350 Mag Mpi low WOT

[silver_power]

Hello. yesterday put the boat in the sea for summer season, and today my brother went for weekend. He called me today and told me that the engine 350 mag mpi (1A348183, MY2009 with 450hrs), has low WOT​​​​​​. The engine with the same load used to reach 5000-5100 but now only reach 4200. No faults at smartcraft display or beeps and the gas is fresh.Everything seems to operate good and smooth , no surging , no miss. The fuel pressure is within range 41-42psi. The engine is just tuned up with new cap, rotor, plugs, wires. I am thinking that something is not ok with the ignition, like it is not advanced. I told him to disconnect the knock sensors from both sides hoping that something is wrong with the knocks. Can the engine operate with the knock sensor disconnected or it will go to limp mode?

 

[alldodge]

Can the engine operate with the knock sensor disconnected or it will go to limp mode?

Yes it can be disconnected without limp mode. If the ECM was retarding timing due to a knock, there would be a code and smartcraft would show it.

Need to start with the basics, compression test. With fuel pressure gauge attached, turn power ON, when pump stops see if the fuel pressure holds, drops to zero, or slowly drops

 

[Scott Danforth]

also check your flappers. if there was any over-heat, they would drop into the exhaust restricting flow

 

[silver_power]

The basics have been done before put it in the sea and while I was trying to solve the misfire issues I had. compression test good to all cylinders (160-165) and the fuel pressure is ok 41-42psi at wot......but only 4100rpm????? The problem is that I do not have scan tool access right now.....and according to my brother everything else is ok. Not surging, no miss fires, ......that us why I am suspecting timing retard because of something...
the cap and rotor are new and phased with the led method (mercruiser sevice bulletin 2011-11r2).
is possible the shift interrupter switch or the main 50amp circuit breaker related with the problem?
I am thinking of something electrical just because the engine is just serviced with fresh oil, fiters , batter cables, and tuned up.....

 

[silver_power]

The flappers also checked just because I had some overheating issues at idle and I checked them. They are perfect. I must say that after I checked the flappers and reassembly the exhaust elbows to the y-pipe, I noticed that if I hit the elbow with my hand, a light metal noise can be heard (engine not turned on) but I did not give a notice. This noise can be heard while the engine idle as well. This is why I hope this metal noise confuses the knock sensor and retard the timing.........

 

[alldodge]

Thanks for the additional info

Would like to know if after the key is turned OFF (motor never run) does the pressure stay up?
Reason- See if injectors are leaking

Put the scanner on the motor and record timing a manifold pressure
Reason- Fuel injection timing is controlled by the MAPT and also would like to know if your getting full timing advance

 

[silver_power]

Update: My brother just made some tests:
1. With the knock sensors disconnected the boat got on plane MUCH stronger but when it reached 3500-3700rpm the beeper activated and the smartcraft screen showed Knock sensor disconnected.
2. He re-connected the knock sensors he got on plane but the engine seems not to have much power and at wot it reached only 4300rpms .

After that I think that the most possible is that something pushes the ecu to retard the timing.

 

[silver_power]

The injectors are not leacking as this was tested 2 months ago...with engine stop and key on, fuel pressure jump immediately to 44psi and after the pump stop drop to about 42. After about 5mins it is almost steady to 40-44psi so everything is good. My problem is that for now I do not have a scan tool access so I can not see the timing etc. All I have and can compare related to vacuum etc is the fuel consuption reading at the smartcraft which seems perfect. 2,3-2,4lt/hr @idle. When I had issue with map sensor,the consumption @idle was 3,8-4,3lt/hr with operation temperature. When there is a vacuum leak or a map sensor problem, the consumption is much bigger even @idle.

 

[silver_power]

How can I test the knock sensor and how can I check with a vom meter if it gives signal for retard the timing while running? There are two cables. One is yellow (or white) and the other is blue.

 

[nola mike]

How hard is it to unscrew the sensor from the engine? You could do that, and run it connected to the harness. I wouldn't think that a likely failure mode is continuous triggering (they usually are piezoelectric and generate a small voltage when knocked, like the starter on a gas grill). If you've got a scanner though, can't it display timing advance? If not, any reason you can't use a good 'ol fashsioned timing light?

 

[silver_power]

Problem solved.
New distributor cap (dizzy cap) and rotor AGAIN and the boat hitted 59mph@gps reaching 5100rpms at a very hot day.
The problem was exactly the following:
1. After hydrolocking, the mechanic did not phased properly the distributor and as a result the engine was surging under load and has low WOT rpms. When i ask him if he had clocked the dist properly he ensurned me that everything was ok
2. I made a full tune up changing plugs, wires , dizzy cap and rotor. result allmosto the same,,,,
3. I decided to check myself the clocking of the dist and i found it off phasing about 8-10degrees (16-20degrees@crankshaft). I re-phase it with the LED method, described at page 15 at 2011-11R2 / 2014 service bulletin. The result was much better with no obvious surging but still no normal WOT rpms.
4. I decided to change again cap+rotor. Result...PERFECT, runs exactly as new.
So two thing may happened. OR the cap+rotor i changed the first time before clock the dist were faulty, OR they became faulty as the engine was operating without the dist properly chased. I made the coclusion that i have to check the phasing of the distributor first and THEN change the cap+rotor. Wrong phasing definately has as a result a pre-mature cap+rotor damage.
So back to normal after 1,5 summer season of searching.
The funny thing is that 10days before i solve the problem, i decided to buy the diacom scan tool as my last hope to find the problem. Yesterday morning i solved the problem and today the diacom arrived.....anyway for now i will play with it to find out it's ability . It will be definately very useful tool......
Thank you all of you very much because you hepled me A LOT.

 

[alldodge]

Thanks, great write up