'06 Merc Optimax 150 rough idle

I've got a pair of these with about 700 hours; I'm trying to diagnose a rough idle on one of them.

I say rough idle but it's more like it idles perfect (out of gear) for 5-10 seconds at around 540 RPM then it stumbles momentarily with the RPM dropping to under 500 then it recovers and is fine for some period of time. At anything above idle there is no problem noted, including wide open.

Compression with my gauge is 115 a cylinder +/- 2 PSI. What the book says to do is check plugs, check air pressure, check injector resistance, check coil resistance...

Air pressure seems the same on both engines - like 80 PSI. The gauge vibrates constantly with the engine running but is centered there. When I shut the engine down it starts about at 80 then you see the pressure draining off. Both engines seem to do this, assume it's normal but worth mentioning.

Plugs were fouled but I did just run a cap of extra oil through the engine (put into filter to winterize) so I don't know what that really means. For some reason though #2 and #4 (port side upper ones) were noticeably less fouled, almost "normal" but on the dirty side for only have 50 hours on them. Maybe that means something. Changing plugs was no help at all.

No issue found with resistance on all 12 injectors, nothing even more than .1 Ohm from the spec - which has much more tolerance than that by the book.

I bought a used coil harness on ebay so I can plug in and check coils, but I don't have much faith in that showing anything as it seems unlikely a coil would be bad without it being more noticeable.

Two issues I'm aware of. The fuel test valve on this engine leaks, doesn't spray but just sorta drips and runs down the fuel rail. When testing though I stopped the leak by putting a better-than-stock cover on the valve (until my new valve shows up)..

The air compressor on this engine always has what seems to be a film of oil on the back side of it - seems to have a weep of some sort from under the pulley. It's been this way for awhile. I have O-rings coming so may fix this but given pressure is good,, it seems like a cosmetic issue; can anyone comment on whether this probably is oil leaking out of the seals under the pulley? Is there that much oil in the compressor to have such a weep?

Anyway - Seems like if I've got a spark problem it's impacting at least 4/6 cylinders, if not all of them. Is there anything that could do this? Else any idea where I should be going next if the coils check out?

I have the luxury of swapping pats from one engine to the other if there are some expensive frequent offenders that would be nice to remove as variables.

Thanks in advance.

Jon

Did you also check the direct injectors?

Some Idle issues with DFI are caused by dirty or bad Direct Injectors. You can put Mercury's Power tune thru the air compressor intake while the engine is running in the water. Just do not let the engine over rev. This will clean a dirty Direct injector and help with rough idle.

Docksidemarineservices said:

Did you also check the direct injectors?

Some Idle issues with DFI are caused by dirty or bad Direct Injectors. You can put Mercury's Power tune thru the air compressor intake while the engine is running in the water. Just do not let the engine over rev. This will clean a dirty Direct injector and help with rough idle.

Click to expand...

I did check the direct injectors, by 12 meant the 6 fuel and 6 air injectors. Of course that's only gonna tell me if they are electrically OK and if clogged will be of no use. The air injectors would be kinda a pain to swap but the fuel injectors I think could be easy - worth swapping fuel injectors across engines to see if the problem follows?

By air compressor intake you mean basically the intake in the front of the engine with the butterfly? Sounds worth a shot to me.

Is there any chance the computer could pickup anything my smart craft isn't telling me? I can have that done fairly easily but given there are no faults showing dismissed that as an option.

Thanks,

Jon

Remove the top cover on the engine and look at the air compressor. You will see the brass barb that is the intake for the compressor. That is where you spray the power tune , not in the engine intake manifold.

Docksidemarineservices said:

Remove the top cover on the engine and look at the air compressor. You will see the brass barb that is the intake for the compressor. That is where you spray the power tune , not in the engine intake manifold.

Click to expand...

Thanks! Will give that a shot.

Also should have my spare wiring harness soon and will checkout the coils like the book calls for.

I think the decarb is on the right track. It's got merc cleaner in the fuel, I took it out on an 80 mile round trip the other day and after the first 20 miles the idle straightened out quite a bit. It's still not perfect but is much steadier - before was bouncing 480 RPM - 620 RPM, now it's more like 520-560.

Jon