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496HO Guardian, overspeed

Old 09-13-2009, 09:32 AM
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Default 496HO Guardian, overspeed

One of my 2002 496HO's went into guardian mode last weekend at WOT. Alarm shut off after 30 seconds of idling. Scanned pcm, was in guardian for 129 seconds. The current codes that show up are for overspeed and guardian. In the overspeed data list it shows 4126 rpms and 39% tps. Rev limit is 5150. The data for the guardian code shows 4792 rpm's and 99.87 tps. Both codes at the same hours. Neither show enough rpm to hit the limiter. Did I over rev or is something else going on?
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Old 09-13-2009, 03:44 PM
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I believe, BUT i'm not 100% sure it works like this.

If something eles is wrong and the engine guardian kicks in, It will lower the engines power out put to within a certain % range, Say from 100% all the down to like 6% Now if you are cruising a long at WOT and your sea water pump starts to fail for some reason "Air bubbles" ECT, Then the water sensor will tell the pcm it's not right and the ecm will lower the engine power % to say 50% but you are still trying to go 100% and now the pcm thinks you are over reving the engine by 50% for example, Then when you go to look it up on a EFI scanner in the history area that is what you always seem to find is over reving. The big test you need to do is go run the boat with the scaner hooked up and see what all the sensors are doing in real time to see if they stay with in the range they need to be in, And then go see if something falls out of range look at the engine % power then look at the engine overspeed count to see if it is counting also. The most common one to fall out of range is the sea water pump sensor and for some reason that can not be picked up in the history some times. This is just my opinion of how the system works But not sure. Good luck with it.
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Old 09-14-2009, 08:23 PM
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Thanks, kind of what I was thinking. I will go over my printouts and see what I can find. I don't have the cableing to use the laptop to read the boat all the time. I have to borrow from a friend of a friend.
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Old 09-14-2009, 11:00 PM
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Originally Posted by boat teck
I believe, BUT i'm not 100% sure it works like this.

If something eles is wrong and the engine guardian kicks in, It will lower the engines power out put to within a certain % range, Say from 100% all the down to like 6% Now if you are cruising a long at WOT and your sea water pump starts to fail for some reason "Air bubbles" ECT, Then the water sensor will tell the pcm it's not right and the ecm will lower the engine power % to say 50% but you are still trying to go 100% and now the pcm thinks you are over reving the engine by 50% for example, Then when you go to look it up on a EFI scanner in the history area that is what you always seem to find is over reving. The big test you need to do is go run the boat with the scaner hooked up and see what all the sensors are doing in real time to see if they stay with in the range they need to be in, And then go see if something falls out of range look at the engine % power then look at the engine overspeed count to see if it is counting also. The most common one to fall out of range is the sea water pump sensor and for some reason that can not be picked up in the history some times. This is just my opinion of how the system works But not sure. Good luck with it.
Yes, you will have a new, lower "temporary" rev limiter if in guardian mode that calls for % power decrease. And it will beep once you reach the max of the lower number causing overspeed codes...
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Old 09-15-2009, 07:07 PM
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I show 8.4psi sea pump pressure. Seems VERY low. Impeller has 15 hours on it, do the pressure sensors go bad very often? That was at WOT, 99.87 tps, 107# boat speed, about 67-68mph on gps. Another oddity is engine load shows 36.74% vs 99.87 Demand.
Thanks for your input guys!
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Old 09-15-2009, 07:25 PM
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Originally Posted by GO4BROKE
I show 8.4psi sea pump pressure. Seems VERY low. Impeller has 15 hours on it, do the pressure sensors go bad very often? That was at WOT, 99.87 tps, 107# boat speed, about 67-68mph on gps. Another oddity is engine load shows 36.74% vs 99.87 Demand.
Thanks for your input guys!
if the system is seeing 8.4 at wot then it is a problem. it should be reading between 25-33 ish..
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Old 09-15-2009, 09:18 PM
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Originally Posted by SDFever
if the system is seeing 8.4 at wot then it is a problem. it should be reading between 25-33 ish..
SD, Thank's for the info on the over reving system the pcm sees, Over the years of seeing the over rev counts here and there, That was all I could come up with on my own with out asking or calling merc about it, And I don't believe I remembered Jim at school ever talking about A new lower "temporary" rev limiter if in guardian mode. And I never stoped to test it myself to see if it really worked like that.

Also [FYI] I Have seen new engines at WOT on the lake with water Pressure at 17.8 psi with every thing working right and no warning horns and no power % down at all. So GO4BROKE if you only have 8.4psi at WOT then you might have a blockage some place in the cooling system. If you had any service work done to the out side transom area by the drive then you might want to look at the inlet water hose behind the bell housing for a kink or twist, It is easy to kink that hose if you are not carefull down there by the ujoint bellows. Or the sensor could be bad or the sensor is not getting the right grounding because of something like paint around the threads of the sensor, Or it could be a short some place in the wires or PCM,Good luck with it.
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