Wondering if anyone has any thoughts on to what might be wrong with my 4Runner. Its a 1998 4Runner Limited so it has the v6 3.4l engine. A few months ago it started to get very sluggish especially at higher speeds and gas mileage dropped significantly plus seemed much louder. I found that going up hills I couldn't get above 50 mph. I got a much needed tune up and replaced the Upstream O2 sensor. No on flat ground I can speed up fairly quickly and get up to speed no problem. But going up steep hills when I hit 50 its like it hits a wall, if I put more gas the engine revs up to 5-6k but there is no extra power. After the Tune up and O2 sensor the 4runner is much quieter and runs so much better at idle and during city driving. Any thoughts on to what is the issue with going up hills and just not having power?