View Full Version : Loses RPM after several minutes above 3000 RPM / Loses Power above 3000 RPM

02-14-2008, 07:55 AM
Hi team. I am a new guy. I am excited and feel privileged to join this community. I have a 1996 Mastercraft MariStar 225VRS with an Italy Hurth Marine MastercrafPower EFI engine.

My problem:

Can pull forever below 3000 rpms, no problem. Cruising at 4500 rpms, after a while, 15 minutes or so (sometimes only 5 minutes at full throttle) the rpm start to slowly drop right down. Put the engine in neutral for a minute, and back to normal. However, tapping back on the throttle when the problem starts will allow it to level out at about 3000 RPM. Think it is a fuel pressure problem. The fuel line configuration is: 1) Fuel tank; 2) Fuel filter A; 3) Fuel pump; 4) Fuel filter B; 5) Fuel injectors, 6) Regulator 7) back to Fuel tank

What we’ve done so far:

1. Checked all the team talk threads, but none of them have a conclusive solution given our investigation below.
2. No faults found when running a diagnostics at the time of the problem (although there is an outside temperature sensor failure, but this problem existed before the RPM power loss problem)
3. Bypassed fuel filter A – still have the problem
4. Pressure tested the fuel pump, fuel filter B and the regulator on a test bench - ran it for over 20min – pressure is constant and within spec – will still have the problem


If you have experienced the same problem or have any good suggestions please let us know. We are planning to run some tests this week-end and would appreciate suggestions.

Thank you,

02-14-2008, 08:25 AM
Do you have a fuel vent?

02-14-2008, 09:39 AM
Do you have a fuel vent?

That's what I was thinking. My engine is different than what everybody else has, but on my high pressure fuel pump, there is a screen in it that has clogged up and given me similar symptoms also.

02-14-2008, 03:58 PM
I was wandering about a fuel vent. Perhaps the vent is clogged. I will try find it this weekend. Thank you.

02-14-2008, 04:43 PM
My friends Moomba had a similar problem, turned out to be a bad MAP sensor. TBI engine (chevy)

He could run the engine all day with the MAP sensor disconnected, but as soon as you plug it back in, no high RPM's under load. (in neutral the engine would rev over 4500 with no problem)

02-15-2008, 08:19 AM
I think checking the MAP (Manifold Air Pressure) sensor is a very good idea. I didn’t think this engin had one (I didn’t select the option when running the diagnostics) and that is possibly why it didn’t pick it up. I will check tomorrow and let you know. Thank you.

02-15-2008, 08:30 AM
I find it hard to believe that the Moomba motor would run correctly all day long, since the MAP sensor is what tells the ECM a lot of what's happening once the motor is above idle. There's no way a check engine light or some indication wouldn't show up unless A) the boat doesn't do this, B) the check engine light was disabled, C) he didn't look for it. Removing the harness from the MAP sensor will set a code and since it has an Indmar motor, it works the same way a MC does.

02-15-2008, 08:37 AM
deonsap- if your motor is original and unmodified, as long as it's a GM with injection and the Delphi ECM, it has a MAP sensor. Look for a clear (although it may be kind of brown, now) vinyl tube that runs from the base of the throttle body to a small rectangular black part that has a harness connected. Make sure this tube is not loose, dislodged or clogged. The ends tend to become stiff and can crack, causing an air leak and this will make the ECM think the throttle is open more than it is, due to decreased vacuum. Also, on occasion, a MAP sensor will develop a leak and this causes the same thing. If you have a vacuum pump for bleeding brakes, this can be connected to the MAP sensor and while it's running, increase/decrease the vacuum (don't increase it too much- the vacuum the sensor sees isn't excessive).

02-16-2008, 03:58 AM
JimN – Thank you for the great advice. I will update our findings after the weekends testing.

02-16-2008, 09:19 AM

He didn't have a scan tool to diagnose the problem and he only ran with the MAP sensor disconnected for a short time. I shouldn't have stated it would "run all day", my bad. :(

deonsap, JimN is the man when it comes to engine/electrical questions. Hope you get it straightened out soon and welcome to TMC! Let us know the outcome.

02-19-2008, 02:33 AM
Hi team. Weather did unfortunately not permit any testing this week-end. I will have to put the testing on hold for several days. I will let you know as soon as I have news. Once again thank you for your help so far. deonsap

03-18-2008, 12:26 PM
Hi team. I would like to update you on how we eventually solved the problem.

1. We swapped out the MAP sensor
2. We adjusted the timing (the distributor fastener was completely loose)
3. We reconnected the fuel tank pickup (the bottom part had dropped off)

The boat is running like a dream. Special thanks to all the guys that helped on this thread.


03-18-2008, 03:41 PM
If you noticed that the timing mark jumped around when you set it, as an FYI- the ECM needs to be put into "Service Mode" so you can set Base Timing. You can't do it right if this step is omitted.

Look on the rear of the motor for the DLC (Data Link Connector). Remove the cover that goes over the terminals and look for the White/Black wire and the black wires- they're in positions A and B. If you have the little metal scan tool (jumper) or a paper clip, insert the ends into A and B. You can do this running, or not. The RPM will drop and it may run poorly, but you need to raise the RPM to set the Base Timing, anyway. Get it to 1000 RPM and set the timing advance to 10° BTDC. Snug the distributor hold-down and re-check it. If it's off, make it 10° and tighten the bolt. Then, remove the jumper from the DLC it will be correct, unless the bushing on the distributor shaft is bad.

03-18-2008, 06:00 PM