I’ve got a 1992 240sx convertible... automatic. Runs great. Several days ago I noticed that my AC suddenly didn’t feel quite as cold. Shortly after I noticed the dreaded steam blowing up from the front passenger side of the engine compartment where the coolant overflow reservoir is. The thermostat gauge on the dash was at the normal operating mark. I pulled over to check it out and noticed the coolant appeared to be boiling/bubbling in the overflow tank. It didn’t take long before it settled down.

I decided to replace the radiator cap first. I drove the car a few days without any issue after doing this. Then I noticed the AC struggle to get cold. Again the steam began blowing up. Temp gauge on dash was still fine. So I flipped to full heat blasting and drove home with the windows down. The steam subsided before I got home. I could see coolant had bubbled out of the overflow stem.

I checked coolant and it was low about 1 quart... I assume from bubbling out. After topping off I drove it again. Same problem. It seems if I run the heat full blast the issue isn’t as noticeable.

I was nervous about a possible blown head gasket because the coolant looked murky. I did a coolant system pressure test. It held strong. Also, my oil in the engine looked great. So I don’t see that as the culprit (thank goodness... unless I’m missing something here).

I drained the coolant and added new coolant. Burped the system. Took it for a drive and sigh... coolant had backed up into the overflow tank again and was bubbling. AC won’t blow cold at all. Engine temp according to dash gauge is great.

I don’t see any leakage from the water pump which I guess could be a possible culprit. Anyone have thoughts on this fiasco?

Welcome aboard!

Radiator failure (typically an internal clog) or a bad thermostat can cause these issues. Less likely would be a bad cooling fan (clutch).

Replacement radiators aren't expensive for an S13, and your larger concern is cooking the ATF (remember, the radiator cools the ATF as well).

Thanks for the advice! We have now replaced the fan clutch, the radiator and the thermostat. Still boiling in the overflow tank. All that is left that we can think is the water pump. We will see where this leads us.

This may also be a small leak in the head gasket allowing exhaust gases to enter the water jacket. This can result in bubbles which would migrate to the overflow reservoir. Do a sniff test on your reservoir.

We are fairly certain this wasn’t head gasket related. It is definitely related to heat. The water pump looks all good. So, we decided to replace the upper and lower hoses. There was a questionably weak bend in the rubber of the lower hose. Possible that there was enough suction to collapse the tube due to an air pocket of some sort? Anyway... we put the car on a good incline and also jacked the front up. We began the burping process again (we were afraid to pull the bleed bolt because it just felt like it was gonna break). After about 30-40 minutes we felt confident we got all the air out. Drove it and it seems the issue is resolved.