John B

The engine feels like it's hitting the rev limiter at 5000. I am running 93 octane. Checked the primary fuel filter, it was clean. Oil level was down one quart. All gages were OK, water pressure was 15 pounds. What sensors would cause the ecm to cut the rpm? Boat speed is down about 4 mph, normally runs 5300 with the same load. Thanks JB

Dennis Moore

The first thing a Mercruiser mechanic will do is disconnect the tachometer from the system. The tach can make it cut out if it is going bad. After that a good mechanic will pull the spark plugs and inspect them to see if there is a dirty one. While the plugs are out he will run a compression check. Inspect the distributor cap and rotor and check the fuel rail pressure.
Some places to start!
Sincerely
Dennis Moore
Merc Master Tech
FAMILY AND PERFORMANCE BOATING MAGAZINE

John B

Dennis, thanks
What does the ECM do if it senses detonation? Does it cut fuel or timing or both? Would a detonation signal show up on the Mecc test device?
Thanks JB

Vinny P

I would start by doing as Dennis said and I would just change the fuel filter. If that yields no results put a scanner on to check if any codes are stored. If the ecm senses detonation, it cuts back the timing.

Dennis Moore

The knock sensor simply backs off the timing until the detonation goes away. A bad knock sensor will set a trouble code and the ECM will go into a rich fuel/conservative timing mode scaling back performance of the engine but it will not cut out cylinders like the rev limiter sound you are hearing.
If you have tested the engine (like I previously suggested) and you found one cylinder a little different than the rest (spark plug color is different) then I would pull of the valve cover for that cylinder and check for a broken valve spring. Not a big deal!
Dennis Moore

FindMe

Not sure what year and model you have, but this is a common problem with EFI motors. Check S/N's to see if it applies to you.

No. 99-8
WARRANTY INFORMATION SERVICE INFORMATION
EFI / MPI Fuel Pumps With Low Pressure
Models
Alpha Sterndrive Engines.
MCM 4.3L EFI: S/N 0L360469-0L360789.
MCM 5.0L EFI: S/N 0L359672-0L376549.
MCM 5.7L EFI: S/N 0L359121-0L375658.
MCM 350 Mag MPI: S/N 0L372105-0L376283.
Bravo Sterndrive Engines.
MCM 4.3L EFI: S/N 0L360474-0L360638.
MCM 5.0L EFI: S/N 0L359653-0L372286.
MCM 5.7L EFI: S/N 0L346472-0L375995.
MCM 350 Mag MPI: S/N 0L359066-0L376282.
MCM 7.4L MPI: S/N 0L362686-0L373270.
MCM 454 Mag MPI: S/N 0L352940-0L370234.
MCM 502 Mag MPI: S/N 0L352957-0L370246.
Ski Engines.
MIE 350 Mag MPI: S/N 0L305670-0L305774.
MIE Black Scorpion: S/N 0L305536-0L305577.
Inboard Engines.
MIE 350 Mag MPI: S/N 0L303829-0L303890.
MIE 350 Mag MPI Horizon: S/N 0L305377-0L305813.
MIE 7.4L MPI: S/N 0L303950-0L304264.
MIE 454 Mag MPI Horizon: S/N 0L303787-0L304120.
MIE 8.2L MPI: S/N 0L303516-0L303517.
Situation
Engines within the serial number ranges listed may be equipped with an electric fuel pump that is painted black with a brass outlet. Under certain conditions, the check valve in the brass outlet of this pump may ‘cock’. This will cause low pump pressure above 4000 rpm and a loss of engine performance.
NOTE: Painted pumps with an aluminum outlet or unpainted pumps with the brass outlet have a different style check valve and spring that will not ‘cock’. Refer to the drawing.

Inspection
If one of the engines listed has a loss of power above 4000 rpm, use a fuel pressure gauge to measure the fuel pump pressure. Any reading below 22 psi (152 kPa) at 4000 rpm and above would indicate this check valve problem. Also, a 0 reading on the pressure gauge shortly after shutting the engine off is another indication. Remove cover from the cool fuel assembly to see if the fuel pump is one that may have the check valve problem.



a - Aluminum Outlet
b - Brass Outlet
c - Area of Pump Painted Black
d - Unpainted Pump With Brass Outlet

Correction
Replace fuel pump with new one and retest pump pressure.
Part Required
(1) 861156A 1 Fuel Pump Assembly.
Warranty
Fill out warranty claim and send it to your normal warranty processing center.
NOTE: The replacement of part (s) and labor under warranty, to the products outlined in this
service bulletin, will expire December 31, 2000.
Warranty Code and Labor: MJ32 2.0 Hours
Diagnosis/Test: MJ40 1.0 Hours
Old Part: USA and Canada: Return with warranty claim.
All Others: Retain for disposition by a Marine Power representative.

John B

I will check the fuel pressure over 4000, and the codes this week.

Motor #'s: port OL 600822. Starb OL415957

I can't tell whether the problem is fuel cut out or ignition retard, it causes a surging. New cap and rotor, new plugs, most of the plugs look very rich, always did. Compression is 145-155.

While I have you GURU'S here.......
One tack sometimes reads erratically, drifting 200-700 more than it should. So I replaced it but it still does it, only at full throttle, and maybe 5% of the time. Any suggestions

John B

Thought I would use 93 instead of 87 to see detonation?? would go away......not.

How does your Formula run?

Sydwayz

iTrader: (1)

I use 87 in my HP500EFI with no problems. I would replace the Cap & Rotor, Plugs, and go over the wires very closely. I had a problem with mine cutting out at midrange, but I could power through it. It was the distributor cap.

ursus

There is a service note from merc not to run 93 octane in 500 efi's. Upon start up the computer runs a check momentarily advanceing the timing for 1 rev to cause detonation. This is to see if the knock sensor is functional. If you run 93 octane under some circumstances this test may fail,, e.g. there will be no detonation and so the computer will now think the knock sensor is non functional and retard the timing / richen the mixture, so the 1st thing to try would be go back to 87 as per factory directions in the owners manual.