350Mag mpi pre-ignition??

[silver_power]

Hello, i am running a 350Mag MPI (s/n 1A348183) with 360hours. The engine operated perfect since last summer that a little overheating problems occured. After that, some issues of rough operation at idle when the engine started every day occured and the same time the starter hardly was cranking the engine....Finnaly the exhaust elbows needed replacement as water entered to No3 & No5 cylinder. I checked the exhaust manifolds as well for leaks but they are both ok with a very little rust and so i kept them. I fixed the problem with new elbows, new head gaskets and the port cylinder head went to a machine shop for cleaning. The 3&5 cylinders of the port side had a little rust internally which i cleaned very carefully as it was very soft. The starboard head was clean and also the starboard cylinders.I also cleaned the oil pan as the oil became milky and i flushed the engine with several oil changes and operation from idle to 1300rpm. Now the oil is clean, the engine starts and operates smooth and perfect with no fault codes, perfect operation temperature and oil pressure. I made also a compression test and all cylinders had around 135-140 psi.
Yesterday (the engine is still out of the boat), i tried to start the engine....turned the key and as the starter turned normally the engine to start, i heard a loud metal noise from the starter and the coupler at this moment stopped the standard rotation and rotated reverse. I then left the key and stoped the starter before the engine started. Obviously the metal noise came as the flywheel hitted to starter teeth while it rotated reverse for a moment.Next to that, i made five attempts to start the engine and the engine started all the five times normally with no problems . The idle is smooth and the throttle response is perfect with no hesitation or vibrations. I cannot understand why the engine ,during cranking,stops to standard rotate and for ONE MOMENT rotates reversly and so the flywheel teeths hit the starter teeths. Is that a pre-ignition problem? Why is this happening?
Thank you very much.

 

[alldodge]

Check the distributer to ensure the timing is correct, also check cap and rotor are clean.

 

[silver_power]

I did not opened the distributer at all, so the timing is correct. The problem is that issue occured only once or twice and never again.....but why???If timing was not good or the valves were not adjusted correctly, the engine would not operate the way it is now....it would ran rough.
I forgot to mention that the spark plugs are new, as now that engine is out of the boat is a good opportunity to change.

 

[Bondo]

I did not opened the distributer at all, so the timing is correct.

Ayuh,.... You had the heads off, so the distributor has been outa the motor, which means it needs to be properly timed again,....

 

[silver_power]

No....I am sure....I did not take off the distributer...... i only had the one head off..and I managed to reinstall the head without take off the intake manifold. So the distributer did not take off.....

 

[alldodge]

No....I am sure....I did not take off the distributer...... i only had the one head off..and I managed to reinstall the head without take off the intake manifold. So the distributer did not take off.....

Wow, wonder if anyone is taking bets of problems later

 

[QBhoy]

Second the distributor theory. Apart from anything else, the MPI engines are well known for having problems with these. Similar thing happened to mine after winter/damp storage.
Think they only go on the one way. Fool proof change out and worth a change every couple of years in any case.

 

[silver_power]

You mean that probably the distributer has been failed?. If yes, which part? The cup or the rotor? ?I have not heard something about mpi's distributer problems.
Besides, if the distributer had a problem, I believe that the engine would have serious operating problem. It would not operate smooth.....

 

[Scott Danforth]

No....I am sure....I did not take off the distributer...... i only had the one head off..and I managed to reinstall the head without take off the intake manifold. So the distributer did not take off.....

that is the most creative way I know of to not do something correctly.

 

[silver_power]

that is the most creative way I know of to not do something correctly.

I do not think that this is the reason of my problem. The head reinstalled with the new head and intake gasket without take off the distributor and all the bolts tightened according the mercruiser service manual. I did not want to disseambly all the staff the engine has on top of intake manifold and so I tried to do the job otherwise. And everything assembled correct. So the timing did not changed. This is why I am asking if anyone can think what can force the engine to rotate reverse for one moment while cranking with starter.?..

 

[Bondo]

This is why I am asking if anyone can think what can force the engine to rotate reverse for one moment while cranking with starter.?..

Ayuh,..... It sounds like it tried to fire, when it shouldn't have,....

To see why, check the distributor,.....

 

[harleyman1975]

The ignition timing sounds to be advanced. Check the timing.

 

[silver_power]

How can I check to see if something is wrong with the timing??

 

[alldodge]

How can I check to see if something is wrong with the timing??

You have what most call the crap cap. Its called this because you never know when it will have issues. It can look fine and still have issues. That said; there is no way to look at it and see if there are issues, other then if there is deposits or damage inside.

Since it did it one time and has not done it since, just go with it and run. My thought is you will have a very good chance with intake issues later. Enjoy the boat and I hope there are no other issues, but if there are we will be here

 

[silver_power]

Why shall I have intake issues and what kind of issues should I expect????
Concerning the timing......Is that something that I should change or something that I must adjust?. I repeat that I never changed anything concerning timing.....

 

[alldodge]

Why shall I have intake issues and what kind of issues should I expect????
.

The mating surfaces of the intake are with in thousandths of clearance. Bothe heads are machined to meet at a specific angle and that angle also has to seal with the valley (where the cam and lifters are). The head was removed, and in the best of cases without the intake being installed it can stick. The intake side of the head can and does stick. So depends on what was done to get the head off and from under the intake. What stress was placed against the intake and the sealing surfaces.

Concerning the timing......Is that something that I should change or something that I must adjust?. I repeat that I never changed anything concerning timing.....

There is no adjustment, this is installed and only readjusted if removed or it does not run correctly. As before it only did it once then there probably is not an issue. If it does have issues, then first replace the cap and rotor. If it still has issues let us know and we will look into it. The distributor is installed using these marks

 

[Scott Danforth]

I do not think that this is the reason of my problem. The head reinstalled with the new head and intake gasket without take off the distributor and all the bolts tightened according the mercruiser service manual. I did not want to disseambly all the staff the engine has on top of intake manifold and so I tried to do the job otherwise. And everything assembled correct. So the timing did not changed. This is why I am asking if anyone can think what can force the engine to rotate reverse for one moment while cranking with starter.?..

It may or may not be part of your problem. However my high school shop teacher would have made an example out of you for doing it wrong.

At best you will have a coolant leak in the Intake to head surface on both heads. One because you disturbed the joint. One because you would have had to force the head into place compromising the gasket.

You will have an oil leak at the china wall where the block, head, and intake meet.

I have done some unique shortcuts over the past 40 years of turning wrenches, however this is the first time I have ever heard someone trying to replace a head without pulling the Intake.

 

[harleyman1975]

I would not have thought it to even be possible. vaccume leaks a distinct possibility as well.