The heat on my 87 4Runner seems to take an inordinately long time to warm up. I'll notice warm air eventually coming out of the vents, it will cool, and then warm back up. It'll do this off and on for about 10-15 minutes and then stay warm. Is this thermostat problem or something bigger?
I'm interested in this too. My F-150 5.0L is doing the same thing.
Sounds more like air in the system/low coolant.
If that ^ doesn't fix it, throw a new T-stat in. They're only like five bucks.
What is the ambient temperature when this is happening?
I have a car that does that when the tempertaure falls to the single digits above zero. Once it gets into the upper teens and twenties the symptoms disappear. Coolant seemed low so I topped it up, but the symptoms didnt go away.
It might be low coolant. I am planning on changing the thermostat soon.
This is what I was worried about. I ran it low on coolant recently. Somewhere there's a leak, small enough to go undetected. The temps do not matter from single digits to 40s. It did not do this last winter. I've tried raising the front of the truck on a steep incline, but the level never seems to change. There is no air purge screw like I've seen on Chevys. Is there a trick to burping the air inherent to a 22RE?
Is the temperature fluctuation based on engine speed? If it cools at idle, and warms up at 2000 rpm, the heater core is probably restricted.
I think Gustav might be a canoe.
No, engine speed seems to have no affect on temps. The temp gauge will come up, indicating a warn engine, but no heat on the other end.
Gustav deleted. 
My brother had this happen recently with his Honda Oddity - it was the heater core. $1400 later he is good.
Check under carpet for missing coolant.
My truck 1994 22re does the same thing, it takes longer than any other car to warm up but still runs well. Don't expect any heat out of the vents during warmup. It wouldn't surprise me if you could have an air block in the heater system preventing it from getting hot coolant. I don't know much about 4 runners though, good luck with it.
Update: Blown heater core hose. Popped it on the way to work. Looped it into itself to keep it from loosing more coolant. Promptly overheated it coming into work. Fixed it after work with too big of hose (3/4in) in -1 degree F. Promptly over heated again. Fixed with correct 5/8th hose at 11:15pm. Filled all the heater hoses and radiator with glycol. Works great (I hope.)
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