2002 Chevy Tracker, Suzuki I4 2.0L engine. 2WD, Automatic, 217,000 miles.
I've had this vehicle about 3 1/2 years, and it sometimes dropped to 500-600 RPMs at stop lights and was consequently a little rough. But that was very intermittent -- would do it three or four times out of ten or twelve stops one day and then not again for a month.
I replaced the valve cover gasket Friday and installed a new PCV valve.
She ran fine Saturday, but had the low idle at some stoplights (not all). Sunday she ran fine, but I only drove a dozen miles or so.
Today, she idles at a solid 1,000 RPM when cold. When warm, she drops to 600-700 RPM and is a little rough, but about one time out of four, it'll drop to 300-450 RPM and is obviously very, very rough, like scary shuddering rough.
I put a vacuum gauge on her. It's 21-23 mm Hg when cold.
What did I screw up?
How do I make it right?
Possibly (but hopefully not) related: when I had the cover off, I pulled the spark plugs. 1, 2, and 4 had only beautiful, thin tan deposits and no noticeable wear after 27,000 miles (I changed them the day I bought her).
I didn't pull #3 because as I started to back it out, it got harder and harder until, maybe halfway out, I would have had to use force. I ran it back in -- it seated fine, but again, it got tighter as I tried to remove it. Rather than risking ruining something, I just left it (it's currently my only vehicle, so she has to stay operational).
The only thing I can think of is it's crossthreaded, but I don't see how that's possible. When installing plugs, I run them in by hand, putting on the ratchet only for final seating. If I had somehow screwed up, I'd remember (no matter how bad my memory might be, I always remember every mistake).
Anyone know why it's hard to get that one out?