Monday of last week, I noticed the heater wasn't heating so great, but coolant level was good. Wednesday, on the way back from work, the temps went up and hovered around the H line. I figured it was finally cold enough that perhaps the heater core had been harboring an air bubble and let it out. Temp dropped back down to normal once I pulled the car over. I supposed the air bubble got pushed around and I continued on my way. Temps varied from normal to almost at the H line the whole drive home. I checked the coolant level and it was fine.

I did an air bleed that night and temps were stable and normal. Thursday about 5 minutes of driving, temps rapidly climbed to the H. I turned around and they hovered around the H the whole drive back. Top radiator hose was at around 215 degrees, bottom hose was around 50 degrees, with ambient temperature around 40 degrees.

I changed the thermostat. I did test the old and new ones in boiling water while they were out. Both opened.

I picked up the coolant funnel thing and bled the air again after putting in the new thermostat. Temps were stable and there was even hot air from the vents now.

I drove it to work Tuesday, acted completely normal, but the ride home, just like last week, temps started climbing to the H and hovered around there. Lower radiator hose cold, top one hot, just as before.

I figured, ok, clogged radiator? Pulled that this evening, it's clean, flushed with no rust or anything coming out. Put it back in and filled up, jacked the car up to burp out the bubbles. Now, normally, when the thermostat opens, it should suck more coolant down from the funnel, right? Well, the level in the funnel increases once the engine reaches normal temperature.

Warming up

Normal operating temp



The exhaust is steamy too.


At this point, I have to assume it's the head gasket, and I don't know if it's going to be worth the effort to repair.

 

Didn’t know you still had one!

I would definitely test your cooling system with a pressure tester too. See if it gets up to 13 psi (or so) and holds for an extended period of time. My last coupe had a pinhole leak in the bottom rad hose I couldn’t see until I got the car up on a lift.

could also do the combustion gas leak tester in the coolant. Do you see bubbles coming up when you do the leak free funnel on the radiator cap?

 

I haven't done combustion gas leak testing, and there's no stream of bubbles through the funnel, just the occasional ones when I give the upper hose a squeeze.

I might grab a pressure tester, but it's all the symptoms. The picture of the steamy sweet exhaust doesn't do it justice. I should have done a laser light show through it. Because of this, I feel there's not a big need to do the things that find small leaks.