Optimax 150hp 2001 misfiring cylinder

Hi guys,

I have an Optimax 150hp 2001 serial #0T281769. I bought it 2nd hand after it had been in storage for 12 months, I was told it ran great prior but after sitting its developed a misfire at idle. I took the gamble knowing the compressions were good - which I verified once I got the motor being 120psi on all 6.
ob

Once I rigged it up and it fired up straight away and seems to sound fine for about 20 seconds before it started to cough. On the muphs the misfire was still present even after increasing the throttle. I did a drop test and narrowed it down to cylinder #6 (bottom cylinder on port head). With that coil disconnected the motor idled and reved great (obviously minus a cylinder though).

Things I immediately tried to see if the misfired changed cylinders:

- swapped plugs
- swapped air injector
- swapped fuel injector
- swapped coil
- swapped coil driver

I thought it might be reeds so I pulled off the adapter plate and confirmed they're all in good condition. I also bled the air valve on the port rail to see if any fuel came out which it did not.

What I found strange is that If I unplug the fuel and air injector plugs on #6, the misfire does NOT go away (although it sounds slightly different). It is only if I unplug the coil itself that the misfire disappears. With the air/fuel injectors disconnected and the plug still in the cylinder I connected a spare plug to the number 6 coil and grounded it out so I could see it sparking. The motor ran fine on the 5 cylinders. But as soon as I hook it back up to cylinder #6 plug she sputters - injectors disconnected.

What I don't understand is why the miss fire would still remain after I disconnected the injectors. The first thought is leaky injector but like I previously mentioned I had swapped it and yet it still misfired on #6.

I bought this since my previous 1998 150 Opti was getting salt water in cylinder #6 and rusted out before we caught it. I know some parts are interchangeable but not as many as I'd like unfortunately.

must have air/fuel pressure tests performed at 1000 rpm and leak down rates at 5,10,15 seconds after shutdown
video posted of pressures running would be helpful, post a link to them here
Recommend rechecking compression, motor warmed up, battery charger connected, kill lanyard pulled, throttle at wide open position, specific readings please
good clear Pics of plugs
then we can proceed further

^^^^^+1 good advice

Pulse pump diaphragm leak, crankcase pressure hose or check valve leak, wrong spark plug, fractured or bad spark plug, or even a plug that is not fully seating against the crash ring. I’ve seen a few that were not threaded all the way through the head from the factory, and this is the way it acts.

how does this engine run going down lake , drop test preformed while under power ? inspect sleeve for rotation not seen one on 2.5 but guess its possible , has this engine always done this from new , may ask previous owner , have you noticed any water droplets on plugs /rust

Thanks for the reply guys. I haven't been able to get my hands on a gauge to test the fuel pressure yet. But in the meantime I did a couple of tests:

- Pulled off cylinder #6 breather hose. From the brass valve out of the head there was very small amount of fuel mist being ejected from it - thats with injectors unplugged on #6.
- Pulled off the crankcase breather leading to the remote oil reservoir and confirmed there is fuel mist being pushed out of it. Not sure how much pressure there should be out of that tube but it didn't seem like heaps. I noticed the remote reservoir wasn't swelling up like it usually does (when I had the last Optimax connected to it).

So seems like fuel is definitely making its way into the crankcase. Which would make sense as a few times I've started it up she reved up quite high which I would assume meant it was burning off some excesses fuel thats pooled in the case.

I haven't put the boat on the water yet, so don't know how it runs under load. The previous owner said it ran great prior to sitting. Haven't noticed any water drops on plugs.

inspect fuel pump diaphram for any tears or pin holes , replace with only OEM parts please

Good news. I pulled off the fuel pump diaphragm and found a spring has broken and put a hole through the gasket. Swapped it over and now no more misfire!

I also decided to check my oil pump was pumping by pulling off one of the output tubes and turning the ignition on. The pump was getting 12v but no oil was coming out. I put the pump into priming mode by engaging the throttle 3-5 times with the ignition on. This seems to solve the problem after it was primed. Hopefully nothing was damaged during my time previously running the motor before checking this - definitely something I should have checked first before even firing up the motor.

Next thing I noticed was the water temp while it was peeing didn't seem warm enough for how hot the heads were - not sure if it just wasn't up to temp and the thermostats hadn't opened up or if there's an issue. I'll put the boat in the water tomorrow and monitor this closely. I'm thinking faulty thermostat or poppet valve if there is an issue?

Thanks again for the replies. Such a great community on here.

Expect to use excess oil because of jockey shifter prime and your engine will be using double oil until the break in clock expires, you'll probably need a new set of spark plugs when it does.

Telltale is the compressor outlet only, you need a Smartcraft device to know and monitor engine coolant temp.

Turns out the port thermostat was stuck open and the starboard side was stuck close.

Good that you found that!

Just an FYI: The PCM actually modulates the GROUND to activate the oil pump and cycles the pump in "counts". So there shouldn't have been oil at the moment you unhooked the line, which in and of itself introduced air into the system.

As mentioned above, your "shifter jockey method" was not a wise decision. It will require significant, heavy-load operation, above 3500 RPM's, in order to expire the 120-minute break-in clock (only runtime meeting these criteria counts). Additionally, you'll be dumping TWICE the required oil into the engine, fouling it and the spark plugs up, and will need to top off the remote oil tank twice as much as normal.

Carbon build-up, oil fouling, etc.