The engine turns over very good, spins fast and doesn't do anything unless you advance the throttle to Wide open possition and then turn it over. While in this possition the engine will backfire and pop through the throttle body, it even flamed up once and I closed the throttle and it naturally went out. This is the only way I can get this engine to start, let it do it's backfiring and then once it starts to stumble and sputter, it will finally clear out and run fine. Once the engine has been run and is warm it will usually re start without a problem. Once running it's like there is nothing wrong, no miss firing, nothing! I've been at this a long time, this one has me stumped. I ordered a new fuel pressure regulator for it, should be in today, going to try that next.

Could it be a bad fuel injector leaking? This floods the engine after sitting some time. Hard to start unless you get a lot of air (full throttle), clears up and runs fine until next time...?

What is the fuel psi on the rail while cranking ? It has a test port. hf...

I will know tomorrow when I test it if the rain will ever stop here in Gulf Shores Alabama, this has been the worst July I have ever seen. I also got my fuel pressure regulator in today and I am going to install it also.
Like I've said, once has been run for a while, it usually starts fine, not perfectly but a hell of a lot better than the cold first start of the day. I appreciate everyone's input, you learn something new everyday!!

Put the fuel pressure gauge on Schrader valve and turn the key on to see what your pressure goes to, then see what it does after the pump shuts off. It should drop a little but hold pressure for a long time. If it drops right away (within 5-10 minute) the pressure is bleeding off somewhere and is either the FPR or an injector. It is flooding and that's why it will start when you hold it WOT.

Baja, what year is the 6.2? Does it have a cool fuel system? If it does, the tiny stainless filter screen under the fuel pressure regulator can clog up , not allowing excess pressure back to the water separator, causing fuel pressure to spike over 70+ psi. It will allow the injectors dumping too much fuel, causing similar problems you've described. Since you're at it, descard this little gem [ 1/4 " in diameter ] to avoid future pressure issues. Merc made this redundant system for warr reasons. There is a filter before the priming [ low pressure pump ] pump, and then the water filter/separator, an 1/2" stainless filter inside the high pressure fuel pump inlet, and finally the little one in the return system under the fuel pressure regulator. Only honest merc mechanics would renommend to descard this tiny filter, cause more money to be made servicing this item!
Fuel pressure should be between 38-42 on the Schrader valve @ idle.

Thanks very much guy's, I will let you know after today what we found. Thanks so much for the input and information!
yes it has cool fuel, and I am not positive on the exact year at the moment, but I think it is a 2005.

Alright this is getting old now, my fuel pressure guage kit will not screw to the test port on the fuel rain, doesn't seem to adapt, but looks exactly the same thread pattern and all. I don't know will try and do something to make that work now, dammit, replaced the fuel pressure regulator on the rail near the throttle body, has a snap ring type keeper. Hooked it up to the vac line same ****, turns over and over, will not start.
tries but never will unless you open the throttle wide open and fight with it. Backfires through the throttle body when this is done, and flames up sometimes, don't like doing this at all.
I found out this engine has another pressure regulator on the darn cool fuel system, think this could be causing problems?
Still lost and ready to say f it.

That is the pressure regulator with the small s/s screen under it for the 6.2 MX with the cool fuel system, controlling fuel pressure!

There is also one on the fuel rail that I replaced and it snaps into place there. it also has a small screen on the front of it. The one down on the cool fuel set up is a solid model that is fastened to the cooling tube. I can't believe this thing has two fuel pressure regulators!
Anyhow, got pressure gauge hooked up today, 45 lbs when key is switched to on position and when spinning it over it still reads around 45 lbs. This engine is getting plenty of fuel and the screen on the old regulator was spotless, so was the fuel water separator filter. I believe that if this engine has had some injector problems leaking fuel into the pistons then these spark plugs are probably fouled badly. Tomorrow I plan to pull some out and check them.