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4000Rpm at Full throttle


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Good day

I'm wondering if anyone can help or has had this same issue
 

 I'm an owner of a Chaparral ssi 265 Running a mercruiser 496 HO Engine with bravo 3 drive with a 24pitch prop

at full throttle my alarm will go off…the  Rpm gage says I’m at 4000Rpm and when I pull back slightly it will stop…I also track my boat speed with gps and I’m only getting around 28-30mph, I know full well that’s not right, I’m pretty sure I should be pushing out around 4800Rpm and getting somewhere between 45-50mph

my temperature and oil gages are reading that everything is ok in that area, and the bottom of the boat was just recently cleaned and painted

does anyone have any idea what my issue could be? 

 

 

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10 hours ago, HailyHypes said:

Good day

I'm wondering if anyone can help or has had this same issue
 

 I'm an owner of a Chaparral ssi 265 Running a mercruiser 496 HO Engine with bravo 3 drive with a 24pitch prop

at full throttle my alarm will go off…the  Rpm gage says I’m at 4000Rpm and when I pull back slightly it will stop…I also track my boat speed with gps and I’m only getting around 28-30mph, I know full well that’s not right, I’m pretty sure I should be pushing out around 4800Rpm and getting somewhere between 45-50mph

my temperature and oil gages are reading that everything is ok in that area, and the bottom of the boat was just recently cleaned and painted

does anyone have any idea what my issue could be? 

 

 

This is a strange one and I ask if you are getting an "Over-speed" indication on the helm display. You failed to mention what year your boat is, but on earlier versions of the 8.1's, Mercs PCM-555 engine management system monitored raw water pressure, and when it was too low for a given RPM, it would put the engine into Guardian Mode, limiting output. The confusing part was this software iteration did not have a low water pressure designation, so Merc kind of stole the over-speed bit. In most cases this is caused by a failing raw water pressure sensor, which is located on the shell side of the power steering cooler. You will find that unit starboard, at the rear of the valve cover. Simple 2 wire plug on connector and the new sensor comes with pipe thread sealant already on the threads. 5 minute change out. The other possibility is that your raw water pump is actually not putting up enough pressure due to a restricted suction circuit, collapsed suction hose, failing impeller, or worn pump casing. The only way to know for sure is to either install a pressure gauge back at the sensor port with a simple tee, or use a scan tool which will give you a digital representation for real time pump output pressure. I like adding the gauge approach as it allows you to check your water pump health instantly anytime you suspect an issue, and for me I schedule my pump impeller maintenance based on the reading at idle. Replacement knock off sensors are available for a fraction of the price of an OEM Merc unit, and I have had mixed success with them. Get a good one, and they seem to last for years. Other-times they fail after a few outings. Pretty sure this is your issue if you are seeing an over-speed alarm.  W

8M6000623 8818793 Water Pressure Sensor Switch Sender for Mercury Quicksilver Mercruiser Mariner Replaces 8818790    On Amazon

 

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Something is way off, my 02 265ssi, merc 496 with 375hp would run dead at 55 mph at 4800 rpm with a Bravo 3 out Drive with 26 pitch props and 2.0 ratio Drive. Are you sure it's really the 496Mag ho motor 425hp or the standard 496 Mag 375hp motor? Alarm could be overheat or low sea water pressure.

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On 10/24/2021 at 10:20 AM, Wingnut said:

This is a strange one and I ask if you are getting an "Over-speed" indication on the helm display. You failed to mention what year your boat is, but on earlier versions of the 8.1's, Mercs PCM-555 engine management system monitored raw water pressure, and when it was too low for a given RPM, it would put the engine into Guardian Mode, limiting output. The confusing part was this software iteration did not have a low water pressure designation, so Merc kind of stole the over-speed bit. In most cases this is caused by a failing raw water pressure sensor, which is located on the shell side of the power steering cooler. You will find that unit starboard, at the rear of the valve cover. Simple 2 wire plug on connector and the new sensor comes with pipe thread sealant already on the threads. 5 minute change out. The other possibility is that your raw water pump is actually not putting up enough pressure due to a restricted suction circuit, collapsed suction hose, failing impeller, or worn pump casing. The only way to know for sure is to either install a pressure gauge back at the sensor port with a simple tee, or use a scan tool which will give you a digital representation for real time pump output pressure. I like adding the gauge approach as it allows you to check your water pump health instantly anytime you suspect an issue, and for me I schedule my pump impeller maintenance based on the reading at idle. Replacement knock off sensors are available for a fraction of the price of an OEM Merc unit, and I have had mixed success with them. Get a good one, and they seem to last for years. Other-times they fail after a few outings. Pretty sure this is your issue if you are seeing an over-speed alarm.  W

8M6000623 8818793 Water Pressure Sensor Switch Sender for Mercury Quicksilver Mercruiser Mariner Replaces 8818790    On Amazon

 

I will get a marine tech check this issue out for me with a scan tool, my boat is a 2002 chaparral ssi265

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On 10/24/2021 at 12:13 PM, Mi3sons said:

Something is way off, my 02 265ssi, merc 496 with 375hp would run dead at 55 mph at 4800 rpm with a Bravo 3 out Drive with 26 pitch props and 2.0 ratio Drive. Are you sure it's really the 496Mag ho motor 425hp or the standard 496 Mag 375hp motor? Alarm could be overheat or low sea water pressure.

Yes it’s The 425 Ho…and yes I know, something isn’t right, seems to me everyone is pointing me to low sea water pressure so I will get that looked at also I’m looking into the 26p props now 

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UPDATE: a marine tech came with his diagnostics tools and has indicated that my issue seems to be the neutral position switch, we took it on a test run while he had the computer hooked up and the diagnostics read neutral over speed limit which will restrict the motor to 3500 Rpm…I will purchase a new switch tomorrow and come back with another update for everyone 

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UPDATE: this is becoming very frustrating now… we changed the neutral position switch and the water pressure sensor, the boat is still doing the same thing…diagnostics is still reading neutral over speed, I’m wondering if I could have bought a faulty switch   

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13 hours ago, Mi3sons said:

When's the last time you replaced the pump impeller and checked the housing for scoring? We were leaning twords that rather than the water pressure sensor

The motor is like new…only 17 hours on it….the issue is the PCM thinking the boat is still in neutral from what we’re getting from the diagnostics…every time I try to give it full throttle, it restricts its self to 3500 Rpm and an Over neutral speed reading comes up on the tech tool 

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1 hour ago, HailyHypes said:

The motor is like new…only 17 hours on it….the issue is the PCM thinking the boat is still in neutral from what we’re getting from the diagnostics…every time I try to give it full throttle, it restricts its self to 3500 Rpm and an Over neutral speed reading comes up on the tech tool 

The shifter position switch is a simple 2 wire SPST unit. Get an OHM meter on it and test it. Not likely a new one would be bad. Check the old one too if you still have it. W

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On 10/30/2021 at 10:19 AM, Wingnut said:

The shifter position switch is a simple 2 wire SPST unit. Get an OHM meter on it and test it. Not likely a new one would be bad. Check the old one too if you still have it. W

I will try this tomorrow when my mechanic comes back thanks 

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21 hours ago, HailyHypes said:

Issue is solved…it was definitely the switch, the throttle leaver on the bracket to the motor wasn’t fully engaging the switch…thanks for everyone’s insight and help 

The cheap fixes are the best. Those switches are very reliable so likely the bracket was causing the issue all along. W

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