What did the scanner show you ? :)

The scanner showed nothing out of ordinary I'm waiting for a free chance to get it on the boat out in the water to see what it says with the engine under load

Both the cam and the crank sensors can have issues without throwing codes.

The Cam sensor is the timing chain cover. There are 3 different generations of Cam sensors and they ARE NOT interchangeable due to interference issues. There are posts on this. If you have the "black" cam sensor, those are NLA new, but can be found on eBay and other places.

The Crank sensor is on the top, aft, just starboard of center side of the motor. It has a single bolt that holds it in, and removing it can be a horrible experience as they can shear off. There are posts on this here on the past.

BAM Marine - Mercury Marine & Mercruiser engines and parts, Cyborg transmissions has great diagrams on where parts are located.

thank you! I'm firmly convinced it's one of these I am going to try it one more time this weekend I cleaned all the connections since it seemed to all be related to getting saltwater on something in the bilge. But if it's no go I'll leave it on the trailer and hop on my buddy's boat for the poker run and go ahead replace both sensors. I do have an early 496 with black cam sensor so I will try and see if BUP has any left or I will just order one on eBay I found

there was 6 or 7 changes thru Merc - each cam sensor is app specific thru engine serial numbers and or block codes. here is the jist on ebay - one is they have no tech support nor really does the selling party on the net or ebay knows which cam sensor that needs to be installed in your Merc.


2 is the unpackage / unboxed cam sensors that is sold from whomever on the net are the ones that did not pass QC from the manu for whatever reason inwhich are minor to junk.. These are bought pennies on the dollar all thrown in one open box unpackaged instead of thrown in the trash.


If you can get the correct app specific cam sensor still in the package or box that is great. I would scoop all of them up even if I did not need one currently. Down the road you are going to need one with out a doubt for your marine 496. Saying this to all 496 owners.


I had other cam sensor runs made - 2 different versions one for the first year Gen marine 496 and the next 2.5 year apps of the marine 496.


For the OP I would look at your 5 volt circuits and see what that voltage reports and is it steady. Also IAC hate heat but this IAC runs very hot at higher duty cycles ( reason part of the failures that's common with this IAC) and since you overheated the engine - I would recheck the IAC and its circuit and connector to it for started


Next is would run the engine on a portable fuel tank like what outboards use. with new fuel line with a primer bulb and fresh gas. Fuel pressure testing as well.


I do have to make a trip to Florida in weeks to come if this helps ?

Boat owner story as he told me and then after the fact brought his boat to me last year, Its a early 2002 496 in a Baja - his cam sensor went bad as he had no engine start. They changed the crank sensor - still no start- his buddy had a 496 early 2003 version so they swapped the cam sensor out - the engine started, Easy fix he thought if he could find the correct cam sensor and did on ebay for I think 100 bucks or something like that. Brand new old stock unpackaged cam sensor. His boat was stored at a lake house above the water, They installed the brand new cam sensor from ebay - he dropped the boat down in the water to test start it. the engine started but ran very poorly and then back fired multiple times, so he shut down the engine. Well the engine sucked water upon that testing and hydro lock. Anyways short jist of it - sometimes you have to watch what you buy. I thought I would pass this info on.

Hey BUP great to hear from you Ive seen you chime in on many forums and i tried to research it on my own but couldn't find something quite like my situation. Today i put in a new IAC valve going to try and test it out tomorrow. My block code is 0M664155 if that gives any insight. Which 5 volt circuits are you referring to? I'm still kinda new to a lot of this but learning quickly and mechanically savvy enough to do them. I will check fuel tank as well, although I might add i was reading good fuel pressure at the rail via mechanical gauge. Im not going to put anything past this motor at this point I swear its more moody than my girlfriend.

If you're in my area give me a shout I may not sucker you into helping with boat but i turn 21 the 25th of this month so we could get a drink and talk boats

Alright guys I finally got it figured it out mostly. Would you believe it was a water pressure sensor it was in some sort of guardian mode without beeping or throwing a code. One of the times I had it out to test it though I had running decent until I tried to go over 3500 rpms it finally started beeping and shutting it way down to nothing but an idle and I happened to have the vesselview app on my phone and it showed low water pressure caused guardian mode. Checked impeller and passages for debris all looked good, put new sensor in and tested it today and it ran almost flawlessly.

Next question. After running for quite some time at 3500 I tried to go WOT for a moment but as soon as I rolled into the throttle I got an exhaust manifold over temp port and stbd. Which put me back in guardian mode. Engine temps never got over 170, there seemed to be plenty of water coming through the exhaust but it did seem fairly hot to the touch. I checked for blocked passages and nothing, when I did water pressure sensor I pulled sea pump off and verified impeller was good. Is it possible both sensors could be going bad? Or are there any other causes of manifolds getting hot besides blocked passages, or maybe some other areas I could check for water flow problems. I do plan on doing a coolant flush as what's in there is fairly old. But with the motor not overheating I don't see that being the issue. And motor temp was read with gauge on boat and with gauge on the vesselview mobile app on my phone

Crank sensor is right by balancer. Cam sensor is, I believe in what goes where distributor used to be.

crank sensor is located engine back top portion
cam sensor is in the front side of the timing cover

Exhaust manifold & riser overheat can be - the heat exchanger end cap gaskets causing water flow restrictions. Also the temp sensor themselves can be bad . Also water jacket area restrictions. You can test water pressure with the scan tool.

Also you can do water flow volume test to see how much water volume the impeller puts out.

Also testing with clear water hose methods is another way to check water flow and if there is any air bubbles in the water flow itself. Cooling water with air bubbles makes for poor cooling. You can test in sections to pin point were the problem(s) lies.