Tidal Sails

My 2003 Donzi 18 Classic with MAG 350 MPI suddenly became hard to start, then faulty acceleration onto plane, before smooth ride at constant rpm. Once I try to accelerate, very unstable engine, but ok at 4000rpm, -once it gets there. Not possible to get higher rpm. Have tapped electronic info-with no luck. Mech state missing "foam piece" from intake ventury. More noisy than usual.
Coil? Ignition? Fuelpress? What can I do?
Mercruiser mechanics in Norway charge USD200+ an hour, and they are hard to find available during summer. Thank you. Are

Plane Silly

I would start by checking your distributor cap for moisture as well as you plug wires. Also check your fuel filter for water and debris.

Tidal Sails

Thanks! Will for sure make certain this is ok tomorrow. Other ideas? Regards Are

stevesxm

i would have the injectors out and pulse them and look for them filthy or overall low fuel pressure. the stumble and stagger sound like a lean condition as the pulse width gets bigger but ultimately passes enougfh fuel to run...

thats where i would go.... if it runs clean on 8 cylinders at 4000 then i would dismiss the notion of cap and rotor etc... it really sounds to me like the efi isn't getting the message to enrichen on acceleration ( map sensor) or it IS getting the message but not getting the fuel

Tidal Sails

Thanks Steve, I believe the injectors most likely are fairly clean, since the stuttering mostly occurs on acceleration. Even from idle rpm to a modest 1200rpm. You mention the map sensor, and I will consult my mechanic regarding this tomorrow, and keep you posted. He is being challenged by this issue, and has set his mind to get to the bottom of it. Why would the map sensor give the incorrect message? What about the coil, or general grounding? My engine looks like new, and shows no faults on the computer...
Another thing I just thought of, is that the engine keeps on running after the ignition key is turned off! This has happened a few times after the problems started.

stevesxm

post ignition run on is usually a function of extremely high combustion chamber temps... this can be a function of a lean condition, timing too far retarded ( which will also give you the stumble and lack of power) ....

its hard to shotgun theses solutions... and foolish to try. a methodical approach will tell you the answer and ill say to you what i say to anyone else that asks how to fix this stuff... you buy the FACTORY workshop manual ( not aftermarket) and a scanner and you do what the book says in the order it says to do it... it is the quickest and easiest way to fix this stuff...

if it was me sitting in your engine compartment, i would do the three things i suggested, then shoot a timing light on it rev it and see if the timing curve looks correct and THEN give up guessing and do it by the book.

im not trying to be a wiseass.... just telling you how , after 25 years or so of diagnosis on this type of system, how i still do it.

when i bought my boat w/ twin 502 mpi's the second check i wrote before i ever drove it was for the two mercruiser manuals, the drive manual and the scanner. that 300 bucks has saved me 10 times that amount.

ALSO nothing happens for no reason.... is there ANYTHING you did just before this problem showed up ... no matter how innocent it seems. so much of this stuff is unintended consequences or a simple mistake made doing something ELSE that is now biting your ass in what seems like an unrelated area....

i had a guy hook up his billion watt stereo to the power lead to the coil one time..... everytime he turned the volume up, the car would shut off..... that sort of thing....

mtgbiker

t.p.s. ???

Dave_N

stevesxm make some good points about the right way to approach a problem like this. But I don't think it would hurt to check the distributor cap and rotor for moisture and signs of crossfiring.

And if you have access to a fuel pressure gauge, that's another thing you could check yourself. You didn't say if the tech had checked it or not. It should be 43 PSI.

By the way, the missing IAC muffler (little block of foam) was probably blown out of place by a backfire and then ingested.

Dave

checkmate DOH!!

as everyone has stated, start with the simple and move to the more complex----slowly eliminating things and reducing your chances of misdiagnosing it by overlooking something.--- as someone mentioned t.p.s??---throttle position sensor----being all electronic does this engine have this setup that would be very similar to a car engine....i have seen tps sensors create a hard start condition and acceleration problems-----once started does the engine idle normally???----if this system uses a cap and rotor setup like gm used forever on their hei systems is there an ignition module under the cap???----or is this a DIS system with a coil pack or packs???-----again--start with the basics and move on---even make a checklist and keep track of things checked and/or replaced-----if equipped with an obd style connector and malfunction indicator lamp with trouble codes similar to cars then diagnosing will be a little easier----also too then hopefully you can get a scanner and have a live data steam as the boat is being driven----if i can help in any way i'll be glad to throw out some feedback----good luck to ya.

Tidal Sails

Thanks alot for responding guys, -and happy 4th of July! Itīs now morning in Europe, and you are probably all sleeping.
First: this happened at the exact same time that my power tilt stopped working late last fall. I changed brushes on that el. motor after this. Trim works fine now. I was also running very low on fuel at the same time. I had, sorry to say, no "condensation fluid" added.
Dave, your foam "IAC muffler" prediction sound plausible. What could possible happen if it was ingested? Are