Overheating Problem

[mattd]

Last few weeks my car has been losing coolant. I tried to pin point where the leak was without any success. I did however see alot of coolant all around the right hand side of the engine bay (around the surge tank ) I finally took the car to a mechanic who pressure tested the car and told me that there is no leaks. After I reassured him that there definitely is a leak he decided to take the car out for a run and then try the test again...He called afterwards and said that the coolant is leaking through the surge tank cap and that will likely fix the problem. After the new cap was installed on Wed the car ran good and I saw no coolant anywhere. Last night though after I came home I caught a wiff of some coolant scent, but didn't see any. Just now after driving for about an hour or so After I pulled into my driveway I noticed a ton of steam coming from under my hood. Once again the coolant is getting out from the cap...what the heck is going on. I guess I'll believe that my coolant is only leaking through here and there is no other leaks but why is it getting through there. Is it the thermostat?? What could cause the coolant to leak through there? Any advice is really appreciated...thanks in advance.

[adallak]

Your coolant is boiling. Are your fans working properly? they should kick off at some 225 degrees? How about water pump? Is the serpentine belt tensioner OK?

Did you check the concentration of your antifreeze? If you monitor the temperature behavior of the car from start to the point it overheats you'll get some idea.

My guess is the serpentine belt slipping and water pump not providing enough circulation for the coolant. It may be fans as well. While you are driving the air flow cools the coolant down, but as soon you stop it can easily reach boiling temps and situation will develop catastrophically when the system gets unpressurized because of failing surge tank cap. It is strange your mechanic cannot diagnose the problem.