Like the title says, I replaced the #3 injector, when I fired the engine it races to 3k rpm, and has a nasty whine, like something is rubbing or maybe a stuck starter.

History on the truck; it's a 93 bronCo 5.8l. 253k miles. I picked it up a few days back, had a dead cylinder, but ran and drove fine otherwise. The PO had replaced tps, iac, egr, and fuel pressure reg, trying to fix the issue. I diagnosed a bad injector after a compression test and pulling the plugs to find which cyl, was bad.

I pulled the upper plenum, disconnected the tps and iac, but left the throttle body on, unhooked the vacuum lines on top of the plenum, and the one 4 way vacuum block in back to access the rear intake screws.

Put it all back together with the new injector, fired it up and the throttle immediately goes to 3k, throttle body fully closed and not binding, everything is plugged in, no vacuum lines missed. I did put a new plenum gasket in.

I'd do a vacuum leak test but at 3k it makes me nervous to let it run. Could the gasket be leaking, would that cause such a high idle? 12 hours ago it ran like a top minus the #3 cyl. I'm stumped.

 

Also I primed the fuel pump with the plenum off to check for leaks, so maybe the computer was confused? I unplugged the battery and will let it sit over night

 

More likely you broke a vacuum line/lines. Those vinyl tubes break if you look at them too long let alone move them around.

 

Found the vacuum leak.

 

Ouch...