2008 8.1 Mercruiser Horizon Alarm over 3,000RPM

[DNB 340DA]

Hello - Over the winter I replaced my manifolds, plugs and wires on my 08' 340DA. I launched over the weekend and it ran perfectly up until I hit 3,000RPM's and my port engine set off an alarm and RPM's immediately dropped and went crazy bouncing around between 1,000 - 2,500 RPM's. As soon as I brought it back into Neutral the alarm went off and I was able to bring it back up to 2,800RPM's and it didn't miss a beat and ran perfectly... As soon as I hit 3,000RPM's it did the same thing... I tried restarting the systems and it did the same thing... (Shut batteries off, put throttles in full reverse and power back up.) I have single lever DTS throttles on my boat as it is an 08' which is the last year SR made the 340... I ran the boat to my slip which is about a 45 minute run and the boat ran absolutely perfect at 2,800 RPM's but as soon as I hit 3,000 the port engine goes crazy on me... Water pressure, oil pressure, and temps were all perfect as I kept my eye on them constantly... I just did complete water pumps and impellers last season. I'm thinking it's a sensor issue, but has anyone ever heard of this happening?? Thanks!

 

[Lazy Daze]

It's tough to make a guess on limited info. When it alarmed, what did the Smartcraft gauge show? What was the reason for the alarm? While that may not tell us exactly what the culprit is, it at least gets into the right system that the computer is complaining about.

But... I would start by double checking that your plug wires are connected properly and that they are clean/snug/tight. Also check your battery voltage - systems off and engine running. Check those battery lug connections as well.

 

[DNB 340DA]

Thanks for your input. There was no code on the smartcraft gauge which was strange... I will check all the plug wires and battery connections...

 

[Lazy Daze]

If the engine alarm sounded, there would definitely have been a notification. I'm not up to speed on Smartcraft enough to say for sure, but is there a way to look back through 'memory'? If not, it will definitely be available to see by a tech with a Merc laptop.

When the RPM's on the tach went wonky, did the engine follow suit? It's sounding like an electrical thing - but that's just a guess at this point.

Might as well check voltage at each battery, directly, with a volt meter and the ground cables removed. DTS is quite voltage sensitive. Just trying to eliminate variables here.