I have a leak around timing chain cover.Is there a gasket i need to replace or something more serious? the leak is on a 1987 Mercury Grand Marquis ,5.0L.

Recheck valve timing and initial ignition timing for correct settings.

check to make sure pet is tight; tighten all hoses. Anything more and you will have to creep under and see where it is leaking. Hopefully not serious.

Are you sure-- they don't fail unless someone
ruins them with improper testing

Have you had every possible component test done,
to resolve whatever is the problem?

Valvoline synthetic would be very good , or even valvoline 10w40 non-synthetic .

There are many things that can cause your engine to overheat. It could be caused by a bad thermostat, a faulty fan clutch, a defective electric fan motor (if your vehicle is equipped with one), torn/broken hoses, or a bad water pump. It could also be something much more serious like a blown head gasket, intake manifold gasket, or a cracked cylinder head or engine block. It could also be something as simple and easy as an inaccurate temperature sending unit making you "think" the engine is overheating when it actually is not.

In any event, proper diagnostics need to be performed immediately. An overheating engine is nothing to mess around with unless you are very wealthy and can afford to replace an entire engine.

Please also review these articles:

What To Expect From An Online Car Expert

What Else Could Be Wrong?

Check the antifreeze for oil, pull your dipstick out, see if antifreeze is in oil or smells sweet or over full, sounds like you may of a bad head gasket. You can also check this by having a compression test done on the cylinders.

If oil was found on outside of plug oil may be leaking from valve cover gasket and it should be replaced

Could be or a cracked block or head cylinder

disconnect battery,,jack car up,,put jack stand on frame,,take starter bolts out,(may not be easy),,take wires off starter,,hoped it helped,,

I'm not sure what the labor time is but technically it doesnt take ALL that long. Besides removing a few parts to clear a way to the covers. I would assume somewhere around 3-5 hours is what you might get hit up for.