Absolutely frustrated with this Mercruiser engine

If it is indeed lacking fuel, how about “adding” fuel when you hit the 2,000 rpm limit.

get a helper and a can of carb cleaner with a straw. Spray it into a vacuum hose or even the throttle body and see if you can exceed the 2,000 rpm limit under load.

Access isn’t easy in a marine application, you may have to get a little creative in how you get the carb cleaner into the intake.
 
hughespat57 said:
Gee, guys...
A thread gets this long and people rarely read the whole story but give the OP a break.
He has gone thru head issues, machinist issues, mechanic issues, and is trying to just get it fixed.

First of course he "hears it" it's weak its only putting out about 50% power.
That doesn't mean it's going to be knocking or sputtering or otherwise noticeable by ear.
These were first generation ECM, they were MEFI, NOT "MEFI 1" because there was not concept yet for 2, 3, 4, 555.
They are extremely basic, very little self diagnostics.
There is NO Guardian mode, that was not introduced until v3

The "Power Reduction" mode is clearly Off
This mode is NOT triggered by the ECM itself, this is triggered by a number of optional external "switches"
High temp:No, Low Oil Pressure:No, Low Oil level:No, a physical switch could also be connected, a "Get home mode"
The E-Stop Input:Off (think fire bottle shutdown)
I have NEVER seen any of these MEFI 1 or 2 options actually used on a boat, early boats from Carbs days had alarms for these items already.

The other external trigger is if the Distributor Ignition Module cant communicate it's status to the ECM, the ECM will stay in a startup mode, I described this way back in the thread.

Yes the engine will run "fine" (but 50% power) with half the injectors out.
These early ECMs had only two injector driver circuits, four injectors on each circuit.
But it is NOT left bank, right bank, it's two on each side. Roughly in firing order.

So as was stated before it becomes a fairly smooth four cylinder engine.
But no 3.7L engine is going to get that boat on plane.

We are back to WHY.
Why low power, Why limited open time,
There are things some of us could do the the OP just doesn't have the tools for.

At this point we know the injectors are good, fuel pressure is good, basic spark is good.
I would break out my oscilloscope and be actually checking the output of the two injector circuits and see if that square wave matches what the computer THINKS it is doing. One injector driver circuit may be bad. But I own one, not many people do.

If I were going to throw a few parts at it I would start with the Ignition Module, only $95 dollars from Sierra
Sierra 18-5107-1 Marine Ignition Module - because it may not be sending the correct signal back to the ECM.

Then at this stage I would just send out the ECM, MEFI 1 are non-existent new.
Buying some E-Bay unit for hundreds or thousands is a crap shoot.
Mercruiser had a kit to use the MEFI 4 but it needed an adapter cable & programming, which I cant find anywhere today.

https://obd2allinone.com/ecmrepair.asp - test and repair them for $100 (test) to about $600 repair.
This is due to the fact it is rarely the computer that goes bad but rather power transistors.

@Hoplite808, hang in there !
Click to expand...

I believe he has already replaced those parts in Post #1.

In regards to ECM focus....the theory is that the ECM is going into RPM Reduction mode and that fits almost exactly with what he is experiencing. As to what is triggering it.....we just don't know....yet. We just know the injectors On duration are not advancing when everything else is. I personally don't believe the Fox Marine tool is able to determine the RPM reduction mode without seeing all the discreet sensors. It only sees two out of six.

I know the OP has done a complete top end rebuild and the title of the thread says it all. It was all of our collective wisdom who told him to have the injectors serviced....and while it was a good thing to do....it didn't solve the problem.

I'm hopeful we are on the right track with the ECM but the real challenge is being able to override a set parameter in it. @Ericinmich has first hand knowledge of the system and I am hopeful that Service mode will do that.

I appreciate how much money and time he has invested in this engine but this is a tough problem to solve without having your hands on the engine and most importantly the right diagnostic tools to work with.
 
Why not replace all six sensors? That would only leave the computer in this scenario. Now I can officially be labeled as a parts cannon.
 
Here is some data if this helps from my starboard 454 horizon, idle, 2000 rpm then 3600
 

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blaster said:
Why not replace all six sensors? That would only leave the computer in this scenario. Now I can officially be labeled as a parts cannon.
Click to expand...

That certainly would be one approach and may be the answer but I doubt he has all six installed.

About the only two things that fit the problem now are the ECM/sensors and a timing chain that jumped a tooth.

He is getting closer to where the problem is.
 
If timing chain jumped a tooth I don't think it would run as good as it does now. I personally have never seen that happen in 25 years of street racing and building motors
 
He has a good MAP vacuum reading at idle (compareble to chris380 printout , assuming similar ambient conditions ) .
I guess with timing a tooth off the overall volumetric efficiency would be bad and so the map with closed throttle higher .

My gut feeling is the timing is ok .
 
A long time ago, I was working on a friend’s LT1 engine he had rebuilt. In the process he installed the timing chain a tooth off retarded and it would start fine and idle smooth as silk. But when you put a load on it it fell on its face.

That is my one and only experience with it in a hundred engines. 4 hours later it was running fine.

I have never seen a jumped tooth on a Mercruiser but this engine is still fighting us.

I agree it is a long shot and I certainly wouldn’t waste my time with it when the ECM seems to be in play.
 
PlayDate said:
Chris,

Great work pulling the data! 3600 rpm with almost 1,000 hours and you are only burning 20gph!

I like that!
Click to expand...

Keep in mind that is just one engine :)

This is what 7.4's look like in a 400 with 1,500 hours on the clock. According to the internet these motors should have given up the ghost 500 hours ago! Sipping along @ 38gph. :):)

52897151879_3b6507c30f_h.jpg
 
Since we're reaching for straws - Is the injector electrical harness oriented correctly? Left bank vs right bank?
 
ttmott said:
Since we're reaching for straws - Is the injector electrical harness oriented correctly? Left bank vs right bank?
Click to expand...

These are not direct inject but each ecu injector driver fires it's 4 injectors twice per revolution (banked double fire) so it would run fine if the banks were swapped. The fueling isn't that precise, if you looked into the intake runners it's more of a fog of fuel getting sent in the back of the valves before and while they are open. You actually can get some crosstalk of fuel that can cross over to other runner but these run rich (not at stoic) so it's not noticed.
 
ttmott said:
Since we're reaching for straws - Is the injector electrical harness oriented correctly? Left bank vs right bank?
Click to expand...
Ericinmich said:
These are not direct inject but each ecu injector driver fires it's 4 injectors twice per revolution (banked double fire) so it would run fine if the banks were swapped. The fueling isn't that precise, if you looked into the intake runners it's more of a fog of fuel getting sent in the back of the valves before and while they are open. You actually can get some crosstalk of fuel that can cross over to other runner but these run rich (not at stoic) so it's not noticed.
Click to expand...
370Dancer said:
Funny, I just posted something about this on my thread "1999 MAG MPI Just quits". Easy to get a couple mixed up.
http://clubsearay.com/index.php?thr...just-quits-videos.101750/page-11#post-1403763
Click to expand...
You guy's may be on to something here. I would scope the injector's side by side with a compression gauge. You can even perform a running compression test by removing the Schrader valve from the compression gauge hose.
Actually, if you don't have a scope you could just use an injector noid light.
 
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