Thought I would post an update to leave a trail for anyone else with similar problems.

To review, I have a 92 940 which wasn't heating well, especially idling in stop and go traffic. I was concerned about it during snowy, icy winter weather because I don't think it was heating enough to melt off the accumulation. It would heat some but it just seemed like it was undersized. I'd been fighting this for at least a couple of years- never had been good since I owned it.

I went through everything in the system and came to the conclusion that all parts were functioning correctly. Engine temp was good, it heated some, and the AC functioned well which implied that all the flaps and recirculation system was good. I had the system flushed to make sure there wasn't some kind of blockage.

The only thing left was the Nissens heater core I had replaced shortly after buying the car. I remember it fitting fine but looking different than the original- more plastic, aluminum instead of brass, etc. I decided that I needed to try changing it out or get rid of the car. I took the old leaking Volvo core to a local radiator shop to see about repairing. They said it wasn't repairable but they sold me a World brand core that looked like the factory core and I made plans to change it.

In the meantime, I took the car on a 4 hour road trip. Going through traffic in a mid-sized town, I heard a pop and steam started pouring out of the vents. Heater core had failed catastrophically. Rapidly couldn't see anything- we rolled the windows down and looked out the side until we could find a place to pull over while the majority of the antifreeze was pumped into the floor pans. Fortunately, I was able to re-route the hoses to the core and we grabbed some antifreeze at the convenience store and were soon back on our way.

I spent 7.5 hours last weekend changing it out and it heats now! We've had some freezing mornings and I sweated on my 15 minute commute to work yesterday even without recirculating.

Looking at the Nissens core, the failure was a split in the plastic end caps that was big enough to drain the system quickly. This core had a very different design than the Volvo or World cores. In those cores, pretty much every fin is a flat passage that the coolant runs through so the surface area should be much higher compared to the volume. The Nissens had 9 pairs of approximately quarter inch tubes with a bunch of fins attached to them. I can see why this might have trouble transferring the heat. The core was definitely the problem- I just can't be certain whether it was the design or if I got one that had some internal restriction that wasn't apparent.

Given the time involved in changing it, in the future I will stick to a factory or "factory-like" heater core.