1999 380 Mercruiser MAG MPI fall off after 2000 rpm 2450 MAX

New to me, 350 hours each repower I did late last year. LA Fire Rescue engines, but they did sit for a couple of years without being run.
I've been fighting some issues where things got mis-adjusted, and some stuff just didn't work.
I did refresh most of the ignition just because it's been 20 years.
I'm still dogged by what is probably a miss, because the transmission is rattling at idle (not as bad as it used to), and I cannot get top end out of it.
Since I have the technology, I did some samples of ECM parameters at various TPS and RPM settings to see if there's a glaring issue, or a pattern.
The results are attached as an Excel file. If you need it in another format, just ask.

While you are looking at this, I am going to recheck fuel pressure compared with Starboard, and put another fresh set of plugs in it. There's a txt file of next stuff to try. The only mechanical thing I found so far was a valve rocker that had been punched thru by the lifter push rod, which was preventing the valve from opening much. I thought that was really going to be it, but I still can't top out in RPMs.

Ask away, I'm all ears.View attachment mercruiser ignition fuel tests.txtView attachment mercruiser ignition fuel tests.txt

update:
Fuel pressure gauges on both engines. Both prime to 38-40lbs. Hold that at idle
I'll be taking it out under load and going past 2000 rpm to see if there's a fall off
Vacuum gauge on both engines, tapping into the intake plenum where the fuel regulator goes.
Both are at 22-23 hdg, right in the middle of the green. If I pull on the throttle cable on either to run up to 1200 or so, the vacuum increases to 25 and stays there on both.
Once again, I'll check this under load to look for variance.

what you attached in the first post were not .xls files but bit maps ... they aren't legible when I zoom in on them.

would also be of benefit to idnetify the variable on the x-axis and any color codes used to segregate the data series.

Yeah, this board doesn't like non picture attachments.
Let me see if I can clean the screenshots of the .xls into something better.
Stand By

If you can turn the bitmaps into PDF's, they usually zoom legibly...

Assuming the graphics are 'paired' (left and right engines), the weird thing to me is the drops on both the MAP and TPS....almost like the reference voltage is sagging.....are the alternators functional?

Yeh low voltage can do all kinds of crazy stuff to you. It's odd that both engines behave identically-ish in regard to the poor performance. Check the grounds...be sure all the connections are clean and tight.

View attachment Engine Run Data 5-19-2019.pdf

Here's a PDF. Both engines are not operating poorly. The Starboard is operating fine, and reaches 4600 RPM without issue.

PDF's much better...at least on this end.... the RPM curves make no sense to me, as plotted.....the text below (full/half) adds to the confusion factor.
The TPS & the MAP curves, assuming they are both done w/ RPM on the X-axis and Port on the left, Starboard ont he right, don't make any sense with the drop-off in the upper RPM end. If the one engine is fine, I'd suspect your scanner has some configuration issue.
Without valid data, any assessment/analysis is subjective at best.....

I took the boat out for a ride.
I have Fox Marine real time engine monitoring gateways for each 1999 MEFI engine that display over NMEA 2000, and a Bluetooth app on my iphone. During the ride, I took 5 samples (screenshots of the app displaying all these parameters)
the notes below each set of readings on the excel page describe what I was doing.
In the first sample engine run time of 11.5 minutes, I set the throttles so each engine's TPS was reading the same, 10.16%, looking for variations in other readings
In the second sample engine run time of 12.5 minutes, I set the throttles to synch (or near it) of 1671 RPMs, looking for variations in other readings
In the third sample engine run time of 15.5 minutes, I set the throttles to around 2000 rpm, where the Port engine starts to really need more throttle than the Starboard to keep that RPM.
in the fourth sample engine run time of 18.5 minutes, I tried to plane out the boat. The Port engine got to 2430 RPM at full throttle, while the Starboard easily got to 3006 RPM, with only 45% throttle being applied. I saw no point in running the Starboard up to full throttle as I know that already is doing fine at 4600+. Fearing potential damage to the Port engine, I backed off to my normal slow cruise where both engines are "kind of" acting the same.
In the 5th sample engine run time of 58.5 minutes, I took one more reading at around 1800 synched RPM to see if anything had drastically changed over time.
The charts were just me looking at various parameters over the samples comparing the Port to the Starboard engine, looking for patterns.
The list of observations under the sample data are my notes of what I see looking at the comparisons.

ok...so your plots are just samples in times.....

Looking back thru the table, the first thing that jumps out is the indicated battery voltage is LOW.....this can cause many issues....I'd be inclined to get that corrected first...it should be pretty close to the alternator's output voltage.

The port using more fuel is consistent with the indicated MAP values....but the port engine appears to have a higher MAP reading for a given RPM....I'd tackle that issue next. I'd suspect a vacuum leak as all readings appear to be biased high...

another observation....from the table....MEFI4 on the Port and MEFI3 on the Starboard....???

you may want to see if you can get the checksums or other ID'ing data regarding the tune in each controller.....

makomark said:

ok...so your plots are just samples in times.....

Looking back thru the table, the first thing that jumps out is the indicated battery voltage is LOW.....this can cause many issues....I'd be inclined to get that corrected first...it should be pretty close to the alternator's output voltage.

The port using more fuel is consistent with the indicated MAP values....but the port engine appears to have a higher MAP reading for a given RPM....I'd tackle that issue next. I'd suspect a vacuum leak as all readings appear to be biased high...

Click to expand...

My vacuum gauge was connected to the fuel regulator port in the plenum. 22-23 HDG both sides, didn't flinch on some run up, but I haven't checked that under load yet.
Roger on the Batt voltages. New batteries, but the wiring is 20 years old, even if most of it is 0 or 00. I am getting suspicious of the alternators, as batt voltage never rises, even above 2000 rpm. Still puzzling that the STBD runs so good at such crappy voltage.

makomark said:

another observation....from the table....MEFI4 on the Port and MEFI3 on the Starboard....???

you may want to see if you can get the checksums or other ID'ing data regarding the tune in each controller.....

Click to expand...

I have the checksums for both engines. The MEFI-4 was changed probably in the first year or 2 of existence because the engine hours are so close. I never found any info in Mercury's database other than a manifold replacement due to water ingestion around 2000. By that time, MEFI 3 was discontinued, so it was replaced with the 4B kit, including the harness. They did not repin the connectors. I have no idea why the ECM was changed out.
MEFI-4 checksum is BF9B
MEFI-3 checksum is CB5A
They were a matched set of engines in a LA fire rescue boat until it got decommissioned around 2015 or 16

Vacuum at idle won't tell you much.....under load is where it matters and you can see it in the MAP data....and the pulse widths, too....

I'm pretty sure the 4 will replace a 3 but not the other way around cause they added a few options with the 4....and as long as it was flashed correctly, that shouldn't matter. I don't have access to the dealer database so I can't validate the checksums....

If the alternators are running with the regulators working, you can do a voltage drop test to see where the problem connection(s) is.....and it they are bad, they will get warm....the ohm meter resistance test doesn't really tell you much as the current it uses is real small....

Does the port engine sound 'normal' when running....just not making any power?

Vacuum at idle won't tell you much.....under load is where it matters and you can see it in the MAP data....and the pulse widths, too....

I'm pretty sure the 4 will replace a 3 but not the other way around cause they added a few options with the 4....and as long as it was flashed correctly, that shouldn't matter. I don't have access to the dealer database so I can't validate the checksums....

The engines were a matched pair from a LA Fire rescue boat. The only warranty claim I could find was a manifold replacement around 2001. I have to believe the 4B was swapped in around the same time as the engine hours are only 50 apart.

If the alternators are running with the regulators working, you can do a voltage drop test to see where the problem connection(s) is.....and it they are bad, they will get warm....the ohm meter resistance test doesn't really tell you much as the current it uses is real small....

very familiar with ohm’s law. Will chase power drop down.

Does the port engine sound 'normal' when running....just not making any power?

No. There is a definite vibration at idle( shakes the damper springs, making a racket at idle). I can hear a different cadence compared to the very smooth running Stbd engine. Want sone video with audio? I can put it on Dropbox and send you a link.

maybe recheck the ignition leads - make sure they are tight on both ends...and in the correct terminals....if thats got the crab cap styple distributors, you may wanna change the cap on the troublesome one -- they are known to have short lifespans, especially if aftermarket...

Another thing to check is the Cam Retard...especially if you moved that distributor....pretty certain you need a scantool for that.....

Standard round HEI distributor. Mercury parts. The 4B does show CAM Retard. The 3 does not. CAM Retard was 0 throughout all of the samples.

The 4B unit likely has an old tune in it that doesn't use the cam retard then....i don't think the engine would run if it was really zero....and I reread your earlier post & caught the 4B with the cable adapter replacing the -3.......so that leaves the low voltage issue and a potential intake leak driving the MAP higher than normal = more fuel flowing thru the injectors...

Gonna get back on it this morning. Been 98 in the shade lately.

brutal....good luck