496 Mag/HO Newbie Help! Please!
 

Hi guys, I am new to the forum however I have always lurked around reading up on your good info! I have a problem with my 2002 29' Envison with a 496 mag. I have had a worry free 100 hours of service over the past seven years but now things have gone amiss again! When I bought the boat new in 01 I had to have mercruiser replace the motor since it was one of the very first editions and I broke off all the pushrod tops! My experience was a little jaded since the local performance marine shop here in Tulsa screwed it up royally! The boat ended up in Stillwater (short drive) where the techs replaced the motor. The shop is now out of business and I am a little leary of who to take it to!

Now for the meat of the problem! I do have LS motor experience in my hotrod but nothing on boats! I was running pretty close to wot last summer when all of a sudden the boat took a nose dive (single drive). It felt as it someone jerked the throttle back and I lost about 600-700 rpms. The boat starts and runs but has no power under load. I finished the weekend off and parked it for winter in the shop! I can hear the fuel pumps kick and my exhuast is always open. Could the computer have thrown me into 'Guardian' mode? Is there a possibility it is the dreaded crank sensor everyone talks about? Is there a way to test the crank sensor? Also what is the proper scan tool for this motor and where can I get one? The boat did not come with the smart craft readout and I am getting no warning beeps from the boat. Thanks for any help you can give me!

Shawn Tillotson
Split Decision Customs
Owasso Oklahoma

I felt I needed to add some info on the problem for the best help possible!

* No audible alarms, just the normal beep when the key is turned on

* Running in normal operating temp

*Normal oil pressure

*All fluids are checked out

*Had an new seapump put in about 45 hours ago at the dealer in lake of the ozarks

*RPM normal at WOT is 4950, currently will not exceed 4400.

*Lack of power occurs all the time, from try to get up an plane and at plane

*Motor sounds a little rich at idle, not as crisp as usual (possibly a coil?)

Thanks again for any help!

Fuel pressure? Do you think you could be running short on gas at higher rpms? There is an in line strainer on the side of the pump that caused me some trouble.

I wouldn't even guess without scanning it. Pay the diag fee (usually 1 hr) and see what the ecm has to say.

You should get an alarm if it were in Guardian Mode. If you are not getting an alarm, a scan may not tell you much. You might try changing the fuel/water seperator and checking fuel pressure.

After reading his post again I doubt it's the fuel issue I talked about as he says it isn't coming out of the hole either. My problem was she ran fine for a few seconds til she started starving for gas and would run but just seamed off of what it should have. After you would bring it back down or shut it off it could recover and would run fine again until you got back in it. This was also an 01 ho and if my memory serves me right my preasures would run around 42 psi and then drop to about 8.

First off, fixing a computer controlled engine without a scanner is like using a computer without a monitor. It can be done, but its not efficient.
Stupid question, have you checked your throttle cable? Is the throttle working properly?

Here is the rinda scan tool:
http://www.rinda.com/marine/marine.htm
I think its about $400
I'm going to buy one this summer

The throttle and cable attachment at the motor was the first thing that I checked. All seem to be going in the full range of motion. When this first happened I was running perfectly at 4900 rpm , but as I said it was as if someone threw out an anchor. There was an immediate RPM loss that I have not regained since. Is there such thing as soft faults vs hard faults on an 01 496? Will soft faults not sound an alarm an put it in a minimal guardian mode? I will check the fuel pressure when I get over to the shop, but it will have to be a static check with snow on the ground here there will be little chance to get to water. I have no strainers in the fuel pumps after fighting a huge fuel pump problem after the new motor in 2001. Ended up my pumps were getting clogged up from fibers coming from a leather welding glove in my fuel tank!!!!

A side note question.

How were the fibers from the welder's glove getting past the water seperating fuel filter that the fuel flows through first before getting to the first fuel pump?

Take a second and check your plug wires. My boat had the same symptoms last year and it was because a plug wire had fallen off the number 4 cylinder. Most likely not the problem but easy enough to check and eliminate. I also had a fuel pressure problem when my boat was new (bad booster pump) and I could not even begin to get my boat on plane with low fuel pressure (17 lbs. as it was trying to roll onto plane).

Do a comp check...Sounds like a rocker nut backed off or a broken push rod...That will kill the power and you would be running on 7 cyls. That would not throw any codes either. (it might be and easy fix. Pull the valve cover and fix it)

When a Fault Code is set there should be a soft alarm (two beeps per minute) and there will also be a reduction in Available Power (Rev Limit) to 90%.

With a hard alarm (constant beep) there can be an additional reduction in Available Power, and depending on the fault, it could go as low as 0% (engine shutdown).

You said that the WOT RPM dropped to 4400, which is exactly where 90% Available Power would put you. However, you should get an alarm and your performance at idle would not be affected. But don't get me wrong, I completely agree with the guys above who say that a scan is the best place to start.

I think you would have heard it if you had a rocker nut back off or broke a pushrod, but you could have dropped a cylinder for some reason, either mechanical, ignition or fuel.

Not a CKP, you would have a no start condition if the CKP was toast. It sounds like a non-critical sensor has "failed in range" meaning you will have no alarm, but possibly have a sticky code set which has limited your RPM range and does not sound an alarm. Could be a CTS, or an ETS, but definetly not a CKP.

If you want a scanner for yourself get one from:

http://www.rinda.com/marine/marine.htm

Get the Mercruiser Scan Tool, and you can scan your motor, and all your buddies motors too, as well as reset your IAC's, and perform a variety of other diagnostic and maintenance issues.

i had that happen to me a few years ago. i got back to the dock and opened the hatch. i was so pissed off and everyone knew it and they just kept making it worse. everyone had something to say. im checking everything and making sure everything was right and i remember this guy just standing there telling me" your not going to find the problem like that" then bam!! there was the plug wire. plugged her back in closed the hatch and told the guy to have a nice day. i love easy fix stupid things.

Being in Owasso, I would say take it up to Morton Marine. They are mostly a bass boat dealer, but also sell runabouts with the merc 496 and HO engines. They are a certified mercruiser service center. I had them winterize my boat in 2006, and they also helped out with getting my boat on its new trailer in 07. They have always treated me right and are good guys. If your engine is stock merc, take it by and have them run a scan.

If it isn't stock merc, I'm not sure who I'd send you to. My engine builder, Brad Smith, is in Joplin, and does a huge amount of high end marine engines. If you want to go that far, give him a shout.

The alarm could be broken.

I think you would have heard it if you had a rocker nut back off or broke a pushrod, but you could have dropped a cylinder for some reason, either mechanical, ignition or fuel.[/QUOTE]


I had a rocker nut back off completely and there isn't a sound except a very slight rough idle. Only loss of rpm down about 300 -400. Motor "carried" very well as a friend put it. Would even plane and run very fast on 7 cylinders.

I'm not at all saying from a distance that this is your problem. I am saying however, that it could be the problem and your description matches my past symptoms.

Good luck & borrow a scan tool

Yes I went to them and they said that there was not enough facility to work on a 30 ft boat. I was sure hoping they would take a look at it. I am going to see if they can pull the codes if not I am going to contact Ugly Johns and have a "interview" with them and how I expect my boat to be fixed.

Sunday I will pull the valve covers off and see what I can see, I just hope the gaskets are reusable. I am also going to pull the plugs and ohm the wires also and also check the fuel pressure statically sitting in the shop with the motor running.

check the spark plugs have seen these go bad for no reason motor will still idle and run pretty well:)

I would eliminate all the fuel, ignition, sensor, ECM related causes before I would pull the valve covers. Just my .02.

Depending on the year of your current motor at least the ones with Gen II cool fuel system often suffers from a bad low pressure fuel pump if not the inline fuel filter has been installed. The engine will still run even with the pump disconnected, but it will not rev out. No fault codes will show a faulty or clogged pump.
Check the fuel pressure at wot

Any updates?

He is correct. I personnally had a spark plug wire come loose on the plug (most likely not pushed on all the way). When you loose that cylinder everything works fine except you loose a % of the power and therefore max rpm's at WOT is reduced as well. In a car with full mufflers you could probably hear the difference when one cyluinder is dead but in a boat, especially with non muffled above water line exhaust I can hear no difference from in the boat.

Thanks everyone for your help! Sorry I have not got back with you on the update but I just figured it out today! Been working allot of hours and plus I have several muscle cars and the past couple of months have been consumed with going on the Hot Rod Power Tour. Too many toys not enough time. I have not had the boat in the water yet or even pulled it out of the shop this year. I changed my oil tonight and while poking around I did find a plug wire off of the plug. When this all went down I had a guy check all the wires while floating in the middle of the lake but should have done it myself. A local mechanic at Ugly Johns told me to change the water/fuel filter again, so I did. With a fresh oil and all holes alive now she sounded great in the drive way with the water muffs on. I will see how it runs this weekend, should be normal again! I hope so since I have been dying to get out on the water! Thanks again everyone, hope I can hang around more!

Shawn Tillotson
Split Decision Customs
Owasso, Ok.