496 Mag surges on idle
 

My 496 started surging on idle at the end of the boating season last year and the local mechanic told me it was a common problem with the 496 in cooler water; it's still doing it this summer in warmer waters. I've replaced the IAC valve, spark plugs and ECT Sensor with no luck. I've also looked for corrosion problems at every sensor/connection I can reach and have not seen any signs of corrosion. I've seen this problem discussed on several forums but have not seen any solutions so I recently purchased the Diacom software. I have a couple of recordings to review if any one can help diagnose please let me know?

The spark advance, MAP pressure and IAC position all vary while the engine is surging and it makes the infamous "sucking sound"; it only shows fault codes for sensors I don't have. It runs fine on the throttle but the varying RPM's makes it difficult to pull into a slip.

Any help would be appreciated!!!!

who IAC and what procedures in detail did you do for the install of the new IAC cause there is more to it than take it off and replace -

what is your PWM percentages of the IAC when scanning ?

what is TPS readings at rpms ?

What is your MAP readings ?

I installed the IAC and didnt follow any specific installation procedure which is probably the issue. What's the correct procedure or can you provide some direction on where to find the correct procedure?

The IAC varies from 25% to 51% (approximately).

The TPS remains constant at 0

The MAP readings vary from 5.3 to 7.3

This recording represents a "mild" case of surging; it's worse when decelerating off-plane.
Thanks for helping!!

Following... I have the same surging issue at idle. I also replaced my IAC.

After BUP responded, I did a lot more research and decided to try another IAC and to clean the Merc paint off the IAC flange to ensure there wasn't a vacuum leak. That did the trick and it runs great!

The first IAC I installed was not a Mercruiser part! I purchased the new Merc part and compared them side-by-side and noticed many subtle differences. Lesson learned!!

Let me know if you want to use my Diacom software. My boat (and computer) are at LOTO and we're headed there this weekend.

Thanks for the reply back for the fix. And right there in the first statement was the issue, Who IAC ? and . Procedures. there is more to it than just removing the old IAC and putting on a New. Have said many of times and can not say it enough.

fixed 100s in the same boat as to what you did at first,. And the aftermarket IAC s are not always going to work for you either. Seen that many times.

alot more to it than removing IAC and replacing with new. Then the next question is who IAC did you install ? Many aftermarket IAC s do not work correctly for the Merc 496 app.

What is PWM %'s showing when scanning the engine thru out the rpm ranges ?

so your research found my advice procedures on the proper install and not to use aftermarket IAC s. - My guess is very possible --- If not - its still all posted here on OSO for the taking to get it done correctly.. haha. and again thanks for the reply on the FIX as it validates what I have been saying may times. Proper procedures and aftermarket IAC s can be the issue

Yep. Hindsight being 20/20, I should have started on OSO. I would have saved $60 (which is 1/2 price of the correct part) and a big headache.

Thanks BUP!