alarm going off at wide open throttle

T

tugger

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Nov 11, 2019
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boatless
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Hello, I have a survey on Monday and no way to get to a mechanic or diagnostic reader for a code before then so I hope some of you have some ideas with all your experience. I have a 2007 320 Sundancer with 2 350 mercruiser mag mpi with v drives. My starboard engine will go wide open throttle with the trim tabs up to about 4550 rpm and my port engine will go approximately 4800-4850 rpm at WOT. If I have the trim tabs down and go WOT the rpms are about 200 less on each engine and maybe 2-3 mph slower but no problems and no alarms. When I put the trim tabs up and go WOT at the full rpms I described above the starboard engine is fine at the 4550 rpms but the port engine at 4850 runs great but after 5-10 seconds I get an alarm and the engine doesn't shut off but goes into safety mode. Any ideas of what this could be and how to fix it. Thank you for your help
 
Tabs being down on that boat will slow it. Normal. Alarm sounding isn't. Smarter guys will chime in here, but my gut says 4850 is a hair too high. If I remember, max WOT was 4800. Gents, help the guy. Will 50rpm set off an alarm?
 
What about temperatures, oil pressure, and water pressure? What did they look like?

What if you back the port RPM's down, does the alarm stop?
 
without serial numbers cant look up the exact engine but there are at least two specs
4600-5000
4800-5500
neither should be a over rev alarm at 4850
As @JimG points out what are the temps and oil pressure?
Have you got Smart Craft display that you can check for low water pressure?

Is this your boat or one you are looking at?
The difference in revs seems like a performance issue or mismatched props?
 
thanks all. I called mercury and they said the rpm spec for my engine is 4800-5200 rpms. It is my boat and wanted to see if I could solve it for the people buying it. The mercury rep said the water pressure should be between 11-20 psi. Does anyone know what the oil psi should be at 4800 rpms? serial numbers are 0W694854 and 0W694855. I figured since I was at WOT with the trim tabs down so assumed the oil and water pressure was adequate. Temperature was spot on at 154f so figured the water pressure was ok and according to the analog gauge the oil pressure was just above 50 psi, right where it has always been and consistent with the other engine. Could a faulty water pressure sensor be the cause giving the ecm a false reading even though the temperature is good? Also, could it somehow be a result of about a 250-300 rpm difference between engine output? Again, all is good with trim tabs down and WOT. u guys are the best, really appreciate your help.
 
Need more info, read post 6, obv. you dont have a scanner which would be very helpful to your solution.
 
This is why you need to scan the codes, otherwise you can start throwing a lot of parts at a problem like this.
Water sensor is about $165 for OEM
Sometimes they just get clogged it has a small hole in the sensor.
Sometimes water gets in the sensor (Mercruiser started sealing the newer ones better)
But there was a period many had issues with that sensor.
The water sensor should be at the trans cooler on yours above the gear. 1" deep socket to get it off.

p.s. You could swap port and starboard to test

0148739_mercury-mercruiser-8m6000623-water-pressure-sensor_550.jpeg
 
Last edited:
I have a brand new oem water pressure sensor I will try. Answer to post #6 and other questions I missed are it one long alarm and goes into guardian mode, doesn't shut off but I back the engine down to idle and then she's good to go again.
 
tugger said:
Hello, I have a survey on Monday and no way to get to a mechanic or diagnostic reader for a code before then so I hope some of you have some ideas with all your experience. I have a 2007 320 Sundancer with 2 350 mercruiser mag mpi with v drives. My starboard engine will go wide open throttle with the trim tabs up to about 4550 rpm and my port engine will go approximately 4800-4850 rpm at WOT. If I have the trim tabs down and go WOT the rpms are about 200 less on each engine and maybe 2-3 mph slower but no problems and no alarms. When I put the trim tabs up and go WOT at the full rpms I described above the starboard engine is fine at the 4550 rpms but the port engine at 4850 runs great but after 5-10 seconds I get an alarm and the engine doesn't shut off but goes into safety mode. Any ideas of what this could be and how to fix it. Thank you for your help
Click to expand...
Do you have Vessel View or Smartcraft?
 
I don't know what you have for electronics in the boat but the North Star GPS in mine from 2007 is capable of reading smartcraft codes.

I actually wound up adding the gateway to connect it to the smartcraft after the fact but it was an option. If your boat has any of the North Star or navman electronics from that era you may have the capability to read them
 
I have a garmin and led smartcraft readouts on both of my tachometers and thats the extent of my electronics
 
I would check the water pressure. I've never had a wide open throttle pressure issue, but when I blew the hose after the pump before the pressure switch on plane the alarm sounded and she fell right on her face in guardian mode. It did not show an alarm on the smartcraft screen but I was able to find it in my Northstar.

I'm wondering if that additional 2 to 300 RPMs when the tabs are up and the bow isn't pushing as hard as enough to push it over and threshold. Could be as simple as a fouled strainer and you pick up a little more air with the bow up a bit. Could be worn plates in the pump and it's cavitating if it has the bronze pump with the stainless inserts.

Could be the hoses collapsing under vacuum, or it could just be a failed pressure switch.

I'd see if I could get it scanned, the code should still be in the ECM with an hour stamp
 
UPDATE: I spent all day at the boat, the trim tabs have nothing to do with my problem, only that when they are up the engine rpms go up from 4500 to 4800 which is where I have my problem. I payed close attention on a test drive to the water pressure based on all your great feedback and the engine water pressure is fine up to 3500 rpms, it's over 8.2 at 3500. Then as the rpms increase the water pressure goes down to 6.5 at 4500 and then falls to the low of around 6.2 when I go WOT, 4800 rpm and alarm and guardian mode shuts that engine down but it does stay on, I just go back to idle to stop the alarm. So I replaced the water pressure sensor and it didn't make a difference. THEN I saw that the connection from that engine to the water heater(which doesn't work) had a huge leak in the connection, the connection on the water heater corroded off. So I took the hose and bypassed the water heater by taking the hose that came out of the engine to the water heater and looped it back to where it goes back into the engine. I was sure that was the fix so I went out for a test drive and there was no change! I was so disappointed. I replaced the water intake pump 6 months ago with a stainless steel hardin pump so i know the impeller is good. Any other ideas? Thank you so much
 
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