mercruiser 2.5 runs great on muffs but overheats in lake

[bunky2004]

so this is a perplexing problem. When i run with the muffs on in the driveway it runs great, always around 140 and thats all. I put it in the lake last night and she overheated rather quickly. I disconnected the hose on the manifold side of the thermostat housing and fired it up briefly. Managed to soak the inside of my boat. oops. Any thoughts as to what would cause this?? Thank you so very much in advance!

 

[Rick Stephens]

Possible anything is plugging the intake slots or under the pump? Your pressure feeding the pump on the muffs, and drawing in the lake.

 

[fhhuber]

I'd suspect the pump impeller.

A flaky impeller will give ALMOST enough water... add the pressure feed from the hose and you have plenty.

You can get similar results with a NEW impeller with the pump miss-assembled.

 

[Rick Stephens]

I'd suspect the pump impeller.

A flaky impeller will give ALMOST enough water... add the pressure feed from the hose and you have plenty.

You can get similar results with a NEW impeller with the pump miss-assembled.

Agree

 

[thumpar]

My first thought was impeller also. When was it replaced last? I would split the drive and look. Also check for any parts of an old impeller in the thermostat housing. When on the hose you have pressure from the supply. On the lake the impeller has to do it all on its own.

 

[bunky2004]

ok so i took impeller out of drive unit and it is infact brand new. i seem to have no water supply from the intake finns to impeller. how do i get the lower unit off to find a blockage?? do I have to take off the prop first??

 

[thumpar]

Take the whole drive off as one unit and then split the halves. It will make it easier to put back together. The only time I take my prop off is to get to the lube drain/fill screw.

 

[bunky2004]

ok how does the drive come off?

 

[thumpar]

Put it in forward gear. Remove the 6 nut that you see facing rearward on the upper part of the drive. Remove the trim pin that attaches the rams to the drive. You can then pull it off. It only takes about 10 minutes.

 

[bunky2004]

so i dont know if it matters but this is an OMC cobra outdrive.....same rules apply??

 

[thumpar]

Well you don't have a Mercruiser than. I know nothing about OMC.

 

[bunky2004]

thats what strange about the boat, it has a mercruiser engine block with an OMC outdrive

 

[thumpar]

The only thing that is Mercruiser is the label. OMC used the same block as Mercruiser but everything on the outside is OMC. At some point someone just swapped the block.

 

[bunky2004]

gotcha, that makes sense

 

[Bt Doctur]

Cobra is basically like the merc. Fwd gear is best because you have to turn the prop to align the shaft splines for install
remove the 6 nuts and pull rearward.

 

[Bt Doctur]

https://www.youtube.com/watch?v=yaAj_RrRKGk

 

[bunky2004]

so basically put it in forward gear, remove 6 nuts and remove?? that simple?

 

[bunky2004]

of course remove trim rams

 

[Bt Doctur]

Yes