PCM555 alarm
 

Buy a scan tool and suddenly you're the neighborhood marine mechanic.

Anyway I'm trying to help a friend diagnose an constant alarm on a 2002, 5.0L Mercursier with PCM555. The symptoms are a constant double beep alarm, hard starting, stalling and rough idle. He says it runs fine at WOT except the alarm. Using the Rinda scanner there are no fault codes other than the normal ones. All the scanner conditions looked normal except available power which was only 89%.

But... I was unable to run the IAC test. When I tried the scanner simply said it was unable to execute the command. I was also unable to run the cylinder dropout test on any cylinder except 1 & 2. With any other cylinder I got the same error on the scanner. I suspect a faulty IAC but any help would be appreciated.

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When my IAC was bad, it idled rough but no beeps, power reduction, or codes.

Can you see the IAC level with the motor running with the scan tool? With Diacom you can scan real time and see if the IAC is functioning. At idle it should be bouncing up and down. If it is flat lined, then you know the IAC is bad.

BTW, I was watching some auto show that showed that you can clean IACs with brake cleaner to free up the plunger. When mine went bad, I just replaced it. that was before I saw the show.

Yeah the IAC level would sometimes bounce around and sometimes flatline. It was at 99.9% at one point and 50% at another. If I moved the throttle a bit I could always get it to change but when I let it alone it would then settle at some new arbitrary spot and sit there.

it sounds like faulty iac be sure to replace the filter as well as far as watching the iac move on the scan tool you can only see where the pcm wants it to be not where it actually is