I have a 1999 chevy blazer with 124000miles that overheated on the highway,. Now the engine runs rough at an idle but seems ok on the highway. Several different mechanics said different things from a cracked cylinder head to a cracked head gasket or bad injectors.The scanner shows cyl 1,3 4 misfire> . how can i find out for sure before spending big bucks for something that may not work?

How long since a tune-up? Any applicable trouble codes?
The basics for a rough idle, check for a leak of some kind, fuel--egr--vaccum.
Check/clean idle air system and anything that has to do with mass air flow like, maf--map--tps, something of that nature.
Does it run ok going down the highway?

I Have a couple of questions. First question does engine surge when idleing cold? 2. Does engine start quickly without a lot of cranking, Cold or warm? 3. Do you have funny smell at highway speed, or after you stop? With out this info and your imput, I would only be guessing. I am retired mechanic of 25 years. and would like to help if u need me.

That is a little bit broad, but, to help you narrow it down, I would first check the plugs and wires, and also the fuel pump/filter.

To answer your first question; Yes. I just had the exact same issue with my Ford Explorer. I would be running down the highway like nothing is wrong, and then suddenly it was like someone shut off the engine on me. Except my problem was that my fuel pump was overheating from being over worked. The strainer on the bottom of the fuel pump was getting plugged up.
To fix this issue, you will need to replace your fuel pump.

it can very greatly depending on the engine, miles, tempture.... but a good rual of thumb is any thing over 15 psi at hot idle is ok and any thing over 30 but under 70 at cruse is ok

Converter is ruined, as are the O2 Sensors

Now you have to fine out why

Rich Condition or Overheating,
Engine Misfires

Have fun !!

in order for the engine to pop back in the intake it has to be firing when an intake valve is open.The best place to start would probably be the distributor cap replacement.the toughest part of this job is the smal torx bit screws on the cap and not breaking the distributor. the screws were threadlocked ,and the distributor is made of plastic.be careful not to over tighten.

changed harmonic balancer reset time

change the fual filter and if that doesnt help try the fuel pressure regulator

Barring any mechanical issues you most likely have a failed injector.