# Mercruiser standard cooling system question



## a1SlowHand (Oct 6, 2007)

Howdy All,

I have a question about an 1985-87 I/O Mercrusier 5.7 cooling system flow. I'm trying to understand and trouble shoot a cooling problem. I know the impeller picks up the seawater but what I don't know is how far that water will travel in the cooling system. Does the impeller have enough pickup to push that water all the way to thecirculating pump? Or does the circulating pump pull that water from the impeller area? Or am I totally out of the ball park?

Thanks


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## Tuna Man (Oct 2, 2007)

The foot impeller does most of the work. The pressure output of the motor water pump is only about 5-7 psi. I have a water pressure gage hooked up to my motor water pump. 350cui/I/O

Be specific....what seems to be your cooling problem, I've diagnosed a few on here. Have had my own also.

If you have installed anything (engine wise)prior tothe cooling problem be sure to mention it, no matter how insignificant you might feel it is.


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## a1SlowHand (Oct 6, 2007)

Yes you have Tuna Man and I have been reading some of your prior "helps" for some clues as to where I should test next. 

The Problem: I installed a new sterndrive (<--- read not rebuilt) I then started it up with the earmuffsand ran it through froward and reverse a couple of times at low rpm.I ran it for about 30 minutes at various rpms never execeeding3000. My maxrpm should be in the 4600 - 4800 range. Everything worked great. Next day I was again running it with the earmuffs and had worked my wayup to about 2400 rpm. Everything sounding and running smoothly. Then I noticed the temp gage a little high (about 160), so I just keep an eye on the gage to see if it would cycle back down. When it reached 200 I shut it down and started tracing water lines. 

Well when I got the input water line that run through the transom I noticed no water coming in. So I pulled the lower unit off and found a melted water tube that runs from the impeller housing to the copper tube. I changed that out and reinstalled the unit and tested it again. Still no water coming through the transom.

So I guess you are going to tell me I need to change out the impeller, right?


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## Tuna Man (Oct 2, 2007)

NO, not at this point.First thing to check is when the cups are on and the water hose on...is there ANY water coming from anywhere around the outdriveGimbalassembly (outside the boat) . There is a hose that runs from the back side on the port side of the outdrive that goes up to the *pipe *that goes through the transom. It is hard to see and attached with a worm gear clamp. I have seen this hose slip off, and DON'T ask me where I saw it either.:banghead:banghead

NEW stern drive...Was it a COMPLETE outdrive including the transom mount housing, where this hose was disconnected. This hose has to come off if a new gimble housing is replaced.



















So with a melted tube and the lower unit off, you failed to pull the impeller housing and check the impeller.:banghead:banghead:banghead

If the hose is on where I told you to look, and no water flows out anywhere else but from around the cups...go for a new impeller...it got pretty HOT.


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## a1SlowHand (Oct 6, 2007)

I did remove that hose and hooked up the muffs, water shot a good foot and half in the air. That was why I got to wondering if the circulating pump pulled or the impeller pushed the water.


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## a1SlowHand (Oct 6, 2007)

> *
> 
> NEW stearn drive...Was it a COMPLETE outdrive including the transom mount housing? *


*

Sorry missed this question. No it was just the drive, I used the original housing.*


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## Tuna Man (Oct 2, 2007)

PM phone #


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## The Maintenance Shoppe (Nov 5, 2007)

Your water hose cannot supply enough water for the pickup pump while revving the engine.

Only run your engine below 1,200 rpm while on the hose or risk damaging your water pick up pump.


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## a1SlowHand (Oct 6, 2007)

Thank you TMC for the information that was something thatI wasn't aware of, in fact all I have ever heard was don't turn the hose on too strong or you risk blowing seals and such.


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## sosmarine (Sep 28, 2007)

It really depends on your water pressure and size of your hose as to whether or not it will supply enough water. I run motors all the time above 1200 RPM and seldom have a problem. 

How old are your Exhaust Manifolds? Why did you change your O/D? What has the temp. when you ran it last before changing the O/D? Check your P/S cooler, if equiped, for blockage.

FYI, that plastic tube is for alignment purposes and doesn't carry water.


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## a1SlowHand (Oct 6, 2007)

> *sosmarine (2/17/2008)*It really depends on your water pressure and size of your hose as to whether or not it will supply enough water. I run motors all the time above 1200 RPM and seldom have a problem.
> 
> How old are your Exhaust Manifolds? Why did you change your O/D? What has the temp. when you ran it last before changing the O/D? Check your P/S cooler, if equiped, for blockage.
> 
> FYI, that plastic tube is for alignment purposes and doesn't carry water.


I don't have the history on the Exhaust Manifolds, I did pull them off flush water through them that came out clear, new gaskets installed and flush to insure water flow. I change the O/D because I broke off a water pump stud while in the process of installing a new impeller (I didn't knowhow old or conditionof the old impeller) and did not get it drilled out properly.... so just bit the bullet and got an new unit so I would start to have some history. 

I think the temp was in the 140 -160 range, I'm not real sure it's been a while, before replacing the O/D. No P/S cooler installed. Interesting point on the tube being used for alignment purposes only. I was thinking it was a VIP (very important part) ): . 

I want to thank all that has offered suggestions, tips, time to explain the cooling system to me, etc. It's been educational.


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