OK... Here are the facts:
1989 Blazer S10 4.3 4x4 (white w/ blue trim... just in case that's important)
New plugs & filters in February
New theromstat in June
Checked my vacume hoses, and as far as I can see no leaks.

It seems OK when I first start it up, but as it get's warmer, the power goes.It feels like it's laboring, or towing. When I'm running the AC, and I put any kind of load on the engine(like going up a hill, or trying to pass)the air switches from the panel vents to the defroster vents, and I either have to take it out of over drive, or keep my foot in it. I'm getting about 30-50 miles per tank less than I was. Pep boys sez that they can't find any problems (no codes and such) but problems there be!

Oh.. I took it to Pep Boys to get the 02 sensor replaced, and they were unable to get the old one off! Any thoughts?

Please ask questions.

 

First off dont go to pep boys. Secondly it sounds like it might bejust dumping fuel in, if you only get 30-50 on a tank which maybe related to the O2 sensor. Maybe the engine is getting flooded and causing the loss in power not sureabout that though.

 

Sorry about Pep Boys. I wanted to go to another shop, but they were closed. Driving to work this morning my "Service Engine Soon" light came on as well. I'll have to take it to a "real" mechanic to get it fixed. Can anyone reccomend one in Santa Clarita, CA?

 

I recommend you.
Find autozone and get the code read.

Acts up when warm, could be a couple things, although it gets worse under load, and your vents change. Vacuum leak. Has the vacuum hose ever been replaced? Do your hands turn black when you touch the hose, if so, replace it, all of it. Look for my article in the self help section on 4x4 no worky. I go into some detail about the vacuum hoses and where they go.

 

Turns out it's a bad timing chain. The new shop (not Pep Boys) has to order a new one from Chevy, as apparently they have upgraded the design. They will also do the O2 sensor that Pep Boys said they could not do. They also said that the gasket between the Throttle body and manifold is bad. They said that would cost $220 to replace, I said I'd do that myself. Kragen had it for $4, though I might splurge and get the $9 one.

 

Let me guess the new timing chain has a tenisoner, that gm failed to put in the 4.3s until like 98. Supposedly the slack chain will not shorten engine life. Yah sure GM it will only wear out the chain twice as quick. I would go for the better gasket at 9 for you manifold.