Can You Run 2 Raw Water Pumps

PGtoy

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Jan 1, 2005
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I have a 1996 5.7 Merc Alpha 1 that I changed to fresh water cooling. with salt water it used to rum at 160 now with fresh it will run to 200+<br /> I have changed the engine pump, new out drive pump. Flushed the system a hundred times. All with no luck below 3000 rpm it will run all day at 175 - 180. Could it be I need a engine driven raw water pump for more water flow with the Alpha drive? and can I "T" them to feed the engine? I have a through the hull fitting in place for the AC so that is not a problem. Need some advice on this one........ Thanks
 

Don S

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Re: Can You Run 2 Raw Water Pumps

That is exactly the problem with changing over a salt water engine to a fresh water cooling system. About 50% of the time the rust and corrosion inside the engine flakes off and plugs up the heat exchanger. Most of the time, you can't get them cleaned out because the flakes get wedged between the tubes and you can't get there to clean them.<br />You could try an engine driven pump, but don't T it in with the drive pump. There is another post that someone else can tell you about that explained how to bypass the water from the drive pump.<br />Basically, if the water is cold (below 60°), then you shouldn't need a belt driven pump on the Gen II drives unless you have fresh water cooled manifolds also.
 

WillyBWright

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Re: Can You Run 2 Raw Water Pumps

To run a motor mounted pump, you have to put another thru-hull in and eliminate the Alpha connection. You'll still need that pump in there to cool the drive, but it mussn't go any further. You'll have to plug off the fitting where the hose attaches inside and then eliminate the hose on the outside. You can't use the AC thruhull or you're asking for supply problems. But I gotta agree with DonS. If you don't do the change within the first year, you're guaranteed to have clogging problems forever.
 

PGtoy

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Re: Can You Run 2 Raw Water Pumps

Sounds like I may need to think of going back to salt untill this engine is done then with a new engine run the fresh water cooling. What would be the average life of a salt water engine ? It could be the exchanger is pluged up, That is why at low RPM the engine will cool right down. Just another headake ( Boats )
 

tommays

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Re: Can You Run 2 Raw Water Pumps

the motor will last a long long time in salt as long as the manifolds and risers are not allowed to fail <br /><br />they will last 5 years at most if the boat is left in the water longer if trailered and flushed<br /><br /> not changeing them before they fail is what kills the motor <br /><br />the motor block can stand a lot of salt<br /><br />tommays
 

PGtoy

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Re: Can You Run 2 Raw Water Pumps

Since I went all last year trying to fix the over heating problem, it looks like the best thing is to return to salt water cooling. You can buy a new GM marine enging for 1900.00 it may be best to repower at a later date.
 

jimmythekid

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Sep 21, 2004
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Re: Can You Run 2 Raw Water Pumps

just a thought: try running an oversized engine oil cooler <br />200-210 degrees really isint all that hot, by adding the extra cooling in the form of an oil cooler, it may just bring you down to the 180-195 degree range where it is just where you want it for best efficency
 

waterone1@aol.com

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Re: Can You Run 2 Raw Water Pumps

In my opinion, an engine driven pump circulates more water than the lower unit impeller in the alpha. Before I got rid of the heat exchanger, I would switch over to a "bravo type" or engine mounted pump.
 

PGtoy

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Re: Can You Run 2 Raw Water Pumps

So you think 200 is not really that hot I can get it to 200 - 210 but thought I would hurt the engine in a boat. At 3500 RPM it will run at that temp. I could do a oil cooler, got plenty of room . I even thought of hooking up the water heater to the engine cooling it has the places for engine heated water heating.
 

Bondo

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Re: Can You Run 2 Raw Water Pumps

How big of a heat exchanger are you running ????<br />Did you use a "Kit" ?? <br />From Who ?? <br />Have you discussed this issue with their techs ???<br />What T-Stat do you have in it now ???<br />There's no reason you should need more water than the alpha can supply......<br />Your issues lie somewhere else......<br />There's no reason not to hook up your hot water heater either.... <br />I just plumbed in a nice Heater out of a Mack truck, into a Highliner for cabin heat..... Works Great....
 

PGtoy

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Re: Can You Run 2 Raw Water Pumps

One big enough for a full closed system, I changed it to a half system when I started thinking I was overheating.<br /> Yes I used a kit from Peformance Product Technologies, Yes no help<br /> 175 - 180 I think at least by the temp gage and heat on the exchanger that is where it opens at.<br /> I just keep thinking there is something I can do to fix this, at 0 to 3000 RPN it runs to 180 after that the temp will rise just like the RPM<br />I have had it to 210 all gage readings. You drop it down to 1000 or so and it will fall back to 180 and run there all day long. Now keeping in mind this is a 7 year old engine in the 8th year I went to fresh water cooling. I did flush the block several times before the kit was installed<br />and several times after. I guess the block may have been in salt to long although the engine only had 50 hours on it when I got it a year ago.
 

Don S

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Re: Can You Run 2 Raw Water Pumps

200° is too hot for a marine engine in an enclosed compartment. You SHOULDN'T be running more that 180° or 185°<br />If you've dropped it down to a half system, the drive pump should keep it cool with no problem.<br />When whas the last time you replaced the raw water pump impeller in the drive. A marginal impeller or one with a set in it will cause your problem. You have an Alpha I Gen II drive which has a bigger pump in it than the older alpha I drive pre 91.
 

PGtoy

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Re: Can You Run 2 Raw Water Pumps

I replaced the impeller when this first started about the second time I used the boat. That was my first thought. Then I replaced the engine water pump I just knew that would do it, Then I went to a half system " that had to be it " and I am still thinking of my next move short of going back to salt untill this engine fails. There has to be something I have over looked. That is why I thought of the amount of flow. I did the flow test in the Merc service manual and that was as far as I could tell good. <br /> Could it be there is just to many years of rust built up in the engine that it will no longer transfer heat good. or as some one said in another post, the exchanger was cloged from the first time I used it.
 

Bondo

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Re: Can You Run 2 Raw Water Pumps

the exchanger was cloged from the first time I used it.
Take it off,+ Back-flush it into a bucket,+ See what's there...........<br />If there's enough Crap in it to hinder cooling, you Should see Lots of it......
 

skyking_22

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Jul 1, 2002
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Re: Can You Run 2 Raw Water Pumps

Phil, I put a San Juan Engineering closed system on my boat using just the stern drive pump and it works fine ran around 160-170 degrees. I had a 2000 Mercruiser 5.0L. I recently changed the engine to a Mercruiser 5.7L (my other post) and installed the belt driven pump with a through hull fitting. Runs great 155-165 degrees. Several things I notices was there was a new gasket for the exhaust elbows that if not installed correctly will decrease the flow, the other was changing the thermostat of course, and I needed to put a new temp sensor on, at first I was getting false high readings.
 
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