early HO's no DTS, IAC, 335 hours.
353 owned 13 years

rough 550-600 RPM idle
idles poorly, occasionally stalls like you turned the key off, (first "near stall" event yesterday tach dropped then came back)
This is all only on cold start 4-5 minutes later after it starts warming up and comes off rich mode and attempts to do the warm 650 idle
will crank but not fire for 1-2 minutes, then it fires right up and resumes poor fluctuating ~600 RPM idle
runs fine otherwise. RPM, power under load no miss, no tach flutter
runs appropriate above idle all thru RPM to WOT right with the other one

NO CODES (Merc laptop) other than the normal open non smartcraft, fuel tank 1 and 2 ect.

coincidentally just prior to this idle issue I noticed this side's volts on gage approx. 12V at idle goes right to 14 with any RPM over idle.

I don't see this being fuel delivery volume or pressure.
I threw an IAC at it just because it is so typical
as well as new foam mufflers in TB

no good. gonna scan it again tomorrow my bet is it still tells me nothing
saying electrical?
battery? (new this year but could be junk I'm not impressed t all with the other side's reserve with stereo on it...
I just thought to parallel them tomorrow for the voltage and see if it makes a difference.

boat is slipped with battery charger on.

thoughts?

Thanks.

Slightly different than my problem last week, but check all your ignition wires. #7 was off.

Have you ever replaced the Crankshaft Position Sensor? They can go south without throwing codes. I know this first hand.
My indicator was RPMs not matching between motors, when all else was the same.

my understanding has been that when a crank sensor is bad they either won't run at all, or down on RPM throughout?

above idle it runs fine, right with the other one, not short on RPM or power.

it's the indicated low voltage on the gage at idle that I feel is trying to point me.
My concern here is that I either don't have enough voltage available to power the PCM at idle, or a voltage loss somewhere?

at this crappy ~600 RPM idle it indicates ~12V, bump it up with the throttle to above this problem say 7-800 and the voltmeter jumps right to 14V

a total fail and or a no signal at all of the crank sensor is - stalled engine - no restart and NO TACH signal - the tach signal comes from the Crank sensor / reluctor ------ with that said improper reads can lead to engine running problems and lagging rpms to hard starts and overall poor performance for a jist not to go into details.

in your post explain "2 ect" you mean a fault (ect) and "no dts iac" what does that mean you changed the IAC ? or you had no IAC but I see you changed one - what brand IAC also there is more to it than just changing the IAC - you need to change the gasket with new plus the mounting surfaces need to be straight and perfectly clean amoung other things. The hose connections need no leaks and no restrictions you need perfectly clean flame arrestor as well. No leaks for the mounting block and IAC itself as well. Everything needs to be cleaned ie: hose, IAC cavities, mounting block, throttle body & plate and so on.

DTS apps have no IAC nor IAC muffler among other things for the jist

Mechanical throttle and shift have a IAC and a IAC muffler - just an FYI Volvo Penta does not use a IAC muffler nor a FORD IAC - they use GM - IAC's. I TOTALLY understand we are talking Mercruiser here but I like FYI's.

Spark plugs and what brand and gap plus what do they look like ???

Fuel injectors ?? Tested and flow tested and cleaned ??? I do that service as well.

"2 ect " -- ect = engine coolant temp and if that's on the scan that is a fault -

Running voltage on the scan is ??? especially at idle ??? needs to be above 12.7 v more like 13 and or above volts and your 5 v running on the scan is ???

Load test your batteries before running the engine.

TPS reading on the scan is while running ???

And what are these readings from cold engine not running just key on. More to it than looking for faults - many times faults do not show up for a poor running engine. Just saying

IMPORTANT - Also leads me to spark plugs and possible wires - are all coils firing properly and or THIS 4 SURE **** if you are sayin - low voltage if you only have 12 v at idle speeds while running at 600 to 700 rpms ? If so very weak battery and poor voltage output for a running engine at idle. Connections all TIGHT & CLEAN and rust free ? All of them including back side grounds

Next - overcharging batteries is one of the worst things you can do. Hopefully you battery selector switch is turned off while charging batteries.

yes early 496 no DTS, has IAC.
New Merc boxed twice the price IAC...
new gasket, clean surfaces, no harness plug melt, or bad pins, hose to TB appears fine

the faults showing are always there for Smartcraft functions not used
the "2 ect" was in reference to fuel tank #1 and #2 and other smartcraft options not in play.

stall and then crank with no re-start for the same 2-3 minute interval every time, then at 3 minutes BAM fires right up.

So if I am reading you correctly, when it does stall, during the interval that it cranks and won't fire, if the tach is dead when cranking >>>>crank sensor?

so load test battery cold
scan cold for TPS and available voltage
then repeat running

remember it idles rock steady@ 750 RPM or so COLD RICH MODE, it is when it sees coolant temp ect. and comes off rich it won't idle steady @650 it can only hold a fluctuating 550-600 with the occasional stall, or as noted for the first time this week a "near-stall".

real quick - fully warmed idle in N gear needs to be 650 rpm --- read with scan for rpms, can careless about your boatside tach. TPS, IAC along with coolant temp (160) and real position setting of your throttle body plate and VOLTAGE play into idle rpms and running conditions. read all this on the scan and what are the readings - first from cold engine NOT running and then from a fully warmed running engine.

spark plugs look like and what brand ??? how old and the gap ? If DENSO throw them in the trash.

Voltage and load test the battery is ??? Connections and grounds condition ???

MORE INFO ---- HARD STARTS AND HARD RESTARTS can = crank sensor and or cam sensor or both or grounds and or connections especially moisture in the connections. Rust as well.

as noted it's not 650.
I get you about the tach, but it is indicative at the least
I've owned the boat 13 years, warm idle is down ~100 and erratic
.
will scan live data
plugs are old and need changing but not a single blip in running quality above 7-800 RPM all the way to WOT

no "hard start" anytime
it either fires right up or doesn't fire at all (in the 2-3 minutes following a stall, as mentioned above)

what I don't get is the "wait interval" of those couple minutes till it will refire. That's not a plug.