My boyfriend has a 91 4Runner with a totally junk tailgate. The window only goes up, you have to push it down, the seals are shot and there's rust everywhere. When we drive it with the front windows even CRACKED, the whole truck stinks like exhaust. Boyfriend claims it's not exhaust. I won't ride in it anymore, even though I love it, because it gives me headaches. He really thinks nothing's wrong with it.

I'm not asking you to tell me that I'm right and he's wrong, but what the heck could be causing this! It only happens when the windows are down at all. We don't drive with the tailgate window down.

Typically, with all generations of 4runner or any truck that the back window goes down and other windows open.....exhuast DEF comes in. It probably is exhaust, even though your rear window isnt down, its seeping in through the rust and busted seals. time to fix that rear tailgate. There is a suction effect with the front windows open that pulls the exh through, right around your head and out the front window.
Ive heard of people finding tailgates (recently on here) that were in good shape and not expensive. Good luck, I hope you can ride with him again soon

on a side note: my tailgate and seals are in perfect shape, and with the rear window down i still get a little exhaust in. was the same on my '05 but a little less fyi

The manual does state, in several places, NOT to drive with the rear window or tailgate down due to exhaust coming in. I did it once (before I read the manual) while moving some big stuff and about choked. But yeah, even the slightest opening in any window in front is bad news.