North Idaho

Have owned our 2003 stingray 230SX since new. Have 210 hours on the stock 350 mag and bravo 1. All has run flawlessly since new with merc mechanic shop doing all work on it. Over 4th July weekend we developed a miss between 2500 - 3100 RPMs. You can hear and feel it but multiple on water runs with mechanic and his laptop shows no issues. Motor was tuned up before start of season and shop has talked with merc help desk and switched every ignition part they can with same results. They Sent ECM back to merc last week to have "flashed" so maybe that will fix it. This is week 3 of boat being in shop and I need other ideas on what the miss issue might be. As I said it is only noticeable between 2500 - 3100 rpm and there are no fault codes. Summer is getting away from us so any thoughts would be appreciated.
Thanks
Tom

BUP

iTrader: (9)

Cap and rotor on these are a problem a lot of the times. Try another one even if yours looks great. They have and had huge problems with them cross firing between 5 and 7. Also have to ask was the distributor ever taken out and or rotated. The timing is non adjustable on these apps but proper distributor and rotor phasing is very important.

Trash

I had the exact thing on a 1995 EFI motor. It was injector related. #1 injector was not flowing correctly (gummed up) causing a lean miss. Pull the injectors and have them flow tested and cleaned. You likely have one or more injectors on the fritz.

This won't throw codes nor be detected except if you are looking at spark retard/knock you may see that if scanning with ScannerPro or something equivalent.

BUP

iTrader: (9)

With the ECM 555 and you had old MEFI totally different set up with all ignition and injectors and fuel delivery parts plus how it works. The current diagnostic testing especially for ECM 555 / PCM 555 his mechanic should have or done as it would have been wise to is - an injector balance test - this will tell a lot about the injectors and related components.

Also the scan should have or would have shown any faults 59 thru 74 to pinpoint a fuel injector problem(s). With that said it can NOT depict improper flow patterns in which could cause a miss or running problems. It will or can detect open or shorted / stuck closed or stuck open fuel injectors.

Also fuel pressure testing and fuel regulator testing can also detect fuel related problems. Anyways just added info. IMO cap and rotor and possible spark plugs or an injector or two with high impedance but really do know what was truly tested by someone else nor having eyes, hands and ears on it..

Hopefully with Merc on the other end of this they all have ran the proper diagnostic testing and decided it was the ECM. So maybe its fixed.

North Idaho

More details....The shop pulled all the injectors and sent them out for testing and cleaning, no change after reinstall. Fuel pressure tested good but I am not sure where or how many places they tested it although I think they tested every injection point. Multiple scans have shown zero faults. Cap, rotor and plugs are all new and the cap was replaced a 2nd time because of the failure reputation. To my knowledge I don't believe the distributor has been moved but will ask. The shop I am dealing with is the top merc shop in my area, very good reputation.
Forgot to mention above that the miss is only noticeable under a load, it won't do it if boat is not in gear and running down the lake. It is also not noticeable above 3100 or below 2500 rpm.
Thx everyone for your feedback, any/all ideas are appreciated.

BUP

iTrader: (9)

These dist cap can be bad right out the box especially if it is left over stock. There was huge problems with these HVS distributor caps (we call them flat caps). What diagnostic equipment are they using ? Merc CDS, or diacom or Stats or MEDS or MEDS 6.0. Hopefully they are not using Merc DDT. just asking.

Can I ask who is the shop as I might know them and where, Idaho ? It sounds like a real lunch eater but I like these kinds of puzzles. Post your serial # of your engine.

Have them run your boat at the lake with a portable fuel tank, new fuel line and fresh fuel. Also I would check fuel vacuum hg. Merc spec is no more than 2 Hg. What are fuel pressure readings? Ask them about the fuel vacuum readings and run boat on a portable fuel supply plus report fuel pressure readings thru out the rpm ranges. Did they test your fuel pressure regulator ?

If your engine app has the shift in gear switch (engine serial # dictates) have them check that for out of adjustment or poor connection or possible bad switch.

Also check orange power lead on back side of the alternator. Mercrusier has a service bulletin and recall about that and the poor connection and wire routing of it. I am very well OEM factory Mercruiser & Volvo trained just saying (not bragging) so hopefully they listen to what I am saying here unless they have huge ego problems from someone telling them how to fix a boat over a public internet site. So the ECM checked out fine but did not change your running problems ?

Also you can not rev these ecm 555 motors up unloaded on the water hose because it will set off a overspeed fault & warning horn plus kill spark.

At 3000 to 3600 rpms somewhere in that rpm range will set off the overspeed faults running unloaded. FYI

North Idaho

BUP,
Thx for the detailed reply. I am on the road this week so will have to try and get serial number when I get home. Would any of the suggested issues explain why the miss is only noticeable from 2500-3100 RPM? I am forwarding this chain and future replies to the repair shop.
Thx again.