good, pick a different one. They should have a cooling system presure tester. A lot of places now seem to have the ability to check for gasses in the cooling system as well. Shouldn't be that tough for a shop to diagnos. I'll give the first shop the benifit of the doubt at this point and assume they have a pressure tester, tested the cap and it failed. Tested the cooling system, but didn't look close enough or long enough because they already found the defective cap.

The heater not working when there's not enough coolant in the system is a normal condition.

The other guys gave a good list of what the could be's are.

My flying blind guess would be the water pump leaking while driving down the road. No reason, just a hunch. Given the many possiblities, don't take that guess to the bank.

You most likely have a very small leak. I would look very closely for coolant staining around the water pump first. Also look a the radiaror seams for wetness.