Hi, Thanks in advance for any help. I have a constant alarm going off on twin 2005 525 efi when running fairly hard. Port tach bounces when over 3000 rpm( motor runs fine) but no alarm until higher rpms are reached. Motors seem to temporarily go into guardian mode when alarm starts but once I've chopped the throttles all seems ok again( no alarm and no loss of power) until I repeat the above scenario. No codes on either motor when scanned. All good so long as I stay around 3000 rpm. Thx again.

I chased a similar gremlin last season. at 3000 rpm my tac would jump like the motor was cutting out then i would get an alarm with no code. replaced distributor, coil, modual, didnt fix the issue. turned out to be the wire harness plug, spliced in new harness plug and problem fixed.

Mine sounds a steady alarm when rpms get close to guardian mode which kicks in at 5,400. I don't think I've ever actually gone into guardian mode but I have it propped close to 5,400 in ideal conditions.

Thx for the ideas. I've also been told to check out the water psi switch/sender. I hope to run this weekend after going thru these ideas. Will post results. Thx, Mark

Also check you HP fuel pump. There is a little screen inside and may get some restrictions. I had the problem with 496's. Similar fuel pumps.

I highly recommend running the boat with the Rinda/scanner hooked up. That's going to tell you exactly what is going wrong.

www.rinda.com

Anyone with a modern Mercury Racing/Mercruiser powerplant should own one. It's the best money you will spend on your boat, because it buys you time. Marinas/shops are often WEEKS out on getting a boat in just to be scanned, and all you may find is a $34 part needs replaced which cost you 5-6 weeks of summer.

You would think I sell the damn things as often as I post that.

I totally agree about the water pressure sensor. Also for your motor crankshaft sensor problem / crankshaft position sensor rationale. Both of these are steady horn and guardian can become active with these faults.

I am not dogging anyone here.. Scan tools are needed but a lot of the times the end user can not properly figure out what the parameters mean from the scan nor what the base lines should be beforehand. Very common issue even by some techs who do this as well.

The OP here says no codes - did you scan your engine - if yes with what scan tool - lets say Diacom - do have the latest version or even able to decipher the scan.. . Just asking. Again no intent of a dog here

Problems with ECM and PCM can be traced back to your 5 volt reference signals. Especially if the engine is Smartcraft compatible. These are missed a lot during a scan where the problem lies..