Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Intermittent heat, car occasionally overheats

Hello Car Talk Nation,

Here’s my problem. A few weeks ago my car started overheating and blowing cold air while I was on the highway. I took it to my mechanic and he found that the radiator was leaking. Two weeks later the car was overheating and the heater was blowing cold air when I was on the highway. I had it towed to the same mechanic and the car wouldn’t overheat nor would it blow cold air. He drove it all around and on the highway but couldn’t get the problems I was having to occur.

Now I have intermittent heat. Sometimes I will start the car and it will take 20+ minutes for the heat to blow warm air. Other times the heat works fine for a while and then starts blowing cold air. Usually when it starts blowing cold air I begin to smell a sweet odor, possibly coolant. It hasn’t overheated again, but I have stayed off the highway.

I just had the thermostat changed, and the upper radiator hose warms up quickly. When the heater is blowing cold air, one or both of the hoses connecting to the heater core do not warm up. Sometimes it’s both hoses, other times the hose going in is hot but the other hose is cool.

My mechanic seems to think that the head gasket is leaking air into the system, and that the air is preventing the coolant/water from being properly transported throughout the car. He did a pressure test to see if it was the head gasket when I brought the car in but it wasn’t conclusive. Does it sound like a head gasket leak, or could it be a blocked heater core, or a weak water pump? So far I haven’t noticed any coolant leaks, but I could be missing something. Thoughts?

It Does Sound Like A Head Gasket Failure. The Gasket Was Probably Compromised When The Radiator Leaked Enough Coolant To Cause An Overheat Condition. I Think Your Mechanic Has It Figured Out.
His Description Is A Good One.

When the head gasket is breached, coolant leaks internally into comustion chambers and is turned to steam and goes out the exhaust pipe. Air is introduced into the normally “closed loop” cooling system and proper coolant circulation ceases.

What model-year and how many miles on this Nissan Sentra ?


You know what? Go somewhere else. Get the thermostat changed first by this new place. Run the car around and see what happens.
This is not rocket science. A mechanic should be able to diagnose what your exact problem is and fix it. If yours can’t, find someone else. Today.
I think you have a bad thermostat. I have never looked at your car but I would start with that because it the easiest and most likely solution and would cost you next to nothing to repair.

I have 2002 with ~ 110,000 miles on it. I haven’t noticed any discolored exhaust coming out, so can there be air getting in to the closed loop without coolant leaking out? Thanks.

Thanks. I just had the thermostat changed and I took the old on home and tested it in a pot of water on the stove. The old one opened just fine at the right temp (196 F I believe), so I know that the old one worked, and I was still having these problems. I agree with you though and will take it to another mechanic for a hopefully more conclusive diagnosis.