mmanger

I had an alarm go off when at RPM over 3000.
I scanned motor for codes at the dock and didn't come up with anything out of the ordinary. I cleared the codes and took it out again. After approx. 10 min at 3,000 rpm the constant alarm sounded. This time the Merc scan tool was hooked up to the diagnostic port.
It gave me some codes I'm trying to figure out and having little luck thus far. The codes were:
19 Overspeed
12 low oil ps str
85 oil psi ckt lo
15 map input hi
9 Guradian

It also showed "Guardian unspecified" during PCM data scroll.
Anyone have any idea what these may point towards?
its a 496 HO 368 hours
Plenty of water pressure (sea water pump is not blocked)
Seemed to checkout fine according to standard gauges at helm. I did not see a drop in oil pressure. Water temps were under 180.
I have not run into these before. Any help would be appreciated.
I cant find a complete listing of what the scan tool codes are. I hate the abbreviated stuff.
Just looking for a place to start.
Thanks

SDFever

At end of day, MAP pretty much runs things. You can test the MAP Sensor before replacing it or you can throw one at it. Of the codes you listed and limited info it's a decent place to start. The 555 throws strange codes sometimes that make no sense when it sees something it doesn't like. You can't always take it verbatim. 19 and 9 are very common but not always accurate.

mmanger

I will try the map first because I got a new oil press sensor and it is not something easily changed. I am not even sure how I'd do it. Its in one of those next to impossible spots to get.
Probably have to pull the manifold off to reach it.

mmanger

Changed MAP sensor. No improvement. I am hoping its the oil pressure sensor.
Anyone have any knowledge or advice on how to replace the sensor? From what I see its in a bad spot. Wasn't sure if I need to pull the exhaust off or if there was some way anyone has gotten to it without. Of course its the starb motor so theres about an wrists amount of room between the risers. I was thinking maybe I can reach it from behind but that didn't work out either. Now its raining so I'm waiting for tomorrow to try again.

Raylar

To prevent useless removal of pressure sending unit, you can simply remove the analog oil pressure sender on back top of block and put a manual oil pressure tool on it. If it has low oil than rest is obvious.

28Eliminator

Not that this is your problem, but it's an easy check. Last year my friends 496 had a similar issue and we couldn't figure it out for awhile. It turned out to be low water flow to one exhaust manifold due to sand. We noticed it by touch on the dock that it was way to hot, but water was coming out of the pipe. The scan tool then showed port side was 50 degree hotter than starboard. We tried to flush it and it helped, but still had the problem. At the doc I had him give it throttle too 2000rpm and I pulled the hose of and it blew a bunch of sand out. turned the motor off and reconnected the hose and it has been running fine since. Just saying, those codes are not very accurate because it never did say hight temp on anything.

low_psi

The alarm/guardian at or after 3000 rpm, could very well be waterpressure. My pressure was fine, but had small amount of sand in the sensor, so it didnt see correct pressure. It would Guardian anything over 3000 rpm... Granted your codes don't indicate that as being a problem, I would tend to lean that way by past experience. As a matter of fact, mine did it last weekend. Right as I left the marina. I forgot we had a bunch of standing seaweed in the well where the hi-lo dropped the boat in the water.... As soon as I got over 3000 rpm, on came guardian. THis time it was a simple fix, clear the seaweed...

TWIN-SPINS

wouldnt hurt to check your main harness connections,,,the 10 pin connection