496 mag running issues
02-28-2012 | 07:39 PM
#21
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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!
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02-28-2012 | 10:34 PM
#23
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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!
02-28-2012 | 10:41 PM
#24
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From: Virginia Beach, VA
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
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
02-28-2012 | 10:45 PM
#25
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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!
02-29-2012 | 05:53 AM
#26
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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.
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.
02-29-2012 | 10:31 AM
#27
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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.
02-29-2012 | 11:18 AM
#28
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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.
02-29-2012 | 09:58 PM
#30
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From: Clarkston, Michigan
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.
