Reckless32

I've posted early in the troubleshooting on this.

Synopsis:
IAC Output code on the new Rinda scantool I bought with 2 beeps per minute alarm going full alarm and 89% power at 4600 rpm. Discovered IAC had signs of getting hot. Discovered the IAC on the other motor also had signs of getting very hot (melted plastic around connector on both). Bought two new ones and installed.

Port motor still has the same alarm only now it only 2 beeps when at no wake speed, and after crossing the 4k rpm threshold on the way to full alarm. Previous it was 2 beeps constant up to 4600 rpm.

Further troubleshooting via process of elimination, involved cleaning out via contact cleaner spray of the reachable sensor connectors in addition to ensuring each was fully locked in position. To no avail.

Finally I decided to swap out the two new IAC's, AND the two ECM's (thinking maybe it's a shorted wire harness) to see if it (alrm/code) would go to the other motor. Well in fact it did in identical fashion while the port now ran great?? Okay, next logical step was to swap back the two new IAC's to their original position thinking maybe one of those went bad again. Nope, even worse. Alarm stayed with the starboard motor and the ECM. Not what I wanted see.

Soooo, all indications are the culptirt appears to be the ECM itself has somehow gotten a short or some irregularity in it from whatever it was that heated up the original IAC's (I'm assuming).

The scantool only shows the IAC Output fault. The only other anomaly is when running on plane, or revving the engine at idle with the scantool hooked up I noticed the VDC volt reading was locked on 5.00 without wavering. The other motor will fluctuate slightly to like 5.01 or 5.02, or 5.00 depending on throttle? For all you mech types, is that maybe an indicator of the problem?

Finally, if in fact it's the ECM, what are my options with regards to can it be reflashed/programmed? If so, by whom? or is going new replacement the only option?

Anyone know of a used one for sale?

Reckless32

Ttt

stevesxm

personally , i don't believe it. if the boat runs at all and runs thru the range of performance then it is EXTREMELY unlikely the ecu is bad. i don't have my factory manual in front of me but if memory serves , the iac motor only sees signal at low rpm and is then shunted out and its position at low rpm is a function of several parameters from the sensor array... anyone of which can cause your issue. i know this is a really general statement and obvious as hell but if it were mine, i would be looking at my merc scanner, confirming all the sensor outputs and making sure the " counts" on iac were correct and then going thru the diagnostic procedure for the iac motor as defined w/in the factory manual. i would expect the problem to be self evident from there as it has been for me in the past. oh.... and i found that my mpi manifolds were all leaking at the seals which , once sealed up properly fixed all sort of weirdness.

Reckless32

stevesxm your comments are very positive in my mind, as I don't want to fork out for a new one.

I don't know what your 496HO experience is, but with a '96 Cig you probably don't have any unless you're a mech?....But along your thoughts....Besides the VDC readings mentioned above (fixed at 5.00 no variation) the "count %" showing on the scantool when at idle would show some fluctuation up or down in the low % range. Suddenly gun the engine in neutraial up to 2500, then immediately retard the throttle, as the rpms wound down the count % would jump to 99.9% for a long pause, and would set the alarm in action.

Same indications when running along on plane at say 3500 all things seem normal, no beeps no alarm, the count % fluctuating like normal, then push it to 4000 rpm and the % jumps up and the 2 beep alarm sets in again (none once on plane up to 4k) then push it to the stops and full alarm sets in at 4600 with overspeed kicking in, and the IAC count % goes to and holds at 99.9%, and the available power drops to 89% until I retard the sticks to 1100 or less, then everything back to normal??

You know i agree it doesn't make sense the ECM is bad taking it all in. But as i stated above the only "common denominator" in the end after swapping over IAC and ECM to the other side, and vice versa again with only the IAC, yet the alarm stayed on the side with the same suspect ECM???

I don't have the in-depth manual, just this cheap Mercury basic version, and a SELOC generic manual. Which manual, and where do I buy it online to provide me the info I need to go deeper?

stevesxm

sorry... i missed the part about you having a 496... i have 502 mag mpi's and i got my factory manual from e bay... guy had them for everything... the factory manual is so explicit it is hard to NOT be able to fix it...

as for the ecm... i understand you completely. i did years and years and years of efi work and i would say that of the, perhaps 2500 systems or more that i saw, the number of times i saw the ecus go bad in anything other than a "full stop" condition was only twice. my approach was ALWAYS to look for a sensor or device failure that was lying to the ecu and making it screw up...

in your case my experience stems strictly from the gm racing stuff i did that used iacs... and they would fail all the time... the motors would fail, the seats would get dirty... all sorts of things would happen... and the only way to diagnose them was by the numbers... i read the book and did what it said and they were always right.

in the absence of the obvious ( the device on fire or something) , the only way i know to fix these systems is by the numbers and by what the manuals say. i hook up the scanner, i compare the results to what the book says. i do what they tell me to do... i fix what they say to fix... and it works. for me that is a lot better then guessing .... and a hell of a lot quicker and cheaper.

in your case if you can do a double blind component exchange , then thats hard evidence to ignore... and you may very well be correct... its just way up there in the unlikely catagory for me... and i would be reticent to shell out 4 or 500 bucks until i believed it for real

Reckless32

I agree 100% with that philosophy & usually do the same, but as of right now I don't have the proper manual to walk me thru the PCM-555 troubleshooting cycle, so I have to use the rudimentary methods of elimination. I would sincerely you're right. Should've got the manual with the tool I guess. Well I'm still not going to hang the hat just yet. In fact I'm going to go thru one more swap out evolution just to be absolutely certain I'm isolating down to the ECM.

Pwrbt33 I got your PM and will be calling you shortly to discuss.

Vyper340

Rick - I've figured it out! It's not the engine, IAC, PCM or anything else you've looked at... It's the driver dude.
Hope you get that PCM replaced to put this one to rest soon.

Reckless32

Yeah a PITA, but the learning curve has been beneficial. I just checked it again earlier today by starting veirfying the IAC alarm/code on the stbd engine while the other ran great. Swapped just the ECM, and the alarm/code swapped motors again. Did some experimenting by pulling the hose off the throttle body and holding my thumb over it blocking air. The IAC seems to defintely work and the count goes up, but it seems to react to slow (closing) when I take my thumb off setting off the alarm. Otherwise everything else is fine. Even disconnected MAP and TPS sensors while running and got fault codes on each that went away when reconnected.

Macklin

Rick, have you talked to DaveL? It is possible he may have a PCM sitting around you could borrow for testing purposes. I would also call Tom or Stu at Norfolk Marine / Marina Shores. Remember Tom? He used to work the dock shop a couple of years back. He is now the service writer there. I would drop by and reintroduce yourself rather than call. Tell him I sent ya.

Definitely worth a try.

Reckless32

Saw Dave last night at O's, he didn't have anything (ideas/suggestions) for me other than get another.

I'll take your suggestion and check locally for a used one. I know of another, but it's compatibility is unconfirmed at this point due to it's mfr year. Still researching that lead.

Push comes to shove, I can still boat no problem, just no upper end performance thanx to the alrm and 89% power after 4k rpm.

Since I'm "likely" looking at replacing, I'm almost tempted to open it up and see what's inside....