288sunny

here we go; i have a 03 25OL with a ho in it, its got a 190 hrs on it and use to run like a champ, first things first, i changed out the thermostate and computer because engine wasnt getting over 140 and the reason for the computer was that i was tired of having all that black soot on back of boat (bad mistake).
anyways since then motor now runs around 160-180 which is good, no problems there, since then the boat has had serious problems getting over 4000 rpms, which i use to be able to hit around 5100. some times it 4000 or sometimes alittle more, no more than 4500 though.. so called merc they said, oh its the fuel water sep, change it and should go back to normal, so i did that, along with the little grey fuel filter and then we also put fuel pressure gauge on it, around 40 before starting engine, the while idling it below 40 like 36-38, if i rev the motor is drops below that, according to merc it should be 43 at idle and then when i give it gas it should go up..
Sorry so long, i really need some ideas, anyways i took it out today, and same thing, so my buddy sais man it really sounds like you have a miss in the motor, check your plug wires, so i went through checked all plug wires, all apeared to be on, so then i hit it and holly geeze, it was back to normal, so it rode fine for about 20 minutes, got into a no wake zone, made it through that and then started off again, and it was back, something is not good past 4000 rpms, really sounds like its missing either some fuel or spark, any ideas? does anyone know what the fuel pressure should be? has anyone had to change there mag/HO plug wires again its 03, i wouldnt think they would be bad already but i guess anything is possible, and from what i hear they aint cheap! please help, ideas opinions, thoughts?

Dave_N

I have seen a misfire on a 496HO due to secondary voltage leakage from a under the coil tower boot to the coil frame. The boat was about three years old.

If you had a temporary recovery of your lost RPM's after messing with the plugwires, your problem could be there. Maybe instead of replacing them (yes, they are big $), first try pulling each one and putting some dialectric grease on the ends. That might seal up your voltage leak, if thats what is happening.

Dave

Pwrbt33

iTrader: (2)

It sounds crazy but check the ground wires on the back of the block. We have had a few in the past have loose grounds and that caused a problem. Also double check when you plugged the PCM in that you didnt accidently bend one of the connector pins over. Unplug it and check all the connector pins again. Make sure you plugged both sending units back when you did the t-stat.

Let us know what you find out.

John S.

fountain27ho496

Did u check the screen to the booster fuel pump on lower right side of motor. Mine was clogged with bugs and metal. Had a high rpm miss that no-one could find. Good luck JC

Raylar

Sounds like you must be going in and out of the Guardian mode. Get the boat to someone who has a scan tool and look for trouble codes and download and write down the history on trouble codes. This will probably help find the problem and will save you making any "WAGMARS" Wild Ass Guesses Made At Random. Good luck!
Ray @ Raylar

Dixie Doug

If this is the case(I think it is) the scan tool won't show a fault.

GOODT

Merc had a big problem with the boost pumps clogging and came out with a buliton several years ago its the pump mounted by your water sep. check fuel press at fuel rail.and check intake screen as stated above if found to be bad check the press pump located in the cool fuel assy on the lower port side of the mtr

288sunny

I really want to thank everyone for there reply, im gonna start with plugs and possibly wires, but i dont think thats gonna do it, i really think its fuel related, at idle fuel pressure is 36-38 psi, just before start up, pump prime it goes to about 40 or just a hair above!, seems like low fuel pressure?

GOODT

static with reg disconected should be 43 if I remember correctly

mr mercruiser

I think goodt is right-however fuel pressure shouldnt be dropping down upon accelleration-I have seen several lift pump inlet screens clogged with whatever manufactures assenble their boat tanks with like silicone(i know another member already said this one)-how old is the fuel-fuel is a major problem now containing additives-also-if fuel is any older than a month old -plan on the octane falling out of it..-since fuel contains ethenol-i have seen it eat the inside of rubber fuel lines out-that to can clog the boost pump screen-did you try to vacuum test the inlet side of the fuel line-