Hello all,

Fresh rebuilt top and bottom (2001) 496HO's PCM 555, Port motor rpm's are off compared to Stbd and I'm missing 10 mph, verified new plugs, wires, checked coils are in the right order. Compression is good. Port motor IAC shows 99% when idling and you have to play with the throttle to get her to rev properly. At which point the IAC % lowers, but then back at idle its right to 99% again. I have replaced the MAP sensor and TPS. I swapped the IAC from the known good motor, no change. I swapped ECU's, no change. I have been chasing this gremlin for over a year, even before the rebuild. I'm at a loss. I don't know that the IAC is the issue but what would tell the IAC it needs to stay at 99% all the time. Could I have a coil issue? I am going to check fuel PSI today but it seems to be ok.

If I don't ease in to the throttle on that motor I can hear a pop on the throttle body side of things. Diacomm laptop scan shows no stored fault codes, would a running data log show anything? Missing 1000 RPM on the top end of this motor. Help!!

Thank you!

pull the flame arrester and see if your engine has an IAC muffler, some do and some dont depending on serial #

Ok, what does it look like, a filter maybe? it's in the throttle body somewhere?

Itm #5 at the top, yes filter of sort

https://www.mercruiserparts.com/bam/...31816/11414/70

Thank you. I did notice a little foam type thing in there the other day, should I try to remove it? Crazy to think a little filter like that can cause issues lol

if its clogged it can cause issues with idle , remove it fire it up and see if things change...maybe a good cleaning of the TB is in order

Dead cylinder or two can cause that.
No fuel ? No spark ? Time to fo some diagnostics.

^^Agree
Just refreshed, maybe vacuum leak, intake maybe

OP states
I have been chasing this gremlin for over a year, even before the rebuild.

And that changes what?