I have a 2001 Mercury 496 Mag. Recently I was out on it running about 3000rmp and it shut down. The mechanic told me it lost fire to the plugs. We have replaced the Crank and Cam shaft position sensors. It regained fire but it ran rough so we replaced the Iac. We then replaced the plugs and took a fuel sample. The fuel was fine, the plugs were all fowled on one half, evenly and all in the exact same way. At that point it started running well if you could get it started. I then had the dealer run a scan on it, said I need a MAP sensor. Replaced that, nothing changed. At a moment of part changing weakness I replaced the throttle position sensor. No change. If I try to start it in neutral, it will start but make a solid alarm. If you shut if off and restart, it coughs and backfires. If I move the throttle up a bit it starts like a charm and sounds good, you can then move the throttle back to neutral and all sounds well. When you do this though, it does the beep beep every few minutes. Is there something I am missing?

WELL you can not put a current CAM sensor in the older model 496's - I have written a story book about this up here. All earlier cam sensors are NLA for the 496 and you have the earliest 406 being a 2001.

I have been working on a replacement cam sensor for this instead what Merc wants you to change a bunch of chit out - timing gears. chain, cover gasket and so on to accept the newer cam sensor. GM did all of this for emission reasons among other reasons that I do not want to write a story book about. Serch up here and some of my postings all about the CAM sensors for the 496.

Anyways if you want to be a tester for my CAM sensor let me know and I could elaborate all about as what I have. My problem to test this has been currently, I do not see older 496' s anymore - for the last 3 years nothing but 2006 and up for the 496 and we have been in drought plus this year huge flooding hence service work is not like it use to be in the good old days.

SOLID info from BUP right here:
http://www.offshoreonly.com/forums/g...-question.html

You have/need OLD cam sensors for that vintage 496. It would REQUIRE a BLACK sensor, not tan/grey. IF tan/grey is what you have in the engine right now for a cam sensor: DO NOT run it. It has an interference issue with the CAM.

Also, you should buy your own scanner from www.rinda.com.
If the engine is beeping, it's trying to tell you something, you need to listen to it.
Replacing parts all willy-nilly is a futile effort. That beeping is a "Check Engine Light" on your truck. You wouldn't start changing out parts on your truck without scanning it for codes.
The Rinda scanner is the best money you will spend on your boat. You don't have to wait for mechanics, and you can share costs with your friends with similar motors. All you need is new pigtails at times.

Search "Cam Position Sensor" here on OSO and most of the threads that pop up are about 496s.

Also, IF you had a bad IAC, they can not only take out the wiring harness, but they can also destroy the software driver for the IAC in the PCM.
You need to know if you are still throwing an IAC code.

More info as I have to think about how to write all of this as it is very important info about the early 496's from Mercruiser.

first of all your fault beeps IMO are from your CAM sensor as the PCM 555 is not receiving the correct cam PS signal / reluctor wheel on the gear / the new cam sensor you installed is not reading nor sending the proper input signal to the PCM hence the output signals from the PCM is throwing a fault code / warning beeps along with the engine runs like crap. just saying.

Next the tech BS of this and what is going on. Hope my words make sense of all this.

the cam PS and the crank PS supply the PCM with timing and RPM information as inputs to the PCM 555. When a failure takes place in both and one of these circuits, the engine will run rough and backfire and can even shut down. First and foremost check for continuity between the sensor(s) and the PCM first of all to make sure that is OK plus the connectors / connections are in very good shape with no rust or moisture on them.

It is best to test the sensors as well for resistance values. Most marine OEM's require testing them (sensors) at around 70 degrees for proper resistance values FYI so room temp would be good to.

Values are

camshaft PS - A to B 24.04 mohms then B to C 24.05 mohms
crankshaft PS - A to B 23.2 mohms then B to C 23.21 mohms

I have posted this info before up here.

here is the kicker with Merc and important info for many who even scan the engine and get nothing.

all PCM 555 software prior to level 091, a malfunction with the crankshaft PS and or the Camshaft PS will NOT set a FAULT. With level 091 (PCM 555) if the CAMSHAFT PS is bad or faultly in any shape or form however and does not send the a signal and or the proper signal to the PCM - the engine will backfire and at times will not start in some cases.

Here is what can take place as stop cranking the engine over and then try to restart the engine - operating the engine for 20 seconds to set faults. the (fault) warning alarm will be set 2 beeps per minute. With that said IMO your 2 beeps are the camshaft PS. Hope this makes sense here.

Thanks for the info guys. The camshaft PS that I bought is the old style black one. The dealer first sent me home with the beige one and at a glance I knew it wasn't the same so I took it back and they found the identical one to what I had before. I have scanned the motor and it throws the MAP sensor as being my issue but that is new too. What I will say is, the crankshaft PS has been redesigned, the new one fits but the original has two O rings and the new one only has one. Should this be of concern? I went to three different marine dealers before I settled on the new style, all told me it was redesigned and fit my boats serial number.

Re read post # 2 as what Merc wants you to do replacing the cam sensor - the one that's available currently.

GM block codes 1XA , 1XW. 2XA, 2XW, 3XA, 3XW - No cam sensor its NLA Mercruiser uses theses GM blocks with these codes Next info to make it simpler

Mercruiser 496 Engine serial # 's - 0M025000 thru 0M672040 - The CAM SENSOR - NLA - I explained Merc and GM has a conversion to changed cam gears and chain - timing cover and gasket and then cam sensor to the latest one available along with the new connector spliced into the older harness.

Please scroll down to part # 12 on this Merc parts lists. Look at all the non available listings for the cam sensor

http://www.marineengine.com/parts/me...nd-front-cover

Where did you get a black cam sensor from ? I have 10 that were made for me to test to see how well these are going to work in the older 496 marine engines

You need to post your serial # of the engine to further figure this out . You are close to LOTO and none of these dealers there no anything about this ? If so are they OEM Mercruiser dealers and updated on their factory schools ?

Serial Number 0M095240, I bought the part at Johnson Brother's Marine in Laurie MO. I get that it might not be the same and you have given me way more insight than I have been able to get anywhere else. The darn thing looks (at eye value) exactly the same. I am going back to the mechanic with all of this info. To be honest, some of this is over my head, I really appreciate the info. And yes, LOTO.