Valve cover gasket replacement has lead to new issues
 

My son-in-law replaced a leaky valve cover gasket and now I'm having issues I didn't have before. If anyone can tell me what went wrong and how to fix it, I'd be eternally grateful!

First, my coolant seems to be leaking... There was a copper pipe that was unhooked and then rehooked and it is not leaking at that connection.

Second, when I first take off, whether forward or reverse, nothing happens immediately upon pressing the gas pedal... some seconds later the vehicle starts moving and then it accelerates so fast I'm afraid I'm going to hit something one of these times.

Third, the check engine light is on. It was not on before the repair.

And finally, I think, the àntilock light is coming on every now and then and I can't tell what's happening when it does.

These are all issues that happened immediately after the repair of that gasket, so I'm pretty convinced something didn't go back together properly. Any thoughts from the gurus?

It is nearly impossible to remotely diagnose the issue as it stands. There are too many things going on.
Lets start by - What year and model is your Montero?
Two, post a couple of photos of where you suspect the coolant leaks are coming from (or at least the spot where you can see it).
Three, What is the CEL code? That may help to narrow things down.
With the CEL light and the erratic behavior (sluggish and then surging) I suspect you have vacuum lines or sensors connected incorrectly. It is possible that you have a throttle body installed incorrectly too.

Thank you, HunterD, for the response! I will try to gather more information!

Coolant might be leaking from one of the heater hoses near the throttle body. I remember one of mine was so old that it cracked when I moved the intake manifold.

Check that the throttle cable was reinstalled correctly as it has to be removed in during the process. Maybe the nuts are loose?

Check engine light might be a vacuum hose that was left disconnected.

Not sure about the abs light. Check that all harnesses are plugged in securely. Is the brake fluid level ok?