water pump question, please.

[boatdock]

I have a 5.7 1989 mercruiser with an alpha out drive, within the last three outing i have been through three impellors, the first one was installed by a dealer. what could cause a impellor to melt down. i never run without muffs, the engine has a new top end after the first melt down. the last time i took it out it ran great, the temp was ok until i pushed it up to 4000rpms,however after backing off to 3000 it cooled down to a normal operating temp. i am planning on install a new one tommorrow. is there a certain way to install the impellor?

 

[Boatist]

Re: water pump question, please.

Dock<br />Is the impeller actually going bad or is motor just over heating??<br />What does old impeller look like??<br /><br />Running the motor with out water to the impeller will cause the ends to melt and wear off.<br />Starting motor with outdrive raised same thing.<br />Running with prop hitting a sandbar will also cause it to fail.<br />Bad impeller houseing can cause rapid wear or it not to pump water.<br />Plugged up input water screen will cause low flow.

 

[boatdock]

Re: water pump question, please.

Wow, thanks for the tips, The impellors ends are melted off, the plastic alinement pipe was melted a little. The engine does fine in the water. I have replaced the entire water pump assembly twice. Each time i tested it in the yard with the muffs after assembly, i normally have a stream of water shooting from the inside pipe coming from the water pump about two feet.(So good flow) When running in the yard i only have the out drive 3/4 of the way down.(maybe thats it).Should water run through the engine without the engine being on if muffs are used?(Mine has never done that). The engine does not have a T-stat installed (I am waiting on part for this). The out drive does make noise when running in the yard at times,(I think that is the gimbel bearing). Compression is great(140's), new risers,heads,head gaskets. i do have a broken exhaust bellow, but i don't think thats too big of a deal. Anyway i will make sure to put the drive all the way down, and give it another try.

 

[zzzzz]

Re: water pump question, please.

Dock- watch the bellows ! if its the lower bellows that's cracked...not too bad....if it's the upper bellows replace it ! it can SINK ya' !

 

[2770]

Re: water pump question, please.

something does not sound right here.The impeller will sustain that kind of heat and friction damage only if it is run dry.For the tube to melt it could not have happened when the boat was in the water.The water would not allow the housing to get that hot.You must make absolutely sure that you turn on the water FIRST and then start the boat.Also do not rev the engine on muffs. you must also make sure the muffs dont slide off of the intake screens.Take an ice pick and push it through the muff opposite side of the hose inlet.Push it through the rubber until you feel it insert in to the vent,this will prevent sliding off as you are running. CC

 

[Boatist]

Re: water pump question, please.

Dock<br />about not running with the outdrive raised I mostly ment on the water where when raised might suck air.<br /><br />You dont have kids that like to start boat and act like driving when your are not at home do you. I would check input screens and make sure they are not plugged up. Do you start the motor to get all the water out after you get it on the trailer?? If impeller melted has to be running without water on it sometime.

 

[boatdock]

Re: water pump question, please.

Thanks everyone, the bellow in question is the exhaust(Lower). The kid do not have keys to the boat. It is possible that in shallow water i raised the out drive up too high. What is the normal running temp? Mine runns at 180 at 3000rpms and goes into the red at 4000 rmps but cools back down if i get off of it. I can not find a T-stat that will fit my old style housing so it does not have a T-stat. Can bad manifolds cause any of this, i don't think they are bad ,however thats one of the things that i have not replaced.<br /><br />Thanks again.

 

[sea wolf]

Re: water pump question, please.

it sounds to me that water isn't getting from the lower unit to the water pump. did u take the drive apart? are u sure everything was put back together properly? are u sure the pump isn't air bound?

 

[Boatist]

Re: water pump question, please.

Dock<br />Normal for temp for sea water is 150 to 170 degrees. If have fresh water cooling then sometimes raise temp to 180. In Saltwater get a lot more desposits above 170 degrees. The overheating can be caused be a plugged up exhaust manifold. They last about 4 to 6 years in salt water. Thing is the impeller blades melting in my opinon can only be caused by running with no water on impeller.