Raising this one from the dead. I have an '00 454 Mag MPI, serial# 0M021074. Having issues with fuel fouling plugs on hot starts. We have chased this for a couple years now. Thinking about trying a couple heat ranges higher (45 instead of 43) & the extended tip (MR43TS style instead of MR43T) It looks like the extended plugs will work by what I have read. Not the long plug (LTS) just the extended TS. I like the tops of my pistons, so before I throw a set in, do I have the clearance for the TS plugs. Thanks!
J

The spark plugs are not the problem. The stock plugs are the correct heat range and tip. I doubt an extended tip will help your issue.
If the stock plugs are fouling, it's being caused by another problem.

JW - Griff is right.
What chasing have you done ?

'00 454 Mag MPI, Serial# 0M021074

This started the night I brought the boat home in 8/18, 5 tows later & numerous attempts by multiple "certified" mechanics including the dealer & we are still at square one. Frustrating is a vast understatement.

I purchased the boat in 8/18 from Lake Cumberland with approx. 395 hours. Always a marina boat on a lift, first owner put 125 hours on it in 16 years, second put 270 on in 2.5. Brought the boat back to DFW area.

Boat starts perfectly every time when cold, never an issue. Left us stranded on it's maiden voyage. Started right up, went out, stopped for a couple hours, went to leave & it just cranked, no catch. Get towed in, next morning, fires right up. We were below a 1/4 tank so I thought maybe contaminated fuel. It did live in a marina it's entire life. Pull the separator & sure enough, water. Drained the tank, Seafoam'd, new fuel pump, plugs, batteries & oil change. Took the boat out a couple weeks later, same issue. Start looking at spark, seems weak & coil is original so new cap, rotor, coil. Try again, same issue. In desperation & to eliminate basic ignition, I do the ignition module, typical GM style under the cap. Use the boat several times including a long weekend with dozens of starts, no issue! Not quite, late last season, same issue. This time, it's taken to the dealer. They spend a week with it, bow to stern, find no issues & cannot duplicate problem.. Pull codes, none. We decide I will wait until this season when it warms up & try to duplicate it.
I took it out last Monday, run it for over an hour, back to dock, kill it, let sit 20 minutes & presto, won't start. They bring down Mercruiser Computer, all looks good. Check spark, very solid. Fuel pressure is at 40-45 psi cranking. Pull plugs and they are damp, possibly leaking injector{s**. Take it back to shop right then, new plugs, fires right up. Sent the injectors out for cleaning & balancing. Took boat out yesterday, ran great right up until we were ready to leave out in the middle of the lake. Had been sitting a couple hours, crank, crank,crank, no love. Get a tow back, 45 minutes into the tow, it starts. I had been running the bilge blower the entire time going back, but I don't feel like this is vapor lock. I have tried starting fluid every time this has happened & it doesn't even bump, ether has zero effect while cranking. I have had lots of experience with old cars & boats, ether always does something. I have also added a booster fuel pump & rerouted the return fuel back to the tank. At the end of my rope. Thanks for any guidance,
J

PS: Attached is a log from a MEFI reader. Good start on the left, bad on the right. I have checked all the cables for voltage drop since it is only spinning at 150rpm or so on the hot no starts. Cables are all good 1/0 & fairly new.

Good infvo possibly.^^^

I had a carbed Mercruiser small block boat cone in years ago. Had all ignition components replaced alongvwith new batteries for crank no start when hot.

I take a look and fires right up cold. Drive it gor 1/2 hr...,,holy fk does it labor to crank. It starts I shut it off a little later again and it cranks so slowbitcwon’t fire.

Check for hydrolocking. That’s fine.
Check for over advanced dist setting, that’s fine.

Replace starter (late model pg 260 type) because cables and batteries fine and I have nothing else to think of.

Bang ! Quick turn of the key. Swear the motor jumped outvofvthe bilge and gave me a great big hug. Boat been totally fine ever since. Customer struggled with it for a few years I guess.

I have not run into this before and not since.

Starters usually stop working, not slow crank. Guess not.

Been working on vehicles over 30 yrs.

When you sit do you run the radio etc. Why is the ignition voltage. 11.1 and drops to 10.4. what is the battery like. Cables and ground straps clean. Have they been taken off and cleaned.

Are you running cool fuel ?

Every time it won't start there is strong spark but yet either has no effect. Just don't agree with one of those two, something is missing

Next time it won't start, have a small screw driver, rag and safety glasses on hand. Remove the cap over the schrader valve on the fuel rail. Turn ignition to ON and then press in on schrader valve and see if air comes out. Might need to do it more then once, but I'm starting to thing about vapor lock

https://www.mercruiserparts.com/bam/.../31812/4854/90

Cables, grounds & batteries all check out great.
i leaned towards vapor lock, too so I did the booster pump & Cool Fuel return bypass back to the tank. Plus, the plugs are getting fuel fouled when it tries to hot start. Thanks guys!
J

with the temps you boat in, throw that cool fuel POS in the trash and redo the fuel system,