Random plug fouls 2,4,6

[scrutcher]

1B916729 150 proxs keep randomly fouling a plug on just 1 side, thermostats have been changed water temp is 70 air temp in 80's boat ran fine yesterday but today had a sputter at wot then droped about 600 rmp its seems to have fixed itself then back down it went, got ti the boat ramp pulled plug wires with motor running and found #6 gone, it done this exact thing 2 weekends ago but fouled #2 and #4

[lpugh]

check fuel/air pressures including a leakdown

[scrutcher]

Im just wondering why it would clear up then stop again, I had the injectors serviced about a year ago, almost acts like it might be loosing fuel to a random cylinder but keeps oiling, the plug is really black, I run quickkleen and 87 gas. Would the fuel rail on that side possibly have any effect, I know when I got home the shrader valve for the air had pressure and no gas smell

[JOE54]

is temp of port bank comming up to operating temperature 143

[scrutcher]

I only have vessel view mobile and it dont give me temp per bank just water temp, that was my thought aswell, i might change the thermostat anyways, but it was acted like it might have lost spark or fuel for a little bit then worked fine for about 30 seconds then lost it again long enough for the plug to foul. Of course I dont have an alarm when this happens. I was thinking about just replacing all 6 coil packs, and replacing thermostat. The bigger issues is when I take it to a shop and the water test the boat it seems to run fine which I get because it sometine runs fine with me for a full day sometimes longer, its just random when it happens. I know it will cost more in the long run but I am just going to start replacing fire and fuel

[JOE54]

test for spark when problem occurs on all 6 cyl

[EuropeanAM]

Test- don't just start throwing parts at it. Odds are pretty high you may induce a second problem (or more) with that approach.

[scrutcher]

Would a faulty crank position sensor possibly be causing week spark that ends up fouling eventually mainly at wot without an alarm or code

[scrutcher]

This is the fouled plug but all my plugs are dark like this, maybe getting weak spark im not sure

[scrutcher]

Also the shrader valve on the top of the rail which is fuel when i pressed it today fuel came out then it was like alot of air with some fuel is this normal, I dont have any fuel come out the shrader valve at the bottom just air

[JOE54]

yes normal without engine running

[JOE54]

Would a faulty crank position sensor possibly be causing week spark that ends up fouling eventually mainly at wot without an alarm or code

highly unlikely

[EuropeanAM]

What do you have that plug gapped at?

[scrutcher]

Gap is set to .042

[scrutcher]

I have went through many plugs with this issue, i can run wot shut off the motor and they are still black, it been an ongoing issue and apparently no shop can find an issue, sometime it will run fine for a while then sometimes not, I run the same 87 fuel and use quickkleen, would maybe a poppet valve cause this, I have good water pressure idle and Wot

[SilverFox579]

Getting it to a dealer for printouts of run time history and fault codes would really be helpful.

[JOE54]

make sure MAP sensor is working properly also

[scrutcher]

Wouldnt a faulty map sensor throw a fault code

[JOE54]

nope not if its fouled with oil /sludged up

[aussiejake]

Have you checked the exhaust tuner is still in place?
Randomly fouling plugs 2, 4 and 6 is the main symptom. For some reason the other bank never fouled a plug when it happened to my motor.
A dropped tuner makes the motor run very rich from idle to mid range, so therefore fouling the plugs. The local merc tech said the left bank runs cooler that’s why 2, 4 and 6 were more prone to fouling. Whether that’s correct or not who knows but once the tuner was connected back up it never fouled another plug and ran so much better.