Offshoreonly.com - 496 mag running issues

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Quote:

Originally Posted by downtime270 (Post 3629350)

So on a 496 MPI motor there are 2 knock sensors, one on each side of the block. I attached a picture to help you visualize it, however it will have a bolt through the center. Think of it this way - its just a microphone looking to detect any type of detonation. If it gets interference from the High frequency plug wires - it sends incorrect signal to the ECM and the ECM attempts to adjust the timing or fuel supply to compensate for what it thinks is detonation.. There is a very small greyish/white single wire coming off of it to a black connector. It should be routed very closely along the edge of the block around to the back of the bellhousing where it connects. If the wire is coming off at a funny angle - just loosen the bolt and rotate it to help you route the wire better. All I can tell you is I chased this issue too long to find out this was the problem. I had certified merc mechanics run the boat, tech the boat, and converse with Merc to no avail...Until I stumbled across a technical article that lead me to this...Good Luck!

http://s8.postimage.org/f364xz5d1/MAL9_33502.jpg
jpeg image hosting

thanks for the info. im pretty sure i found it and it is resting on one of my spark plug wires. i also decided to take my water pump apart and found scouring on both sides of the pump on the brass. i read in another post that this can cause problems. my boat has 292 hours on it im the second owner. i was gonna post some pics of everything but cant seem to figure it out. i might not have enough post yet.

There you go. Computer codes would not have shown either of those issues.

Everything everybody said is all good advise. My two cents is to look at your IAC Valve. (Idle Air Control) It is easy to get to (rear of engine, on top). You will likely be able to look at it and see that it or it's connector are melted. If nothing else, remove it, and see if the valve works smoothly back and forth. It is likely rough or frozen. I have two 496's and they both cooked their IAC's at about 500 hrs. My friends have all lost their IAC's. Exact same symptoms you listed. Scanner showed that the Knock Sensor and Crank Location Sensor was bad, but they were not bad. I even carry a spare IAC now. Good Luck!

Change your IAC (Idle Air Control).

At 292 hours i would also change the boost pump cause it's gonna go soon anyway. You should check for fuel pressure using a fuel pressure gauge on the fuel rail. Do a search on this forum and you'll find some good info on both the IAC and the boost pump issues. Nuther thing due to hours and many tanks of gas, is to pop the injectors out one by one and tap out the grit that accumaltes in each one and pop'em back in.

Good luck

Quote:

Originally Posted by RumBlur (Post 3629608)

thank you i appreciate all the help im getting on here i plan on moving my knock sensor wires away from my spark plug wires, replace my water pump with a hardin pump and ill check the IAC. then hopefully take it for a water test. if anyone else has anymore tips tricks ect... shoot them my way. thanks again guys

When my IAC was bad, it only effected the performance at idle. The motor did not go into guardian mode at all. A scan clearly revealed the problem. The iac output level is like a heartbeat line, when it was bad, it just displayed a flat line. BTW, I use diacom software. I didn't have the cooked wire problems though.

I also saw on some car tv show that you can clean iac's with carb cleaner and they will be good as new. I guess it could hurt to keep that one as a spare.

The only thing I would caution - is not to do too many changes at once, or you will never know the actual cause. Troubleshooting and root cause analysis is a critical process. If you have the luxury, clean up the wire routing, go for a ride, swap IAC go for a ride...etc...try to narrow down the cause.

Quote:

Originally Posted by downtime270 (Post 3629862)

Ya I was gonna do my water pump and reroute of the knock sensor first. Then I was gonna take it out and see what happens I'm not too far from a local lake I can test at then go from their. I'll prob leave my rear seat and hatch off during the test so I can get to the motor and check things.

Is their any way to function test the IAC or does it have to be done with a scanner?

Quote:

Originally Posted by 06laser (Post 3630497)

Is their any way to function test the IAC or does it have to be done with a scanner?

I don't think it's the IAC. As mentioned I think that affects idle. Do the other stuff. A steady alarm is supposed to be over revving (not your problem), low oil pressure or high engine temperature. The knock sensor wire thing is interesting too. I'd back flush the heat exchanger when you do the raw water pump. You will like the Hardin SS pump.