2006 175 optimax seems to be dropping a cylinder under load

**kellytheaker** · 10-05-2019, 12:58 PM

Believe gen 2 is 2010 up . So I believe gen 1 and a coup.e post From knowledgeble members state coils and grounds and wire rubs so I would be minamum checking coils and ohms as well as the basic wire connections - rubbing . Or I'm not certain if ohming the coils is adequate When members state replace all . Call Merc maybe or a good dealer - they will possibly talk about coil issue they have seen and a program if Merc is replacing or has found issues with the coils .

**Motv18** · 10-05-2019, 03:09 PM

Has the problem always happened when it was warmer out and not so much when cool.

As the iat sensor will cause power loss if it gets heat soaked or reads hot

that flutter is probably the cause what what was the psi if it falls below 80 no fuel will flow the air will hold the fuel in.

**Fishinmymission** · 10-05-2019, 03:14 PM

Guage had no snubber and was a direct read. It was constant 90 psi . Basically at idle the psi gauge needle is at 90 +/- a flutter of 2 psi. As rpm increases past idk 1500 rpm pressure are more stable and at around 90 +/- .25 psi. When the engine surged pressure was 90 with a slight flutter increase in the needle to +/-.5 psi . This was repeatable every time the engine surged. Been chasing it for two years. Only surges in warmer air and water temps but issue feels like it's gotten worse occuring at more rpm ranges and lower and lower temps. Hard to be certain though.

**Fishinmymission** · 10-06-2019, 11:16 AM

I'm betting I have a intermittent stuck open air injector(less likely but possibly fuel injector). When it sticks open it could cause that slight increase in fuel pressure gauge needle flutter. More air leaves through the injector so I see that slight increase in gauge needle fluttering. Could this be it and this would not throw any codes or alarms? If I get all my injectors tested over the winter will a shop be able to pickup this issue if that's it?

Now I'm thinking next hot day un plug coils and run engine on 5 cylinders to see if I can prevent surging with a certain air injector fuel Injector and coil disabled.

**Motv18** · 10-06-2019, 05:05 PM

Possible I would take air readings as well. The air would fluctuate wildly as only 60* is enough to air the system up during starting, when the injectors are open to backfill and provide immediate air pressure for the system.

I would suspect a fuel smell though if one was stuck since fuel and air are connected at the injectors.

That is honestly darn good readings. Your allowed up to +- 2psi

im still surprised theirs no shrader on you vst to test the low pump for 20-30psi.

Good of luck finding the issue all I can really do is give you maybes

**Fishinmymission** · 07-06-2020, 11:00 AM

So it wasnt the plugs or the coils both of which were replaced but I think i finally found my issue. My alternator would get hot to the point that it would stop producing voltage until it cooled off. It finally got so bad that I was able to see the voltage drop last long enough for me to detect it on my garmin and then by multimeter. I also have a top crank seal oil leak and lots of dried oil in the alternator which may have contributed to it overheating, or its just old. When i changed plugs all were super black and sooty and I run Pennzoil XLF. I think all the cyclinders were misfiring at points when electrical supply could not be met.

Praying that this was my issue. After i let the engine cool off the alternator started making voltage in spec again but the engine made a lot of clanking noises and i noticed the alternator shaking at idle back at the dock where my garmin said it was making voltage but idk maybe it wasnt sufficient voltage or all the plugs are so fouled again it does misfire. The compressor fly wheel has no play and rotating by hand seems like its working exactly how it should. Moving the crank flywheel by hand also seems to indicate good compression.

**CptAJ** · 07-26-2023, 11:10 AM

I'm having this same issue on my 225 Opti. Did you ever confirm if it was the alternator?

**Fishinmymission** · 07-26-2023, 01:38 PM

I do believe it was the alternator but on that trip where i caught the issue i also melted a hole in a piston and i was never able to 100% verify. As the engine would only run on 5 cylinders after that and wouldnt get as hot. Either my alternator was acting up on only the hottest days of the year every year for 3 years or i was loosing compression intermittently with a gap melting and resealing itself on that one piston right around the ring. I find it unlikely that piston would reseal itself and then work perfectly until the next summer season so i do believe it was the alternator. Please post more details and an alternator is cheap so let us know if it works before summer is over.

**25two.stroke** · 07-26-2023, 07:40 PM

Sounds like you had a stuck direct injector. I never read this thread before, but I just read it start to finish and it sounded like an injector the whole time. Thats probably why you melted a piston too. I also had a check valve on the cylinder do this same thing. Check valve screwed into the cylinder was plugged and therefore was not oiling that cylinder. Amazing how just a little check valve clog can melt a piston. And it can stick and un stick in the cylinder as its running. Especially if its an oil issue. But yours sounds more like an injector got you.
As far as CaptAJ...you should pull all your injectors out rn and send them to be flow tested. All 12. For the cost of a single new injector you can have all of yours checked and then you know they are good. If you don't youll probably end up blowing your motor up like what happened in this thread. It happens ALL the time.

**Fishinmymission** · 07-27-2023, 06:31 PM

So I actually spent a great deal of my time to breakdown this engine and took some time looking at another friends engine with a melted piston as well. I'm a Materials Failure Analyst and actually used this topic for my masters degree thesis which is attached and goes over a bunch of the piston melting in greater detail. I have another presentation that is in slide format and easier/quicker/better to read which I will upload next week if I can find the thumb drive it is on. So the RPM drop was intermittent and heat related and I was never able to find any evidence that my injector was running lean on that cylinder which melted a piston. I even flow tested it with a heat gun it never hiccupped. While I do believe that number 3 direct injector or fuel injector probably had a temporary blockage that resulted in a lean condition and the immediate melt down of that piston, that blockage cleared and there was 0 evidence of it when I did all of my injector analysis.

I think that it would be highly unlikely that the injector would act up on only the hottest days of the year and then perform fine at full throttle for extended runs every spring and fall. I did catch the alternator actually turning off and then back on intermittently when hot but sadly I replaced the alternator and never had the chance to stress test the motor again as it was only running on 5 cylinders at that point. I later broke down the engine for analysis then gave the parts away when I had to move apartments.

My best guess is the alternator was acting up and turning off when it got hot enough for three years and then the piston melting was a separate event that just happened to also occur on a very hot day.

There is a remote possibility either injector was acting up on only the hottest days of the year but I did detailed work flow testing and even exposing injectors to harsh heat gun temperatures and they never hiccupped.