Have you tried swapping the ecm's to see if the problem follows?

ok everyone, new update. ecms swapped to no avail. Fuel pressures good, no codes, grounds clean, and cannot find loose connections. Still the same symptoms. I may do plugs n wires next i guess, but i did the cylinder test, and all cylinders seemed fine on the diacom. Motor runs fine over 1500 rpm. Also fuel filters fine

What year model is this Merc 496 and have any other mods been done or added?

Best Regards,
Ray @ Raylar

06 models, bone stock

count the seconds inbetween beeps that will give you a direction to go... they also have a problem with the water psi sensors in the manifolds and also in the rear of the motor fuel / oil log ( not shure which one )

A tough one
 

My belief here is that this is an electrical gremlin that is showing itself as an intermittent interrupt on possibly the ecm power up feed and the ecm is cutting in and out on a very short interrupt basis at lower rpms possibly aggravated by the engine vibrations and loading changes. Other than that idea, I am stumped at this point with whats causing this idle rpm spike.
Hope its not in the harness wiring as that is a pain to find and fix.

Best Regards,
Ray @ Raylar

I think I saw a thread somewhere before, mechanic chasing his tail, ended up being plug wires...finally saw them arcing after it had gotten dark outside. The harnesses are a rat's nest in my opinion.

Have you already tried to switch the idle air valves ?
maybe there is a gremlin there that does the engine rpm fluctuating ? Maybe worth a try

yes, new iacs. Thanks for the suggestions. ill keep hunting

try swapping some of those relays around,,,check fuses and relays for corrosion