pluguglyinhere

I will have to get the engine info when I get back out there. There are no warning beeps until it does this, so what I mean is when it starts to stumble off idle in gear I get a constant beep. The beep goes away after I get back to idle. I dont get a beep if I get in it hard moving up to 4 grand fast. All three times it has stopped doing later and ran fine all the way back in.

Is there something that would create low water pressure, like a valve or something that the shop may have not completely taken care of during un-winterizing. I dont know what the process is for that but could something be causing low pressure and then resolve itself.

I know things can act weird but I would think a fuel problem would be a problem at WOT, middle and cause trouble the whole trip. This is only easing off idle, the same place exactly in the RPM range every time, at the beginning of the day.

What does Gaurdian act like when it trips? I know you cant get full power but does it just shut down, misfire how does it act? This again goes right back to normal nice running motor when you bring it back.

The IAC did not test good. I may have done the test wrong but it was pretty easy. Hit start and add increments on the software and it was supposed to change the motor idle. It changed nothing either way + or - which is a failed test. I wonder also if the IAC causes it to stumble, which a stumbling motor causes a low water pressure code.

Thanks for all the help and I will continue to troubleshoot with your suggestions and post the results

BUP

iTrader: (9)

Your wording "constant beep" as I take its a solid continuous warning horn / beep ? If that is the case there is 8 active faults that can cause this for a NON DTS AND NON CATALYST 496 mag and HO - PCM 555. the biggest one of course is low or no engine pressure

2. over speed
3. ECM memory
3. Multiple CAN circuit faults
4. battery voltage
5. guardian strategy
6. EMCT overheat
7. ECT overheat

anyone of these faults can limit available power to 5% to 90 % depending on the actual fault, again this means the guardian is active.

ECT overheat means = Engine Coolant Temperature and or the sensor, make sure all the cooling systems is providing enough volume of water to cool the engine properly along with the closed cooling system and all related parts.

EMCT overheat means = Exhaust manifold coolant temperature. This code happens a lot with the 496 also and sometimes no code at all. The sensor in the exhaust manifold can be bad. Also any water restrictions and or any water restrictions to or in the exhaust manifolds.

I would change your water pressure sensor as I mentioned before for starters.

pluguglyinhere

All fixed it was the pressure sendor as Diacom said it was. Thanks for all the info and help. I also now know what Gaurdian act like when it happens. We were able to get it to do full Gaurdian (only idle) and after cleaning and checking connections (before just replacing the sensor) got a 90% power with two beeps. I am much more informed now thanks again.

modvp

This is a great thread; can you post the part number for the water pressure sender?

Rdegeo4140

I had a similar problem on my original 2001 496 MAG and it even would go into guardian mode at full throttle at times. After a years (4) of checking I found out that there is a small fuel booster pump on the front of the engine. So the engine has 2 pumps. No more problems.
861155A 3 - FUEL PUMP ASSEMBLY
Fuel Pump - Electric low pressure Fits MCM/MIE 4.3L/LH, 5.0L Gen+, 5.7L Gen+; also used as boost pump on MCM/MIE 5.0L MPI, 350 Mag MPI, MX 6.2L MPI & 496 Mag/8.1S
Part Number 861155A 3
Manufacturer Mercury Quicksilver CURRENT

Sydwayz

iTrader: (1)

Dated post, I know, but:

Check here for parts for your engine:
http://www.mercruiserparts.com/Selec...asp?class_id=1

Sydwayz

iTrader: (1)

The 496HO has this dual fuel pump setup as well. In troubleshooting my RPM issues, a couple weeks back; at one point... I inadvertently left the dock and ran the boat, leaving the boost pump unplugged. The boat starved for fuel over 3000 RPM. I waited for almost 3 hours for everything to cool down so I could change the pump. When I crawled in to change the boost pump, THAT is when I found the connector was either loose and came apart, or I forgot to hook it up altogether.

Turns out, my RPM issue was the Crank Position Sensor, best I can tell.

low_psi

I had the same issue a while back. Replaced the sensor and all was great. Took the original sender and tapped it on the work bench a few times and a few grains of sand fell out... Put the original sender back in and never had an issue. So where I am going with this, if you boat in shallow sandy water, your original sender may still be fine, it might just have sand in it.....