Starboard Engine Issue

glenncal1

glenncal1

Active Member
Nov 6, 2009
158
colorado
Boat Info
2000 380AC
Engines
Twin Merc 7.4 Horizons
On my Starboard engine (2000 Merc MPI Horizon). I can not get the RPMs above about 2600. When you try to advance the throttle past that the RPMs will fall to about 2000 and can then be coaxed back the the 2600 but not much above. The engines have recently been extensively serviced and both run well with the exception of the Stbd only going to ~2600. Any ideas? A couple of questions:

1. Would the engine run if only the lift pump was working and the high pressure not?

2. Same question if the fuel pump relay had failed or was in some faulty.

We pulled the new fuel filters and the fuel was clean with no water. Have a guy coming with a pressure tester. The confusing thing is how well the engine runs at 2600 but wont go any higher.
 
Replace the Ignition Module under the Distributor... Mine did the same thing
 
Do you have injectors? If so, they would be suspect. Also, Plugs/wires. Could just be a bad plug wire or two. I chased around an intermittent problem for 2 seasons. Turned out to be a bad plug wire end. Also had issues with injectors clogged (gen 3 series paint issue) that caused similar issue. Just would not go above 3250 rpms, but did not balk down when going to WOT, just didn't go above 3250. I have different motors (496mags) but these things give you some things to look at. I will say it sounds fuel related, but ya just never know. Another thought, check your voltage going to motor at the higher rpms (including belt tension). Some motors after a certain rpm run off the alternator power and not the batteries. If you are not getting enough voltage/amperage at the higher rpms, this would certainly bog it down as not an adequate spark would cause poor or no burn.
And as West32 stated, the ignition module. Check that first.
 
I have a very long multi page thread about this very issue except with the port engine. I wound up doing multiple things. try searching for me, and fuel pressure.
Fuel pressure checking while running the boat under load, not just at the dock is key to determining what is causing your restriction. There are multiple pieces that can cause the same symptom, making it harder to troubleshoot.
 
Korkie/370D- when you had your fuel injector issues how did the engine run at RPMs below your stopping point?
 
Exactly how you said. There is a test. With just that engine warm and idling, and paying close attention to how smooth or actually not smooth it is running, spray some engine start fluid into the air intake and see if it smoothes out. That is how my marina guy diagnosed mine. He is not a Merc guy, but a Yamaha, Volvo guy with 35 years of working on boats. He did say that this is frowned upon by some folks, but works and he has never had any ill effects to any motor from doing it.
 
West is this the module you are referring to:

ign mod.jpg
 
I would start with some basic diagnostics.... comp ck, fuel press ck, look at the plugs, ck spark at each plug, ck fuel filter, etc....
Report back with your findings and I am sure someone on CSR will point you to the source of your problem.
 
Wonderer- have done those checks other than fuel pressure (having that done this week) I have ordered a new ignition module and will install that before we do an underway fuel pressure.
 
This sounds very similar to what I experienced 2 years ago, but with a 2000 5.7 MPI. The engine was going into some kind of protection default mode (I forget what it's called) by design. The engine had started to overheat due to a bad raw water impeller, and stayed in the mode until the mechanic reset it somehow.

Did this start quickly, or over time?
 
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