Mercury 6.2 MX hard to start
 

Tell us your symptom in detail. Is the engine turns over easy/fast only it doesn't start right away?
Checked the crank sensor [ near the harmonic balancer] wire plugged in properly? Checked the bolt tightness at the 90A fuse block on the starter?

That 90A fuse block gave me havic for about 3 months, mine started but wanted to hiccup, Spectras only lead me to the 90A fuse block, I took mine off cleaned it and tightened it and now no problems, there's also a similar fuse block on the trim motor and it was also loose on my boat.

"Occasional hiccup" , it could well be the crank sensor that you may find at a GM service department cheaper. Too bad you don't have twins to swap them out to see if it's the crank sensor or not. If you have a multimeter, I'd check continuity through the 90A fuseblock, even though it looks clean. Remove the battery cable at the batt post first. You could also, temporarily remove the fuse block and reconnect the cables for a test run. Unplug and re-plug all the connectors that goes to the ECM555 as well ,squirting some contact cleaner into the female connectors.There are some fuses near the ECM to check too.Computer driven engines [ in cars or boats] are voltage sensitive and sometimes a 0.5V drop can cause ignition problems.
I had three Cherokees and the 1990 and later models had this problem with the waterproof crankshaft sensor connector in the back of the engines.All it needed was to spray some contact cleanerin the connector and it was good for another year. Friend worked at a Chrysler dealership as a service manager, told me the horror story that they would replace the distributor charging $$$ to poor customers, finding out it didn't fix the problem. Took the service dep a while to figure it out after hundreds of customers were duped . Same scanario was with the V-6 Ford Probe a friend had. She had the distributor replaced and a charge of around $800,lol.
The small ign coil unit next to the distributor could be at fault as well.

cam sensor . . . this comes up every once in a while with the 08 and up trucks.

Something else you could change pretty cheaply would be the ignition and fuel pump relays I did that also, but in the end it was the fuse block on the engine, I had about a .5voltage drop across the block before I cleaned mine and tightened mine up, afterwards I had only .1 and problem solved, my problem was after it was running underway.
Might also clean and make sure the main round wiring harness is clean and secure on the back of the motor, I cleaned each and every female pin and some where a lil more dirty than I would have thought, also make sure theres a hose clamp around it and it's plugged in tight.

Side note, sure the wires are in the proper firing order??
I once got mine out of wack once while changing plugs I reversed a couple, felt like a winner after I realized what I had done

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...

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.

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!

45 lbs when key is switched to on and when pump shuts off it bleeds down faster than it should.

45 psi when the key is turned on and it is priming, then it shuts off and pressure holds, but bleeds down ever so slightly. When you are cranking the motor over, it holds right at 45 also.

I removed the spark plugs today, all were original, but some black with carbon. # 3 was totally black, I am figuring this would be where my bad injector is. Let me know your thoughts. Thanks again.

6.2 won't start
 

This may be a bit of a long shot but I have been to hell and back with my 6.2 with under 60 hours on it. Pull the distributor cap and have a look at the inside top for both corrosion and tiny hair line cracks.

These engines are becoming commonplace, so I suggest to you getting the basic Rinda scanner [ ca$ 4-500 ] at least. The 6.2's ECM555 monitoring 64 functions. If you hooked the engine up to the Rinda scanner, it would give you valuable information. Also, once you got the engine running ,check the injectors one by one with a Noid light [ available at every automotive store] to see if you have constant pulse flashing.

Bad grounds check for that also check the connections to the IAC and to the ECT for any corrosion / bad connections. Also U might need a new ECT = engine coolant sensor. Have this motor scanned.

Also have them check for intermittent opens or shorts to ground in the MAP sensor circuit. next have them check for proper operation of the TPS = Throttle position sensor.

Anytime a marine shop gets a boat in for repairs the first thing the tech does is get the motor serial #. Mercruiser had two totally different 6.2's engines made. One started in the late 90's and the other started in early 2000's. Again two totally different set ups - one with MEFI 3 (late 90's thru ) the other was with PCM 555 computer controled (early 2000's) Just one example of how they were.

Post ur serial #. Good luck with it.

Had similar issues with mine (a few other things though) and it was the cap/rotor.

Sorry to hear you didn't have any luck. Please post what it was if you find out.

This EFI is really making me miss carb's. Fuel, spark, compression 1,2,3. I just had work done on my heads and now a whole box of trouble has popped up. Mine starts and idles ok but when I give it gas it kind of sputters and doesn't want to throttle up but will eventually pull RPM on the hose but only 4,000 under load. I am reading more about the failing cap and rotors so I really hope that solves my problem.