97 Yami OX66 250hp problem: Missing

[5 Salt]

97 Yami OX66 250hp problem: Missing

One possibly, two cylinders missing but only after running for 30-45mins or so. Lighter or heavy loads doesn't change problem - but definately an issue during extended runs of 1-3hours.

-3,800-4,300rpm Range
-Increase to full trottle and missing stops
-Missing begins as intermitant with slight increase in fuel burn - perhaps 1gal per hour addn'l.
-Over next 10-15mins missing grows to where fuel burn increases by 8-10 gals per hour to maintain same rpm.
-again...increase to full trottle and it cleans itself out and runs great.

Cleaned and re-gapped culprit plug. Checked compression noting no problems for problem cylinder. Bad Plug, wire, or coil perhaps cause by higher temp associated with longer runs. Recenlty replaced all plugs and had about 30hrs of trouble free operation. Could be oil/fuel mixture too rich at the bad cyl or.... ???

Anyone have this problem? Like get some imput before the taking to the dealer only to have them tell they can't duplicate the problem.

[MattPark]

Don't count on cleaning plugs for a cure to a miss, sometimes their time is simply up.

The prolonged running would definitely suggest a coil or wire, and not so much a plug, as it would theoretically be running hot and clean, with little opportunity to foul. Windings in coils are affected by temperature, and wires can be as well. Replace all plugs and wires, and if the problem persists, I'd suspect the coil. Weak coils can help a plug foul as much as a bad air/fuel mixture can.

Is there one coil for all cylinders or are they individual?

[Tim]

Check your fuel/water separator. Might have some water in your fuel. Does it idle ok during these events?

[Onokai]

I'd also say coil /wires. Drain that large silver box of gas on port side of our 250s for crud as well.. This also could be a black box issue-I hope not as they are pricey.. I assume you have a super water seperator already so we won't go there. Was your oil level in external box low?? .Mark

[Chrome Bumper]

Check your wiring diagram to see what electronics only serve those two cylinders. Sounds like an ignition problem. You might need some new electronic component. Might pull the flywheel and make sure everything is clean and fresh looking. I have a johnson that was missing on two holes and it needed cleaning on the flywheel pickups and a new 'blackbox', which you can find aftermarket on the net. 10 years of saltwater got to it.

[5 Salt]

Not a fuel issue as zero water in system and it persists over two tanks of fuel. Oil level in engine tank is right on the normal mark. Will drain engine fuel box and water separator on engine oil tank as well. Separate coils for each cylinder so coil is my first thought.

Someone mentioned cleaning Oxygen Sensor but I having a bit of a time figuring out what/where exactly is the o2 sensor -anyone?

Will give the flywheel a good inspection and cleaning.

[Grady252]

You can also check the electrical connectors between the indivual coils and the black box. They have a little plastic keeper so they are not supposed to come off, but they can get bumped loose from taking the cowling on and off. If it is loose they will idle fine, and top end will be fine, because there is increased power to arc accrossed the loose fitting, midrange will be where you notice the problem.

I also put a little dielectric goop on each one to help prevent corrosion.

Good Luck to ya!

[5 Salt]

boatsetup.com/O2sensor.pdf

Good article on O2 Sensor maint from Yamaha.

[Flatfish]

If all the plugs look good, try taking it out and watch the tach for max rpms.

If it has lost a couple hundred rpm, I would think o2 sensor is beginning to plug up. If you let the o2 sensor go long enough you will lose several hundred rpm(mine would go from 5500 to 4600 with a badly clogged sensor).

Assuming the o2 sensor is the problem....

I would think with tuna fishing being a big draw for ya(low rpm for extended periods of time), it might be wise to buy an extra sensor. They are not cheap. Have NR in Aurora burn out all the crud and test it for ya. You probably wanna pick up a gasket set too, that way you can do a tactical reload mid weekend if need be, if you bunge up the old gasket set, no problem.

NR in Aurora knows what you need.

Mark and the dog

[5 Salt]

If all the plugs look good, try taking it out and watch the tach for max rpms.

If it has lost a couple hundred rpm, I would think o2 sensor is beginning to plug up. If you let the o2 sensor go long enough you will lose several hundred rpm(mine would go from 5500 to 4600 with a badly clogged sensor).

Assuming the o2 sensor is the problem....

I would think with tuna fishing being a big draw for ya(low rpm for extended periods of time), it might be wise to buy an extra sensor. They are not cheap. Have NR in Aurora burn out all the crud and test it for ya. You probably wanna pick up a gasket set too, that way you can do a tactical reload mid weekend if need be, if you bunge up the old gasket set, no problem.

NR in Aurora knows what you need.

Mark and the dog



Chatted with NR - Aurora. Parts are on the way. Have O2 sensor soaking and will go through testing steps later this week. Hopefully the Sensor is good will not need replacing.

Thanks Mark