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

370Dancer

Well-Known Member
Oct 2, 2006
2,472
Suncoast of Florida
Boat Info
1998 370 Sundancer
Engines
380hp MAG MPI Gen VI with V drives
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.
 

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yep saw that. However, the Starboard is actually lower than the Port, and runs like a scalded dog. MEFI 3 vs a 4B, so maybe the 4 is more sensitive to volts.
 
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.
 
Have you manually verified the hand throttle is reaching full throttle at the engine?
 
Have you manually verified the hand throttle is reaching full throttle at the engine?
Smashing idea. The reason I think so is the TPS (Throttle Position Sensor) is reporting 96.88 % and the voltage is 4.18
i just went down, and found i might have 1/8 to 1/4" play at the end. This one starts failing 30% throttle or less.
Keep em coming
 
Ok have you verified timing? I also believe there is a wire that must be disconnected to verify base timing with a light. It will not rev in neutral, correct?
 
I have verified Timing. On this engine, you can only set base timing, and that is by jumping terminals A and B on the 10 pin DLC connector to the ECM. This engine has a MEFI 4B, so it automatically revs up to 1200 rpm, and disables the advance so you can accurately set the 8 deg BDTC, which I have.

Good idea. I just wend down and fired her up. I decided to stop at 4800 RPM at the dock, and I hadn't firewalled it yet, but close. So no, it develops full RPM in neutral. Next question, how's your bottom and running gear? I have a monthly diver, who just did it a week ago. The props are true, as the Port side just got redone after kissing a sandbar (welcome to West Florida) a few months ago.
 
Verify fuel pressure under load. That will eliminate fuel pressure. I would also change fuel filters again. Pull the distributor cap and rotor and verify if there's any corrosion. I'd likely replace the cap and rotors again as well.
 
.5 to 4.5 range - no codes set Cap and rotor are brand new (as are wires)
TPS.PNG
 
Revving in neutral means nothing, have you done a compression test ?
What rpm's are you reaching ?
Is this boat new to you or just the engines ?
Is it possible it' propped incorrectly ?
Is the timing advancing ?
Is the hull clean ?

Not normal for a pushrod to punch through a lifter. Probably more damage in there.
 
Revving in neutral means nothing, have you done a compression test ?
Yep, all 130 to 135 except #4 which is 120 (not the cylinder with the valve rocker punched)
What rpm's are you reaching ?
Port up to 2450 Stbd up to 4600
Is this boat new to you or just the engines ?
I've had this boat since 2003
Is it possible it' propped incorrectly ?
Original props, recently tuned
Is the timing advancing ?
Read the sheet - Port appears to be way out in front of Starboard, then drops off
Is the hull clean ?
last week. Monthly diver

Not normal for a pushrod to punch through a lifter. Probably more damage in there.
Perhaps, but before I tear it down, I'm looking for clues to fuel, air, or spark. Leak Down test was fine as well on all cylinders.
Note that as ECM controlled engines, it will continue to try to compensate for lack of any of the above. Some conventional troubleshooting doesn't work well, like pulling vacuum hoses.
 
Those compression numbers aren't good. That engine, 454 mag ?, in top health should be 160 psi. So it makes no sense that your leakdown is fine. That lost compression is going somewhere.
 
Those compression numbers aren't good. That engine, 454 mag ?, in top health should be 160 psi. So it makes no sense that your leakdown is fine. That lost compression is going somewhere.

Your quoted cimpression number is high...it's closer to 150 psi. With that said 130-135 psi is much closer to what an engine has with some hours. That's not preventing his engine from making rated rpm.
 
Last edited:
Your quoted cimprecompr number is high...its i closer to 150 psi. With that said 130-135 psi is much closer to what an engine has with some hours. That's not preventing his engine from making rated rpm.
Beg to differ, just rebuilt my 502 that was running poorly lost about 500 rpm and 7 mph. 160,150,155 125, 115, 150, 150, 155. All valves were leaking air to some degree.

After stock rebuild it is now 165 across the board. His mag should have similar numbers. There's no way his motor isn't leaking air.
 
before we turn this into a compression thread, a couple of points
I am missing 2200 RPM on the port engine, not 500.
These engines are at 400 hours
I am losing compression. I stated that my test was dry (no oil) and cold, and my leak down test was leaking past the rings. I could hear it in the oil dipstick. I am also aware that this is pretty common for big blocks to leak past the rings on a stationary cylinder, and up to 20% is acceptable.
Today, per my stuff to try list I originally included (the .txt file), I will be pulling the plugs, and redoing compression and leak down. I have a vacuum gauge that I will use to make sure I am TDC on each cylinder during leak down.
numbers to come.
My fuel pressure under load test this morning left me scratching my head a bit. Some of the delta may be the difference in the gauges. I plan on swapping them and doing this again to confirm or rule that out. One is a harbor Freight, that I HAD to use thread tape on the keep the screw on fittings from leaking.
If I start the engines cold at the dock, fuel pressure is 38 or so on both engines.
If I restart after warm up, the STBD (good) is 20lbs and the PORT (bad) is 30lbs.
Once I have left the dock and am underway, pressure on the STBD is around 15, and PORT is around 20
They stay that way throughout the acceleration curve. STBD gets up just fine. PORT starts bogging at 2000 rpm, and never makes it past 2400 at full throttle. The fuel pressure gauges don't move.
Back to the dock, try a restart and the PORT engine is now vapor locked. I have installed the low pressure secondary pumps to both engines to overcome this known issue. So, now I'm thinking I am restricted, or the low pressure pump is inoperative for some reason.
Will change the fuel water separators and check the pumps in a while.
So now the question is, would fuel pressure be higher at the rail if there is a fuel restriction as opposed to lower if there was a leak?
GAWD, I love motors. I know it's going to be something stupid, but not obvious.
 

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