On my 2000 chevy blazer ,what is the part on the passenger side front of vehicle and directly under the radiator?There appears to be a broken vac line on it.Where does that line connect to?

 

Have you tried tracing it to an end that is still connected to something?

 

If it's under your battery and looks kinda like a flying saucer, it's the vacuum actuator for your 4wd. It locks in your front axle.

 

On my 2001 there is the A.I.R. pump and solenoid/valve down low right next to the idler arm, between it and the oil filter adapter. On mine there is one vacuum line to the electrical solenoid, then a line from the electrical solenoid to the vacuum operated valve. I just replaced mine a couple of months ago and the AC Delco replacement kit relocates the A.I.R. pump inlet and the electrical solenoid to up high next to the top of the radiator in order to keep moisture out (and subsequently locking up the pump). There was a tech bulletin on it. Not sure if 2000 has the same A.I.R. system though...... Do you have the check valves at the front of each exhaust manifold? If so, I bet it's the A.I.R. system in some form. My pump/solenoid/valves were 2001 only.

OK I looked it up and yours should be similar to mine.

Here's the diverter valve

https://www.rockauto.com/info/154/DV...mary__ra_p.jpg

 

The 2000 does have it. Mine kept throwing a CEL and blowing the fuse. I pulled it off an it was full of water. I got a replacement from a junkyard and haven't had any issues since.