Since the first outing this spring, my `98 7.4 MPI has been running like crap. It runs EXTREMELY rich at idle and kinda clears up once past 2500 rpm. I have run about 100 gallons of fresh fuel mixed with injector cleaner through it with no improvement. I feel like I have a ECM problem. 3 plugs were very wet, the others were black but dry. It runs OK once on plane, but sounds like it`s hitting on only 5 cylinders at idle. The problem isn`t as bad when the engine is cold. Fuel pressure reads 30 at idle, 38 WOT. I have a K&N and aftermarket exh manifolds. otherwise stock with 225 hrs.
I have new plugs, cap & rotor, new fuel filter...nothing has helped. I need it going for Memorial weekend. Please offer any suggestions. My Mercruiser mechanic is 2 weeks behind. I need my own scanner......
THANKS!

Sorry I wish I could help. We had the same problem, and took that crap off and put a nice carb on.

Sounds like ignition problem, 3 wet plugs. Check the plug wires and cap on those plugs. New wires can be bad, get multi meter and check each one.
Good Luck.

You really need to scan for codes to get more information. You already have done the basics. Make sure that the vacuum line is hooked up to the regulator. I have seen them come off before. If you have the ability to swap Map sensors with a known good one, that may help.

ok here is the poor mans scan tool, first find the timing/diagnostic lead that breaks out of the harness, pull the protective cap off you will be looking at the female pins inside the connector now, take a paper clip and bend into a U install the paper clip into the end two wires that are black/with white stripe and the white/ with black stripe, this will put the ecm into field service mode, then take a 12volt test light connect the alligator clip of the test light to the postive terminal of the battery then take the pointed end and push into the diagnostic connectors brown wire with a white stripe turn the key on and it will flash out codes through your test light, it will start off flashing a code 12, that will be one long flash then two short flashes, this is telling you diagnostics are working, if no faults are stored it will flash code 12 three times, any codes stored will come after code 12, such as 4 long flashes then 3 short flashes will be a code 43, whick is a knock sensor code, codes go from 12 to 51, good luck and let us know what codes you have

That is some valuable information. I will check for trouble codes tomorrow. Is there someplace on the web or elsewhere that has all the codes listed? I will let you know what I get.
THANKS!

I agree with Mate 229, it sounds like ignition. If you have a timing light, connect it to each plugwire and check for proper spark on all the plugs. If there are misses, then you know where to dig further.

im thinking of doing the samething what could you keep ? do you have a parts list? and what gains if any

I was going to try and read the trouble codes by using the method described above, but I did not see a white wire w/black stripe. I have a 10 pin connector with 2 rows of 5. Here is how mine is laid out:
TOP ROW, left to right-----
BROWN/WHITE , BLANK , BLANK , BLACK/WHITE , BLACK

BOTTOM ROW------
ORANGE , ORANGE , YELLOW , BLACK , BLANK

So, do I connect the black/white wire to the black wire on the top row and probe the brown/white wire?

I am running out of time here....Wish me luck.

Do you have a VST tank on the motor or a cool fuel system, if its a VST tank that is probably what is wrong with the boat and needs to be rebuilt.