496 HO on-going issue
 

We did a transom assembly repair last winter and this season have had issues with the starboard motor when started sounds like it is breaking up real bad. It is an intermittent problem though. Sometimes I start it up and sounds and runs great. Other times it starts up and sounds like it is breaking up real bad and possibly down one cylinder. Not the IAC. This happens all the way up to cruise speed and then some. I have changed plugs and wires, done the wiggle test on connectors and even put a computer on it twice. No codes or alarms. Just runs and sounds terrible. Throttles are at the same position from port to starboard. The Oil Pressure gauge is slightly lower on the starboard (problem engine). Took it to a local marine mechanic...wouldn't you know it started fine that day. The mechanic did a load test and checked the fuel system. All checked out ok per spec. The port engine starts up fine every time and sounds great. The starboard may crank over several times before starting and then sounds like the exhaust is breaking up. So now at the end of the season and the problem is still present. Anyone have any suggestions on what else to check? I am just afraid to continue to run it and possibly create an internal issue. I have thought about clogged injectors or possible a coil pack going bad and not firing right...I don't know...HELP!

Jeremy, did you replace the cam sensor too?

Also, there is place in NC that the local guys send their injectors to as preventative maintenance in the winter. I think they charge $300 to clean and test a set of 8. I can get the info for you as I may send mine down this winter.

try swapping the cam sensors,then try swapping the crank sensor and see if the problem follows.

I have brand new cam and crank sensor already installed

Fuel Injector problems are constant problems not an off and on problem here or there. I do injector flow testing and cleaning service all the time. I have a semi custom professional machine built 6 years ago and manyly about 70 boat dealers are my customers so i do know about fuel injectors. .

Anyways your problem - IMO possible 5 V sensor circuit problem - you need to scan the engine with just key on and cold engine and while its running. You need to look into the fault history as well of the scan. I say the 5 v circuit especially because you were back there working. It most likely could be a connection problem or a corrosion issue. You need to look at voltage output as well. Your 5 V sensor reading from the scan should be a steady 5 v sensor reading as well - 4.99 is fine as well. If the reading is jumping all the over the place well mostly your problem

Many things on the 496 are 5 V reference sensor and signals. Even the MAP sensor is to. If you print a couple reports of the scan I bet mostly I could figure it out. 496 is no stranger to me by no means.

Also make sure you do not have low voltage or high voltage for the 12 V reference and batteries. Load test your batteries and check voltage with the scan tool even running voltage. Running voltage should 14 to 14.8

Scan the engine a couple of different ways. You will want to scan both engines the exact same way. it will be 4 different scans per engine. I know this sounds like wtf but this really can help pin point the problems.

anyways I said alot here without enough info on your end.

Post YOUR engine serial number because older cam sensors are NLA. I have written about this years ago and in the past 3 years up here.

Possible wrong cam sensor installed but your engine will not run good period and in other cases will not even start - it will be a constant running problem with the wrong sensor installed.

You need to post your engine serial number and the part number of the cam sensor that was installed. I promise i know what i am talking about here.

Injectors flow tested and cleaned if they can be 100 % corrected is 25 dollars per injector and that includes new basket filters and new O - rings installed back to you so for 8 fuel injector that is 200 bucks plus I pay shipping back to you. A way better deal than 300 plus shipping by far.

And I am picky as hell doing the service plus baby sit all of it, flow testing and ultra sonic cleaning with 2 different types of fluid..

The engine serial number is 0W000298
I replaced both Cam and Crank sensors on both motors so I don't think it is the wrong sensor. I agree it is probably a connection or corrosion issue but it is like finding a needle in a hay stack right now. I did find a corroded terminal in the three-pin harness to the crank POS earlier this year and fixed that and ran fine for a while now this. I don't have the lap-top scan tool but do know of a buddy that does. May be a while before we are able to meet up though. Thanks for your assistance.

Whose cam sensor did you use and what was the part number on it ? Was it ordered thru Mercruiser ? If not who ?

Ordered through boats.net with the motor serial number. If it was a wrong part the other motor should be reacting the same way right? I don't have the parts here to let you know the part # I used.

The following are the sensors that got replaced on both motors:
Engine Serial #0W000298

Crank POS #881667
MAP Sensor #881731
CAM POS #892615
IAC #862998
Intake Air Temp Sensor #805223T

Also replaced plugs and wires, starter and sea pumps

These were changed for preventative maintenance not because of a fault. The crank POS did end up presenting a problem in the wiring harness with the corroded pin, which did get fixed. The starboard motor was the initial transom that was found to be leaking and had the most corrosion on the back of the motor. I do feel that my problem is another harness or some sort of electrical connection that is not making good contact. But have searched and done the wiggle test and found nothing.

496 HO on-going issue - follow up
 

I started a thread on OSO regarding the engine breaking up last fall. BUP - had some good info to offer and thought I would see what your thoughts are for the following:
Now when I start the cold motor, I can turn the key and it fires but breaks up real bad. I can turn the key back off and immediately back on and it fires off and sounds normal now. Sometimes I do need to throttle it up and clear out the fuel and then it idles fine and runs fine. I did a fuel pressure check and it was right at 40. There is definitely a problem with it sending too much fuel (or not being able to burn off the fuel...spark) for it to burn off right away. I can tell it is the #7 cylinder and by looking at the exhaust flaps that side is not quite right. So I checked the cost of fuel pressure regulators and they are quite pricey to just replace. If I am correct, there is one at the cool fuel on the side of the motor as well as one at the fuel rail. Do you have any ideas?

Leaky injector on #7?

Try swapping that injector to a different hole, and if that doesn't do anything, try swapping the #7 coil to a different hole.

I replaced both with brand new.
Last time the boat was out it started rough. Turn the key off and right back on...started better. Rev'd it up a bit and it cleared up and was ok. Did a fuel pressure check and was 40-psi.

Turns out my ongoing engine trouble was from an exhaust elbow with a crack which was leaking water back into the manifold. Ended up replacing all four risers on both motors and one new manifold and the problem is now fixed! This has been an ongoing issue for quite some time and it's good to be back on the water worry free for now. Thanks for all of the suggestions and assistance!