99 Corolla Burning Oil

Hi, looked on here and I've seen a few people with similar problems, so I thought I'd ask and see if a solution has been found.

I have a 99 Toyota Corolla with about 114K miles on it. I had a vaccum switch replaced at about 50,000 and since then, it's been burning oil at a more rapid pace. The Toyota garage checked and said everything is fine with the vaccum switch and seal. They said that the oil consumption was probably due to a stuck ring or buildup. We changed the oil, cleaned the engine, added some oil additive, and $300 later, it's still burning oil.

There's no leak in the engine. The car isn't burning oil on startup, but when the engine downshifts going up a hill or passing someone on the interstate, I can see a puff of smoke come from the tailpipe. My tailpipe is black, as is my formerly white bumper. The plugs are all good, and no buildup other than what you'd expect to see on rather new plugs. I know that a quart of oil every 1500 miles is cheaper than major mechanical work, but does anyone have any thoughts or solutions?

Any help is appreciated.

I have a 2001 corolla that uses oil with only 75000 miles. One suggestion was the valve guide seal. I only have to add a pint or two between regular service to keep it topped up.

I'm probably going to trade this car in on a new tacoma. but I'll post any changes made prior to that.

- JP