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Old 10 January 2014, 08:43   #1
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Mercury 115 4 Stroke Guardian System

I installed a brand new Mercury 115 in my RIB three years ago & after 11 months and 100 hours the engine had a catastrophic failure after about 3 seconds of alarm warning of overheat. I do not drive the RIB hard & is regularly serviced by the dealer and is for cruising, not racing. Mercury replaced the power unit under warranty, but now two years later at 400 hours (300 hours on this power unit) the same thing has happened... catastrophic engine failure said to be due to overheating (a blocked water inlet by plastic bag???)... again, I only got 3 seconds of alarm and I immediately dropped the rpm to neutral and the same thing had happened. I thought the 'Guardian Software' was supposed to give more than 3 seconds notice of an overheat if that indeed is what happened)... Some are telling me that Mercury Engines are prone to this type of catastrophic failure & are best avoided... I am waiting for the engineers assessment of the damage, which looks like I will need yet another new Power unit (makes 3 in 3 years!)... would be interested in peoples views on this engine and the Guardian system that is supposed to protect it...

I will likely buy a whole new engine... not sure if I could bring myself to buy another Mercury though!...
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Old 11 January 2014, 01:41   #2
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warped cylinder head... engine will be dumped... how can that happen when the alarm gave me only 3 seconds and the water temperature was only 3/4 hot ... not even in the red when I switched off... went up to red after switch off... my alarm has only ever gone off twice & both times 3 seconds before engine catastrophe ...
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Old 14 January 2014, 15:09   #3
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Are we talking about an optimax or four stroke? Either way it's very unusual. Sounds to me like the problem has not been solved from the first failure? What caused the first failure was it piston damage? Has the boat had its fuel system checked for leaks I have seen multiple powerhead failures before that have turned out to no fault of the engine but poor fuel supply causing detonation of pistons, the dealer just put a new powerhead on and it went bang again so I would advise find out what's going on and what is failing and what's causing it. So the guardian system is pretty good but it won't pick up on a mechanical failure so if you did say for example have a piston let go the guardian would probably only pick up the last minute over heat transferred through the block rather than the water/Block temp sensor. Hope this makes some sense!
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Old 14 January 2014, 17:43   #4
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Thanks for your comments. My Mercury 115 is a 4 stroke. both power unit failures was said by the Mercury Dealer to be due to overheat & loss of compression due to cylinder head warping, but they were unable to identify why the overheat took place & why the alarm only have 3 seconds notice. I have taken it on longer journeys with no problem & both failures happened after about 45mins to one hours running and about 300 engine hours apart & the engine seemed to run perfectly otherwise...
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Old 14 January 2014, 18:04   #5
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How old is the engine?
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Old 15 January 2014, 05:28   #6
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3 years old & I purchased from new and I do not drive it hard and have it regularly serviced by the official Mercury Agent, The first power unit failed at one year (100 hours) & the new replacement power unit in the same engine failed after a further two years (three years in total) and after a further 300 hours (400 hours in total). normally cruise at around 4000RPM unless rough weather when I slow down. The RIB is used in salt water & the engine always fresh water flushed after every use
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Old 15 January 2014, 15:20   #7
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Was it just a block that was changed? If so it sounds to me that there may be an issue with an electrical component ie fuel map setup in the ECM or something like that if the same failure has happened with a second block. I have never seen this before in these engines and I am assuming that it is an L4NA engine and not a Yamaha based one? The L4's will do high hours with ease so this should be investigated for sure.
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Old 15 January 2014, 17:26   #8
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it was the whole power unit that was changed. I am not sure if that included the ECU map or not. I don't think it did, because the ECU showed 400 hours, whereas the new Power unit had only done 300 hours (+ first power unit 100 hours). If it was a problem with weak fuel, I am a bit surprised it lasted 300 hours with some journeys much longer than the one on which it failed, anyway, you may be right. I am not sure if L4NA, but I do know that it was made in the USA. The general consensus is that the Mercury 115 (4 stroke) is generally a reliable engine and this type of repeat failure is rare?
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