Having engine pulsing problems

[boat101]

I have a Mercrusier 120 HP GM 2.5L I4 engine. When I have the engine at WOT it goes up to 4500 RPM's then back down to 1500 RPM's, and so on when loaded down in water. When out of water it runs at full WOT with no problems. The outdrive uses a 19" pitch 3-blade prop.

 

[vipzach]

Re: Having engine pulsing problems

Welcome to iboats!<br />Have you done a tune up lately? It sounds like water in the gas. Check all your filters and water seperator.<br />Zach

 

[boat101]

Re: Having engine pulsing problems

I cleaned the tank out before adding gas. So it should be water free.<br /><br />hmm can an intake leak cause this too. Cause the intake also has water ports to cool the engine. And like I said the pulsing problem only occurs when in a lake, but not on land.

 

[Alumarine]

Re: Having engine pulsing problems

When the revs are going up and down is the boat speed changing?

 

[boat101]

Re: Having engine pulsing problems

Yes

 

[boat101]

Re: Having engine pulsing problems

Also When I cut off the engine I see some kind of white steam come out of the carbuerator.

 

[QC]

Re: Having engine pulsing problems

Originally posted by boat101:<br /> I have a Mercrusier 120 HP GM 2.5L I4 engine. When I have the engine at WOT it goes up to 4500 RPM's then back down to 1500 RPM's, and so on when loaded down in water. When out of water it runs at full WOT with no problems. The outdrive uses a 19" pitch 3-blade prop.

I have a couple of problems here:<br /><br />1) The speed variation from 1500 - 4500 would be on and off plane type stuff. Are you saying that you can get 5 seconds (an example) of 4500 and then it swings down to 1500 for another 5 seconds and then blips back to 4500, and this is consistent? I'm thinking you would need a HANS device to keep your head attached to your shoulders . . . <br /><br />2) Tell me you really didn't run it at WOT out of the water!! If you did, I, like some others here are very curious as to how high the RPM got and for how long . . . Always wanted to test that, but I like my engines .<br /><br />

Originally posted by boat101:<br /> Also When I cut off the engine I see some kind of white steam come out of the carbuerator.

Warning, warning, Bondo impression . . .<br /><br />Ayuh..........

 

[boat101]

Re: Having engine pulsing problems

Well then tell me what can cause an engine when engaged in the water makes it lose so much power and cause a pulsing of the engines RPM's? I think it could be water vapor coming through the gasket and being sucked into the intakes of the engine, because like I said when I turn it off steam will come out of the carbeurator. Surely someone on here can help me diagnose this annoyance.<br /><br />

Tell me you really didn't run it at WOT out of the water!! If you did, I, like some others here are very curious as to how high the RPM got and for how long . . . Always wanted to test that, but I like my engines [Wink] .<br />

The throttle is limited to a maximum engine speed of 5000 RPM. You cannot push the throttle any wider. So I have not experienced engine detonation as you may be thinking....

 

[vipzach]

Re: Having engine pulsing problems

Originally posted by boat101:<br /> Well then tell me what can cause an engine when engaged in the water makes it lose so much power and cause a pulsing of the engines RPM's?

There is no load on the engine when it sits in your driveway and is running!<br /><br />

Tell me you really didn't run it at WOT out of the water!! If you did, I, like some others here are very curious as to how high the RPM got and for how long . . . Always wanted to test that, but I like my engines [Wink] .

[/quote]The throttle is limited to a maximum engine speed of 5000 RPM. You cannot push the throttle any wider. So I have not experienced engine detonation as you may be thinking.... [/quote]<br />He said because it is not a good idea to run your boat at a high rpm with the muffs on it, the garden hose can't supply enough water.<br />Do you have a fuel\water seperator?

 

[QC]

Re: Having engine pulsing problems

Sorry if I sounded like an a$$. I don't believe the 2.5L has any type of rev limiter, so what is keeping you below 5000 RPM is probably valve float (somebody else want to chime in here?) which is bad.<br /><br />Yes, I thnk the steam is an indication of a severe problem and the RPM swing could be related, definitely not an annoyance. Needs some professional attention. Good luck!!!

 

[QC]

Re: Having engine pulsing problems

Hey Vipdude,<br /><br />You thinking the steam is from water in the fuel? I have experience with water in an EFI and it just stops running, doesn't swing wildly and make steam. Is this common with Carbs?

 

[vipzach]

Re: Having engine pulsing problems

I had water in my gas once before and it would pulse kinda like he is describing. It would start building rpms then choke for a second, like it tried burning the water and would then pickup again. I don't know about the steam though. I suspected water in mine and installed a seperator.....after running fine in the driveway for a few minutes, I checked the seperator. Guess what I found..........WATER! It would run great in the drive and run like crap under load in the water.

 

[boat101]

Re: Having engine pulsing problems

Hmm from what you are describing Vipzach, I think water is the cause. Maybe I should get one of those seperators.

 

[vipzach]

Re: Having engine pulsing problems

boat101, it may not be your problem, but it sure sounds like water in the gas. The seperator is a good thing to have on your boat and it would be a very simple thing to try. Next would be figuring out why and where the water came from. I hope this works for you, let us know what happens. Good Luck! Zach

 

[paultek6]

Re: Having engine pulsing problems

your pickup in your tank is usually located at the back of the tank so when your sitting still in your driveway your not picking up water but when you run boat in the water when you go to get on plane the water moves back to the pickup and guess what ? Remember water settles to the bottom of gasoline its heavier

 

[SILTHUNDER77]

Re: Having engine pulsing problems

I have a 2.5 L chevrolet 153 cid 4 cylinder with iron block, and have experienced a similar problem, i.e running up to 4000, then the boat will back down to around 2500 rpms or so constant and will slow down to 20 some mph, I recently put a temp guage on it and my water temp actually got up to 220 degrees for awhile then cooled back down to around 170 when the recently installed thermostat opened. I am going to now check my impeller to see if it is damaged or I have a manifold restriction of some sort.. I replaced the coil which seemed to help the erratic rpm's, although I may have some valve rattle which stops as I power back to just above wake speed....I am also changing to electronic ignition, and will be replacing a worn fuel pump...I did notice when I had the problem that I could cool the motor down for awhile then it would repeat the same sequence run all out then back down on me...