Stefin298

I have dual 2003 350MPI engines (bravo 3 drives with 26 pitch blades) and my starboard engine is having issues underload. Once on plane, it wont pass 3100-3400. No load it hits the rev limiter. Port engine has no issues getting to 4100 (normal for my boat with those props). If I keep the throttle at 2/3 (3000 rmp) no issues, but if I WOT, it wont increase more than around 3400, then backs down to 3200+/-. I just changed the fuel/water seporator. Plugs were done at the start of the season too. Just replaced the IAC valve on the throttle body due to a code being thrown for this to be replaced.

What else should I try? Would a failing drive cause it to not go past a certain RPM?

n20michael

Did any other codes come up on the scan tool? It COULD be something as simple as a bad sender.

johnnyboatman

iTrader: (5)

in gaurdian mode?

Stefin298

No other codes came up when my mechnic pluged in his cpu 2 months ago. I guess i need to have him back out if it is in guardian mode. When the IAC went out, the alarm beeped twice every 2 mins. No alarms currently.

Would bad fuel cause it to go into that mode? That would be odd that it is doing it on one engine but not the other if fuel.

spectras only

Quote:

Originally Posted by johnnyboatman

in gaurdian mode?

Sounds like you may have a fuel problem and the computer set the guardian mode like suggested above. If you have SmartCraft
it would tell you to throttle back .If you don't, the ECM won't let you go past 3000-3500 rpm . Get a fuel pressure gauge from a speedshop and check the fuel pressure on the Schrader valve on the fuel rail.You should have 43 psi . If it goes much higher you'll run the injectors too rich , hence the guardian mode. It could also be a problem with the opposite , low fuel pressure . I assume you have a fuel cooler that has a system with filters in the inlet of the high pressure pump and a small filter on the cooler under the 43psi pressure regulator.If this small filter [ approx 1/4 "] gets clogged it doesn't allow excessive pressure returning to the water separator, resulting high fuel pressure at the fuel rail .

spectras only

Check this part , # 16 [ filter ]

http://www.marinepartsplus.com/catal...500+99/4856-90

Make sure the low pressure boost pump and the filter before it is good, too.

Stefin298

After reading some other threads, i am wondering if it is going into guardian mode due to the psi in the sea water pump. I noticed that the stbd engine heats up much faster than the port (like stbd 175 vs port of 130) and takes longer to cool down at idle. I might check my impeller 1st.

spectras only

The merc thermostat is 160* , and reading of 175 is OK .130* is too cold and may cause condensation under the valve cover. Those temps you've posted won't cause the ECM to go into guardian mode though. If the seawater pump impeller on the way out you'll hear the alarm go pretty quick besides the temp gauge climbing past 175 .

Stefin298

My engs have always stayed at 175 once warmed up, from 1/2 throttle to WOT (stbd might go up to 180-185 under heavy load, but always backs down). What I meant was when warming up while underload and on plane, the stbd eng gets to 175 very fast, the port takes twice as long. I read on another post about some 496's that went into guardian mode due to low psi in the sea pump. Just trying to do some outload thinking on what could cause it, but I guess the fastest way would be to call my mechnic

spectras only

mercruiser recommends replacing the seawater impeller around 100 hrs or once a year . The impeller can last 200 or even 300 , but sometimes they're gone at 20hrs. Boats in marinas are better off because the pumps full with water. Trailered boats need to be primed sometimes before startup to avoid a dry start that could ruin the impeller in seconds.
When the impeller is dry in the housing it can get stuck to the houssing ripping the vanes right off on start. Operating in silty conditions like in rivers , clay can deposit in the housing and when it dries the results are the same. You may have a worn impeller that still pumps water , albeit inefficiently.
This impeller lasted 188 hrs with consistent 20-22 psi prior to this result on start up.


What I recommend to anyone before starting these engines to go boating is , loosen the serpentine acc belt and see if the seawater pulley can be turned by hand in the rotational direction.It should have resistance but doable. If the pulley is impossible to turn by hand , the impeller must be inspected or replaced period.