350 mag mpi surging under load

[silver_power]

Hello..I have a 350 mag mpi (MY2009) alpha one serial number 1A348183. The engine idles very smooth and with steady 550rpms but as soon as I got the boat on plane and want to keep about 3000rpm , it surges from 2900-3100. If I slow down to about 2800rpm , it surges from 2600 to 2800 and it might go off plane. The only way to ride the boat is ro raise the rpm about at 3600 where the engine surges from 3600 to 3300 and back but can handle a speed range of 35mph.I tried to push to wot and it reached 4700rpm. The normal it reaches with same load is about 4900-5000rpm. The hull is clean.
The ecu does not have any fault codes and the engine starts immediately.
With the scan tool I checked if all the cylinders fire and the result was OK. (Test at 1500 rpm and isolate two cylinders at the time). At 1500 rpm with no load, it surges from 1440 to 1520rpm.
I changed spark plugs, (not iridium for lower cost but new)water separator , distributor cap wirh the same results. I tried feed the engine with a portable fuel tank with fresh gas, the same results.
I cleaned and check the fuel injectors ,the same results.
I will change the spark plugs with used-iridium ngk and the spark plug wires as well because I saw that the springs of the wires are broken and maybe they loose connection with the distributor cap. Just a thought....
The consumption has raised about 25% according to smartcraft mpg reading.
What else shall I check?????

 

[harleyman1975]

I'm thinking plug wires or fuel pressure.

 

[alldodge]

Check fuel pressure while running
Check alternator output voltage

 

[kenny nunez]

You may have either a clogged screen at the fuel pressure regulator or a bad regulator. Make sure the vacuum line to the regulator is not leaking or pinched . Get a fuel pressure test kit from one of the auto stores. The test nipple is behind the throttle body. With the vacuum line connected you should have 43 lbs maximum at full load and around 35 lbs at idle. I just went through this with my Donzi. And to make it a real challenge the Rinda would not show any codes. I did change to the Iridum plugs and that seems to have beeen the problem. I purchased my boat with these problems. Also before I started trying to use the boat the raw water pump on the engine was bad with the impeller broken up. If you do not have enough water pressure that also could trigger the controller into Gaurdian mode to protect the engine.

 

[silver_power]

The voltage both at idle and running is about 14-14.5V.
I was thinking about the fuel pressure but I believe that since it surges at 3000rpm for about 200 rpm it would not reach 4700rpms....it would starved for gas. The engine gives me the impression that something electrical happens. I hoped that with the distributor cap the things wuld be better. Is there any possibility the ignition coil to be faulty?

 

[alldodge]

Is it missing or slowing down then back up?

 

[silver_power]

Slowing down like it works with 6 cylinders with litle vibration and then back up with smooth operation....
And something else.....when I slow down the boat to go off plane, it surges noticeably during decelaration and I feel vibration like it works with half cylinders....till the boat stops and it idles fine!!! Very strange things!!

 

[alldodge]

The coil can be breaking down or the fuel pressure starts falling off. Needs some testing to determine

 

[silver_power]

The fuel pressure regulator placed at the cool fuel module is totally new as the previous found corroded outside despite it worked fine without issues. The regulator placed at the fuel rail has been checked when I took the rail with the injectors for cleaning to a pro and found absolutely ok. I forgot to refer that the engine has 410hrs operation.
The next things I will check is spark plugs, wires, and possible vacum leak with a carb cleaner..

 

[tpenfield]

The problem with MPI engines is that you really need the computer diagnostics to figure out the problems. The best thing to do would be to hook up the computer to the engine and take it out for a test run and let it do its thing in terms of surging. Then all the real-life data would be captured and can be played back to see what is changing or out of whack during the surging.

Many shops will just hook up the computer, check for codes, and run the engine under no-load conditions . . . and usually they come back fine. :facepalm:

If you want, you could get the computer program (Rinda / Diacom ) and do this yourself, otherwise have a shop do it and send you the file.

Fuel pressure, as mentioned, is another thing to check while the engine is doing its surging thing.

 

[alldodge]

The fuel pressure regulator placed at the cool fuel module is totally new as the previous found corroded outside despite it worked fine without issues. The regulator placed at the fuel rail has been checked when I took the rail with the injectors for cleaning to a pro and found absolutely ok. I forgot to refer that the engine has 410hrs operation.
The next things I will check is spark plugs, wires, and possible vacum leak with a carb cleaner..

There are many places which say they do injectors, but unless they have a machine which can fire the injectors to at least 5000 rpm checking flow an pattern, its unknown just how much they actually did. Not saying its the injectors, regulator or pump, we need some testing to determine what it is. A injector shop will give you a printout of what they did before cleaning and after

 

[silver_power]

One more thing that I want to know....why my engine has two fpr's? One on the fuel rail and one at the fuel cooler module ?? I can see a hose connecting the intake manifold with the fuel rail fpr, but where is the hose which connects the manifold with cool fuel module? You can check it with the serial number of my engine given at #1 post...ido jot understand why to have two fpr's!

 

[alldodge]

The one on the rail is a fuel damper, it smooth's out the fuel pressure to reduce the pulsing from the pump and regulator on the cool fuel.

 

[silver_power]

I know that the rail fpr is connected with the manifold via a hose starting from a tube attached to manifold close to the mapt sensor and ending to fpr. But how the fuel cooler fpr is connected with the manifold? Where is its hose?? How is the routing? I am asking because I want to check all the possible point for vacum leak and the cool fuel module is very hardly accessible.

 

[alldodge]

Both should be Teed together under the manifold, item 22

http://www.mercruiserparts.com/bam/subassembly/31752/13884/90

 

[silver_power]

Correct. I have this hose but I am absolutely sure that one edge of the hose (part 21) attaches to the manifold the other to the fpr and the third edge (starting from part 22) is free to the air. So this hose does not connect with the cooler fpr. This is why I am asking .....

 

[silver_power]

I am coming back with my problem as yesterday I took ťhe boat for a ride with the mechanical. Well, after the test he is sure that it is not a fuel issue as the fuel pressure was correct while surging but he is sure that there is a problem with valve lifter. So he is going to take off the intake manifold to check the valve lifters , possibly change some and we hope that this will be the problem. I will post as soon as I have some news.

 

[alldodge]

he is sure that there is a problem with valve lifter

Wonder how is so sure that is the issue? Guess we will find out

 

[tpenfield]

Oh, I hate it when people are 'so sure'

I hope it pans out.

 

[silver_power]

In case that I really have a lifter issue, (just to be prepared) can I buy aftermarket kit for gm small block (roller lifters) or you suggest to buy the mercruiser parts? The original are very expensive!! About 100usd each!
I can find a lot of companies who sell roller lifters for 350 small block gm truck but i do not know if these are applicable to marine use. What are you suggesting?
Thank you very much.