WOT Power issue with 8.1

Korkie

Active Member
Feb 2, 2016
1,329
Leonardtown, MD Potomac River / Chesapeake Bay
Boat Info
2006 340 Sundancer, Garmin 7610, xHD 24 Radar, VV703, SeaStar Hydraulic Steer, Shadow Drive AP
Engines
Mercruiser 496 Mags
Bravo III Drives
New owner of a 2006 340 with 8.1s. Did the Sea Trial and everything good. When bringing it home down the Chesapeake, started to loose acceleration in the starboard engine. Was able to maintain 3300 RPMs with the throttle maxed. It was about a 3 hour journey and this started about a hour into it. I have changed the fuel filter and om waiting for a decent day to take her out and see if that helped. (weather not been cooperating). If no improvement, was going to change plugs and see if that helped. Could this be a fuel cooling issue ? I have seen bits and pieces about fuel cooling, but nothing as to what problems it causes.
 
check fuel pressure, it that is good do a compression test - thats what happened to me - I was low in two cylinders due to a blown head gasket (between the cylinders) and the valves were not sealing very well either (bad seats)...
 
With a 2006 I'm thinking you have SmartCraft/VesselView...3300 RPM sounds like guardian mode to me. Any alarms?
 
Check the "do no harm" stuff first! Oil levels and condition in the offending Engine and V-Drive.
Coolant level and condition. Then is there water flow out both exhausts? Is it the same?
A scanner will show you raw water pressure, but you may not need to go that far. If everything is visually good, I second the Spark Plugs. It is a reasonably common failure and the scenario is 'idles fine, fails under load (at higher RPM's), sets misfire code, puts engine in guardian mode'. Plus, plugs are cheaper than head gaskets
 
Check spark plug wires and plugs. Very similar thing happened to me during a trip (about an hour into it too) and it was a crack in a spark plug lead causing arcing to the exhaust manifold which lead to the rpm dropping on one of the motors.
 
Sea Ray Version of limp-home mode. Good question on how to clear it. I just cleared the codes with my scanner, but I bet there is something on the smart craft menu to re-set as well.
 
If the cause for the code is fixed, then guardian will shut off. Old code might still be readable in the history via the software.

here are some of the likely causes for reduced power:
bad spark plug
bad plug wire
bad coil
low fuel pressure - plugged filter, failing cool fuel module, clogged pickup, failing antisiphon valve
water in fuel
failing injectors

went through this last year on a friends 8.1. No codes, no alarms, reduced power that continued to get worse. Tried plugs, wires, coils, fuel filters, coolfuel module, new gas.... Did all the troubleshooting and swapping we could. Ended up using the computer and shut down each cylinder while under load and found that 3 of them hardly made a difference when shut down. Bingo - injector issues. New set of injectors and good as new.
 
You want to know what's wrong...well that depends, as you see causes could range from low oil, bad plugs, to blown head gaskets. Why not get a Merc tech to throw the scanner on and do some troubleshooting, chances are then that you'll only fix what is needed.
 
In my case I did firstly involve a merc tech but nothing showed up as wrong on the computer. To his defence the boat was out of the water by this stage so limited ability to take it for a run and test other than hook up the computer. He also didn't charge me for his time.
 
That was my plan. even had one lined up only to be stood up twice. Only issue is I have no Merc mechanics near me. I have mechanics, but so far none are not actually certified Merc mechanics. Have even been trying to find one that would come to me and I would pay travel. No luck there either. I was given a marina I didn't know about to try, hopefully that one will work out. Nearest auth I have foubnd so far is a 3.5 to 4 hour one way trip. I am one the Potomac in the area of St Clements Island.
 
Consider ordering Rinda software and diagnose yourself. I think it was $500 or so with cables. Great tool to have in the bag.
 
It also might be the IAC sensor. That's a real easy fix. It's located on the back of the engine and looks almost exactly like a thermostat cover. It sits on the top back of the engine. You can swap with the other engine and see if the problem moves or not. Also it's a good idea to keep a spare IAC sensor on board.


Sent from my iPhone using Tapatalk
 
FYI - If you have DTS, there will not be an IAC.
 
Had the same problem. Bad plug wire. Your computer can adjust and mask the problem for awhile. Do you smell raw fuel at idle? That's a tell tale thing.
This is what I found after doing and checking everything. May not be your answer. Good luck.
George
 
Check all the ground connections especially the multiple ones that go to the lug on the engine. Lack of a good solid ground can really confuse the engine's computer.
 
Thanks for all the suggestions. Plan on taking her out tomorrow with a replaced fuel filter. If problem persists, change the plugs n try again, still persists - swap plug wires - then try swapping the IAC. Paulswagelock, What is DTS ?
 

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