Back fire on start stock 496HO
 

Unless you want to just throw money at it, I would do some testing first. Compression and leak down test would be my first, then check fuel pressure

Do the above tests, pull the codes, check IAC, water separator filter. Having injectors tested, flowed, and rebuilt can never hurt and they are very suspect. And for what it costs from Bup, not expensive to do.

scan and knowing the parameters. fuel pressure testing is the first 2 steps for pin pointing issues or not. Then move on to comp and leak down. Easier testing first to rule out then move on. When was the last time you changed fuel filters and did fuel samples ? How many hours on the engine(s) ?

Have to jump in on this --- Pulling spark plug wires on DIS set ups is asking for huge trouble - whom told you to do that? thats 50 year old carb testing methods. And if you have gas fumes in the engine compartment - we will say good bye to you now. 4 years ago people working on a 496 in my area burned down a complete section of the docks and boats in the marina from pulling plug wires off and the spark / gas fumes caused a huge fire. I am posting this as a huge safety issue for everyone AND NOT TO DO THAT FORM of backyard testing plus possible ignition / ecm / pcm inflicted issues.

And for the record --- NOT A SALES promotion but injector testing and flow testing / cleaning IS PART OF MAINTENANCE along with fuel samples. I have never had a used set of all 8 injectors come in that were completely balance across the board. NEVER. And on a 496 regardless MERC - Volvo - Indmar - PCM - Marine Power - it needs to be done. NO way around it. Its really not the injectors themselves but the contamination issues are the factor here especially from improper fuel maintenance and storage.

Added info on top of --- Keep in mind internal fuel pump wear happens on all electric fuel pumps. Guess where that wear ends up ? Then you have the rubber fuel lines that no one ever changes. Currently the range for those are 4 to 7 years max for change out .

2 contamination issues right there that happen in every marine engine even carb engines, the same happens.

300 hrs on spark plugs and wires man I would start there as well --- How many hrs on the app totally ?

<div style="text-align:left;">describe runs fine after start up ? can you reach full rpms at WOT ? 5000 rpms HO mag and 4800 for mag 496 is WOT</div><div style="text-align:left;"></div><div style="text-align:left;"> I would not rule out CRANK SENSOR here either for starting issues. That signal and cam sensor have to sync for start up among other things</div>

Have no idea what happened here with that mess -- OSO ?

will fix in the next post

Post 8 have no idea what happened. fixed now here

describe runs fine after start up ? can you reach full rpms at WOT ? 5000 rpms HO mag and 4800 for mag 496 is WOT

I would not rule out CRANK SENSOR here either for starting issues. That signal and cam sensor have to sync for start up among other things


FIXED from earlier mishap

A really good reply from me would be --- I bet the crank and cam sensor fault codes are NOT real fault codes --- they happen for a reason and that is common as to why they happen. Anyone want to take a guess why ?

Even without a real fault code I would not over look the crank sensor. They become an issue even for a hard start for a mild issue to even 300 lost rpms for a medium issue to a complete stalled engine to a no restart and no tach signal for the worst ends of a completely failed crank sensor.


BLEED down fuel pressure - I would really look at the issues here. and what is fuel pressure while the engine is running ?

Man your having a hell of time with your boat right now. We have all been there.

Hey BUP -Ican't remember, do the 496's have wasted spark ? Prob not because individual coils,but still worth the ask. :)

sequential injector firing with 8 coils near plugs. DIS

early coils gen 1 were prone to fail here and there.

gen 1 crank sensor more prone to fail than the later version without the ribs. And the rib crank sensor ( early version ) can even break when pulling it out. Not common but can happen

crank and cam sensor signals are very important

2 versions digital throttle n shift and mechanical throttle n shift.

early years 10 pin wiring harness later years 14 pin wiring harness

IAC can play a role for stalling as well.

more than half IAC failures come from lack of proper maintenance.

More than half PCM repair that I see have a bad IAC circuit. ( H bridge) for PCM 555 and ECM 555

are you sure the coil harness is on correctly. my buddy put his on wrong one time and had the same issue it sounds like your having. it would run just a touch rough and afterfire out the exhaust while idleing.

We have had this come through the shop before and kept getting a cam sensor fault. Throw a sensor at it and problem disappeared for awhile but always returned. Changed crank sensor 3 years ago and no more issues. Seems the computer cant tell where engine phasing is so it will constantly throw a cam sensor fault if the two dont match up. Good luck!!

Resolved
 

Crank shaft sensor
 

have them for 160 shipped. Same exact Merc uses - Not knowing your engine serial number yours possible changed. And I will be in Florida working on boats next month and even a couple of 496 s .

to set the record straight not all aftermarket crossover IAC's work as they should per PWM matching what MERC has installed And for the record I can prove all of this first hand on twin engine scan reports just by same day changing the IAC s and gaskets out from Standard motor products and BWD to what Merc is selling for IAC they use.. Anyone what to take that bet ? because I have the sameday scan reports from idle to WOT of the differences of the PWM between the IACs. You are going to be surprise at the difference between the PWM s. Apples to Apples testing. Plus how many people up here have I told to try the IAC merc uses and they reported their running issues cleared up completely.

I have many OEM IAC s as well for way less plus the proper info and proper related parts that need to be installed on top of.

For the crank sensor the early version were RIBBED versions and are NLA they went to a NON ribbed version. Like I said I have them that are the same as MERC .

https://www.offshoreonly.com/forums/...rges-idle.html

the link above is one about switching to the IAC Merc uses - yep 60 bucks more he had to pay but like I said the aftermarket cheaper IACs do not exactly work out alot of the times. Really do not care if you think im wrong cause If I have to Ill post the testing results i did for the day all about this and the difference PWM results from one IAC to another to what MERC uses, I was even shocked at the different parameters when scanning

Yeah, thats very true. Store bought IAC from Autozone and the like never worked on my496/8.1 Raylar 600's. They caused a lot of crazy issues.

Don't see an IAC causing a bouncing tach or no high performance. Bouncing tach, maybe loose connection and/or misfire, and performance would be misfire, loose connection, fuel, bunch of other things

Stalling, maybe IAC but could be other things and may still be loose connection

Nope not even pointing a single finger at whomever but pointing out for the ones whom want to argue with me. At times can be common.

Also for the OP I was pretty sure has a auto store bought aftermarket IAC because all the forums recommend the $ 60 to $ 70 dollar aftermarket IAC. Not the way to go. And my point is alot of times the aftermarket IACs do not work how they should for the MERC set up.

The same goes for aftermarket fuel injectors they never flow like the OEMs and ignition parts are the worst from the cheap aftermarketers.

That same IAC is used on on other Merc apps as well. I change all them out - you can tell when you look at the bottom side of the aftermarkets from what Merc uses, the difference of construction.. I sold 43 IACs for 2018,.... 15 of them were replacements of fairly new aftermarket IACs.. I would not use the aftermarket IACs period for those exact MERC set ups.

Its not a big deal paying an extra 30 bucks per what I have and install compared to the aftermarkets. I always op for an exact replacement regardless of the 30 to 40 bucks more without the headaches is priceless and less down time. Also the advice about the IAC what should be installed is priceless. Also there is alot more to just changing out the IAC for all that to work correctly...

if the engine stalls on its own a crank and cam sensor can be set BUT ITS NOT AN ACTUAL FAULT. The IAC itself will not cause a cam / crank sensor codes (real actual cam / crank codes). Many times the IAC does not even show a fault code when its bad but there is reason for that - there has to be certain events seen taking place within the PCM for the IAC fault code to show. Sometimes they take place and show a bad IAC fault code and sometimes they do not.

Added info -- the 1 st very early gen Merc 496 s do not even show the cam or crank sensor faults period. Just saying. If I recall its was the 2001 models only.

I do not know if he ( the OP ) has a stalling issue or a jumping tach or poor running engine or whatever at this point.

Possible corroded and or wet connections / connectors being the main issues - is this a storm boat per mentioned or I am miss reading that info ?

The TACH signal comes the crank sensor - a bad crank sensor can produce a NO ENGINE START and NO TACH signal at the very worst case. Lessor issues from a bad crank sensor can cause loss in engine running rpms, back firing, poor running conditions, poor tach signals among other things.

Prob best to have your boat look at by someone who is well trained on this stuff. The forums are a huge dart board for getting things properly fixed . - its all guesses without hands / eyes / ears and the array of diagnostic tools needed to fix boats correctly. And IMO I believe you have more than one issue going on.

100% correct ,
The ecu does not realize the engine has died because all the other sensors are providing a input to the ecu,except the crank and cam sensor (because the engine is not turning ) so it will trigger a false code.
another way to trigger a false crank sensor code as well as a knock sensor code(which I know we are not dealing with) is EMI ,depending on how the wiring harness is routed especially if its close to any ignition wiring you can be picking up Electromagnetic interference,