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 2 water pumps on Mercruiser
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Author Previous Topic: Prop Tuning? Topic Next Topic: Norcold cooling unit instalation  

PatSea

RO# 12641

Posted - Oct 02 2011 :  18:00:16  Show Profile  Reply with Quote
I just replaced the impellor on the raw water pump on one of my 1987 Mercruiser inboard FWC 454CI engines. It got me to thinking about the fact that there are really 2 water pumps, the raw water pump and the engine circulating pump. The raw water pump supplies water from the lake to the circulating pump, correct? Why are there 2 pumps? Couldn't one pump do both jobs, supply the water and circulate it through the engine? How do these pumps work together in series? Is there a rubber impellor in the circulating pump? Does that impellor ever need to be replaced? Sorry for all the questions, but I'm just trying to understand how this sytem works.

Homeport: Catawba Island, Ohio

JVM225

RO# 28365

Posted - Oct 02 2011 :  19:17:39  Show Profile  Reply with Quote
The circulating pump (Water pump for us car guys) circulates the water in the motor. The raw water pump brings the water up in to the boat for the circulating pump.

87 Sea Ray, 30' Sundancer
Merri Mack
MMSI #338122779
95 Eastern 22'
C6 Corvette Convertible
68 GTO


Homeport: Farmingdale NY Go to Top of Page

j-d

RO# 15782

Posted - Oct 02 2011 :  20:39:48  Show Profile  Reply with Quote
The circulating pump has an impeller but it's a paddlewheel-looking thing. A car water pump will work on a boat engine, but if it's raw (salt) water cooled the car version's impeller will corrode away. The marine version has a non corroding impeller and probably different bearing/seal assembly. Not sure about the latter, but definitely the former (impeller). This pic is a Buick pump. The ones on Chevy and Ford have a plate on the back so you can't see the impeller.


God Bless, jd
1996 Sea Ray 215EC
Alpha One GEN II 5.7L/350CID/EFI/220HP
14-1/2*19 Stainless RWC

Edited by - j-d on Oct 02 2011 20:49:17

Homeport: Sunny Florida Go to Top of Page

zane

RO# 19940

Posted - Oct 02 2011 :  20:46:12  Show Profile  Reply with Quote
On raw water cooled perfomance engines many people,myself included have done away with the horsepower robbing circulator pump. Cleans up the front of the engine too.yes the raw water pump can do it all, but you have to put dumps in. On a day to day family boat the stock set up is best.


Homeport: long island, n.y. Go to Top of Page

Comogene

RO# 9942

Posted - Oct 02 2011 :  21:01:55  Show Profile  Reply with Quote
Pat:

The pumps do NOT work in series. Inboard and some I/O engines can have either of two basic cooling systems. A so-called, raw water cooled engine only has one pump - the internal circulating pump which sucks raw water into the engine, circulates it and then sends it out the exhaust. The raw water does all of the engine cooling. A fresh water cooled system is a bit of a misnomer-it is really a "closed cooling system." In fact it is two separate and distinct cooling systems. Raw water enters by means of the external pump which heads to a heat exchanger before exhausting out. The engine itself contains antifreeze which is circulated by the internal pump. It also heads to the heat exchanger where it is cooled by the raw water much like an auto radiator cools your cars antifreeze and then recirculates.

From what you describe, you have a "closed" system which has two pumps and a heat exchanger. Your lake water or raw water typically does not go into the engine itself, just the heat exchanger and then to the exhaust risers. As noted before, the external pump has a rubber impeller which needs replacement as necessary. The internal circulating pump has a metal impeller and rarely needs servicing.

Gene



Homeport: Beverly, MA Go to Top of Page

SCORPIO

RO# 4810



Posted - Oct 02 2011 :  21:02:25  Show Profile  Reply with Quote
Dumps? please elaborate.

Chris USPS AP

Homeport: Lewes, Delaware Go to Top of Page

meide

RO# 13472



Posted - Oct 02 2011 :  21:10:52  Show Profile  Reply with Quote
Gene,

Even a raw-water cooled marine engine usually has two water pumps (the circulating pump in the block and an external pump to supply the cooling water). The circulating pump cannot do the work alone since it is not self-priming.



Homeport: Pasadena MD Go to Top of Page

Chief Alen

RO# 31229

Posted - Oct 03 2011 :  04:40:59  Show Profile  Reply with Quote
Just wondering how many ponies does the circulating pump run by a belt takes away from the flywheel ?

A guess is all i want to know.



Be good, be happy for tomorrow is promised to no man.

Homeport: Manalapan, NJ Go to Top of Page

PatSea

RO# 12641

Posted - Oct 03 2011 :  06:37:07  Show Profile  Reply with Quote
Thanks for the responses. I have a much better understanding of the system than before. My engines are definitely raw water cooled and has both a raw water pump and a circulating pump. Sounds like the primary failure mode of the circulating pump is the front bearing failure.


Homeport: Catawba Island, Ohio Go to Top of Page

j-d

RO# 15782

Posted - Oct 03 2011 :  07:46:42  Show Profile  Reply with Quote
...the primary failure mode of the circulating pump is the front bearing failure...
Correct! There's usually (and I mean nearly always) a "weep hole" in the lower front nose of the pump, right where the shaft comes out. A trickle, drip or stain there indicates failure to follow. May also rattle or clatter.


God Bless, jd
1996 Sea Ray 215EC
Alpha One GEN II 5.7L/350CID/EFI/220HP
14-1/2*19 Stainless RWC

Homeport: Sunny Florida Go to Top of Page

ronp

RO# 23477



Posted - Oct 03 2011 :  08:16:58  Show Profile  Reply with Quote
I would think the issue with NOT having the circulator pump would be at times when the tstat is closed - most raw water is going right back out through the manifold and nothing would circulate water around the block. I expect this would lead to hot spots, maybe enough to boil and make steam.

Thanks,
Ron

Homeport: Amityville, NY Go to Top of Page

zane

RO# 19940

Posted - Oct 03 2011 :  10:04:44  Show Profile  Reply with Quote
its called a crossover system and u can find them on performance marine sites. hardin marine sells a good one.. You do actually drill holes in the thermostats to keep circulation. You have to put dumps in because when the stat is open the raw water pump brings too much water into the system and the engine doesnt heat up correctly. Its an estimated 15-20hp gain. I wouldnt do it on a day to day cruiser but did do it on my stinger when i had it. doing that ,changing exhausts and intakes can wake up a stock motor relatively cheap. I wouldnt do it on a family boat though.


Homeport: long island, n.y. Go to Top of Page

Comogene

RO# 9942

Posted - Oct 03 2011 :  11:17:01  Show Profile  Reply with Quote
Learn something new every day! Never had a RWC engine, except O/B.

Gene



Homeport: Beverly, MA Go to Top of Page

Sandy

RO# 1159

Posted - Oct 03 2011 :  12:24:33  Show Profile  Reply with Quote
To give you a better visual idea of how others above have described your 2 separate but related cooling system circuits , you could check in the Merc 454 service manual.

http://www.boatfix.com/merc/Servmanl/16/16B6R2.PDF ( From the very handy Boatered "Mechanical How-to Articles Forum" ,p.2.)
Even though it doesn't look like a link since it is not underlined , scroll down to & click on "6B" to open up the cooling system section where there are good diagrams of the whole system.


Sandy

Homeport: The Vineyard Go to Top of Page

PatSea

RO# 12641

Posted - Oct 03 2011 :  13:25:09  Show Profile  Reply with Quote
Thanks Sandy. I have the seawater cooled system but I found that diagram. I've learned a lot about this system through this whole exercise including this thread.


Homeport: Catawba Island, Ohio Go to Top of Page

Sandy

RO# 1159

Posted - Oct 03 2011 :  19:48:05  Show Profile  Reply with Quote
Sorry PatSea, I had read your description as FWC in the 1st post and missed your correction above. That's Fresh-Water Cooled with the separate circuits , as opposed to RWC , raw-water-cooled with one circuit that you now are indicating is what your is. Either way , it really helps to understand your engine's cooling system .

I have FW & RW & flow -direction arrows on my engines' hoses marked with a paint stick to aid in tracking down temp/flow issues to/from each component with hoses whose proximity to each other can deceive the tired brainium.


Sandy

Homeport: The Vineyard Go to Top of Page
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