Hesitation/RPM Hunting on 496 Mag HO

[Bootkie2]

This season after logging 4 trouble free hours, I've now been chasing power/RPM/hesitation issues on my 06’ 496 Mag HO with 920 hours.

So far, I've done the following:
-scanned computer using Rinda for active fault codes / freeze frame data. There are No active codes or occurrences.
-Tested fuel pressure on the rail, getting 40-41psi even when it the engine bogs down.
-Removed/Inspected plugs, all looked pretty normal (not drenched in gas/oil)
-Replaced all spark plugs (since they were already removed, why not)
-Replaced IAC and IAC muffler
-Fuel and oil filters replaced right before spring launch on 5/15
-Ran engine connected to a clean external 6 gal fuel tank with fresh gas.
-Alternator testing FAILED, voltage all over the map, so threw in a new alternator and at least fixed that issue.

Still after all this work the hesitation is still there. I'll be cruising at 3000 RPM and then suddenly bogs down and hunts for RPM, (almost feels like when doing a cylinder drop test on the scan tool) so I slow down to headway speed and the engine sounds really really rough. Then it will recover for a few minutes, I'll add some more power then ,it bogs down again and sounds really rough. Repeat.......

I have not tested or done anything with the injectors. but it feels like that should be the next move. What do you all think think?

I'm trying not to think bad compression just yet but probably going to have to test it at some point.

 

[alldodge]

Tested fuel pressure on the rail, getting 40-41psi even when it the engine bogs down.

What is pressure at idle and WOT?

Your motor is in the area where paint was peeling inside the fuel pump and clogging the injectors

 

[Bootkie2]

Yes, we had the dreaded fuel module paint delaminating 5 summers ago. It’s run fine since then but maybe it’s catching up with us a bit now.

 

[tpenfield]

Something like what you are describing really needs the full computer diagnostics to figure out. It sounds like you did the code scan, but often these issues will not create a fault code. I see that you have taken a few shots at it with the 'parts cannon' to no avail.

If you already addressed the fuel module issues, it may be something else. You could try the injectors, but it would be interesting to see what the TPS values are when the engine is hunting RPM.

Post a serial number of the engine, as that is always helpful. Is this a cable throttle or DTS?

 

[Fun Times]

Maybe run the engine at night and look for a light show under the brackets/bolts, by the valve covers, etc. from the spark plug wires as these engines tend to that from bad wires leaking spark.

If you didn't have the injectors removed and professionally cleaned and evaluated from the paint, it's time even if it's not the issue. Also inspect the fuel pulse damper on the fuel rail in which it looks like a fuel regulator but really isn't a regulator on the Merc marine engine setups.

http://ww2.justanswer.com/uploads/keysync/2010-05-08_005804_EN_09.pdf?r=impact%7C2003851%7CBlogger.com%20Tech%20Support%20-%20Text%20Link%7C&clickid=z3uVWDXLoxyIUWPy4FS%3AWXYSUkFwDZ2gKRiu0E0&aff_sub_id=EdgeBingFlow&aff_sub_id2=oc5A05L8E_cFrofF6SHThPTQCERYtosjsWESdn5_59y_5mYJkpnT2DJw2C2nxnwf&aff_sub_id3=&aff_sub_id4=&chat_text=&irgwc=1

 

[Masinc]

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Yes, we had the dreaded fuel module paint delaminating 5 summers ago. It’s run fine since then but maybe it’s catching up with us a bit now.

Maybe not a solution in your operation but my 350mag had a very similar sounding glitch. Tried several ideas. The cure seems to have been the replacement of the high pressure pump and regulator. That job requires attention to detail during assembly. Runs very well now. And I worked it HARD last weekend. The low pressure pump replacement yielded no change. I put old pumps in the parts washer and energized them to watch and low Px pump will flow a lot of fluid. But the same experiment with the high Px pump resulted in very low flow and oddly if you choke the intake of the pump it would flow better but still not steady. Mine had no previous maint records so there was no way to know how long the pumps were truely in operation. Gotta figure those little pumps run anytime the engin runs so thats a lot of revs.

 

[Bootkie2]

The main issue ended up being the rubber flapper got ingested and stuck in the exhaust and the riser gaskets needed to be replaced.