I currently don't have any of the heater stuff on my car, although I still have the stock HE's.

Should I be capping off the outlets on the engine cooling fan housing? If so, what specifically can I use for caps?

I did a search, but couldn't find much. I would think this would be a pretty common question.

It’s a good idea to cover the holes in the engine tin and use rattle can paint caps to cover the holes that lead into the longs. It’s not nessesary to cap the electric fan.

The smell of dead mice will last for weeks. Ask me how i know ...

It's the two outlets on the fan shroud that I was concerned about the most, as it seems like most of the air would just go out there instead of over the engine.

Click to view attachment

As air is always going to flow along the easiest route, I think capping those outlets would be wisest. Then the air is all forced over the engine (which is a good thing).

When I got mine it was a heat delete conversion and they'd made some aluminum caps held on with SS hose clamps to cover those 2 outlets.
I've now converted back to full heat OEM config.

That would seem logical but consider that the fan housing outlets were designed to have air flow at all times to have air flow through the heat exchangers.
I think Jake Raby found the air flow is better if the fan housing outlets are left open. Capping them off can disrupt the air flow in the shroud by having two dead ended outlets.
I would run an oil temp gauge and check the difference.

Bruce

Please report your findings running them open or capped. It would be good to know, but I’d trust Jake Raby’s advice. If the car isn’t overheating with them open, it really shouldn’t be an issue.

I've never had any of the heater tubes/fan/valves in my car, so I'm not really clear on the OEM configuration. If you aren't actively blowing heat into your car, aren't the outlets just dead-ended further downstream anyway? At those flapper valves or whatever they are called?

Unfortunately, I don't have enough historical data to provide good findings. I've basically driven my once for an extended period, and the oil temp just kept climbing. BUT... I also have no engine tin seals, I've got uncapped holes everywhere in my engine tin, my thermostat isn't hooked up. So I want to drop my engine again soon, and try to get all those issues resolved at once.

Sounds like a good idea. I will give that a go.

If there is nothing connected then Cap them off. In a functioning system and the heat for the cabin is off, the air will go through the heat exchangers and exit through a cap on the flapper valve