R.P.M. gowning up and down under load.

[Fenwick2]

Last two trips strange R.P.M. changes occurred. First time crushing at 3,000 rpm the motor surged up to 4,000 rpm without moving the throttle. Backed it down , ran steady then dropped down to 2,000 rpm so I bumped it up and it again to 3,000 rpm and within a few seconds surged back up to 4,000 rpm again. Last trip it was flat calm so I decided to open it up and ran at 5,400 rpm for around 10 minutes. Then again without touching the throttle it dropped down to 4,500 rpm. I backed off the throttle and tried to get back up to W.O.T. but could not get above 5,000 rpm. Where do I start to look for a fix? I will pull plugs today and look for fouling. Gas is good. I'm thinking plugs or injector problem. Maybe gas and water separator? This is a 1999 Mercury Optimax.

Thanks in Advance

 

[Fenwick2]

Re: R.P.M. gowning up and down under load.

Someone mentioned it could be a (MAP) Sensor. Anyone had this problem.?

 

[Jimmy70]

Re: R.P.M. gowning up and down under load.

Almost sounds like a sticky throttle cable hanging up and then relaxing or letting go under tension. Might try lubercating the box or and the cable.
Jimmy70

 

[Fenwick2]

Re: R.P.M. gowning up and down under load.

Sounds like cable slack but it's not. The throttle stays put for the most part and my hand was on it at WOT when it lost about 600+ rpm. I'll check a few things, change plugs and check water separator and fuel filter. Then it's off to the shop. $$$. I'll reply after I get the problem solved. Thanks for trying to help.

 

[Fenwick2]

Re: R.P.M. gowning up and down under load.

Well it took me long enough but I have things running correct now. Did it myself. I have to assume my problem was twofold. Dirty injectors and fouled plugs. I changed the fuel and water separator, added some Sea Foam and gas before re-installing. I also replaced the plugs and they were very dirty. Shame on me for not running Sea Foam every tank like I know I should with this new gas and for trying to run plugs for more then one season. I also cruse at 3,500 rpm and this may cause fouling also. Happy to say after I did the changes the boat now is running like new again. Cost including a new tool for removing filter is $139.00 That bottom left plug is a hard one to get back in. Good thing I have small hands but I still have blisters on my fingers, The Beatles.