Me and my wife bought a 1990 4runner 5speed v6 280,000 miles (body is in great shape one little dent and missing a spare tire and gas door broken off) little over a year ago for me(hunter and fishermen) and the family. I really have grown to like this rig alot. When we first got it. It needed brake's no prolem jumped on that got it done. Then 3months later the slave cycilender for clutch when out got that done. Last three months it been having some issues rough idle and a loss of power. So did some checking on this site replaced maf sensor, o2 sensor, muffler only cause it needed one, plugs, wires, cap and roter. Now it's backfiring on deaccelration. And when going up hills shift into 4th gear it bogs down and will not gain speed only loose it and have to shift in too 3rd gear. I know she's got alot of miles on it but if i dont get it running better the wife is going to make me sell her. I would like to work on it and get it running better. So any advise would be helpful or if anyone lives close to me would like to help a rookie work on his yota send me a pm. I know if I am making a effort to fix her she(wife) will not ask me to sell it. Thanks alot. Really like this site alot of useful info. Allen

I'm not too familiar with the V6 engines but your symptoms sound alot like the TPS (Throttle Position Sesnor) is bad or out of adjustment.

Edit: Have you checked for codes?

I would look at your heads. Do a compression test and I bet you find you have a burned exhaust valve. Mine had one burned on each head/side. I had both heads reconditioned at the local machine shop.

if the motor is going out on you it would be a good idea to do a motor swap from your 3.0 to the 3.4 out of another runner or tocoma. there will be a couple mods to it but not much and it would cost just about the same to put a newer 3.0 in. with the 3.4 youll have alot more power and a lil better gas miles.

Backfiring on deceleration is a good indication of a TPS (Throttle position sensor) problem. There was just a great post on adjusting it, and the dashpot that slows the throttle closing to prevent backfiring. TPS issues also cause shifting issues in automatic 4Runners. The post is all about this problem.

Also see the article after (also linked from the post):

End of this post: 22RE Throttle Position Question

Toyota 22RE Throttle Position Sensor