470 Merc over heating issue

i need some help here please. I have changed the outdrive impeller and when driving my boat the engine will tend to overheat. There is no coolant leaking issue out of the circulating pump area. I am at a loss right not. Anyone have any ideas?

Re: 470 Merc over heating issue

sparke444 said:

i need some help here please. I have changed the outdrive impeller and when driving my boat the engine will tend to overheat. There is no coolant leaking issue out of the circulating pump area. I am at a loss right not. Anyone have any ideas?

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is your thermostat opening up, is your heat exchanger clogged up is the riser clogged, you need to tell us what you have done so far to narrow down

Re: 470 Merc over heating issue

the out drive impeller has been changed. We have checked the weep hole from the water circulation pump and are getting nothing there.
I am looking to get help with process for flushing out the heat exchanger and for checking out how the water flows throught the exchanger so I can see where it should be hot at and cool at and where to find a possible issue at.

Re: 470 Merc over heating issue

Take the endcaps (have new gaskets handy) off of the heat exchanger and take a look. There is only raw water there so don't worry the antifreeze will not leak out. The heat exchanger works this way: Cold raw water flows in through through the regulator to the front hose, through a set of tubes to the back then through another set of tubes back to the front, then back through a third set of tubes to the back where the warm raw water exits up through the aft tube power steering cooler (if you have one) up into the exhaust elbow where it circulates around the elbow and is dumped into the exhaust. in the meantime the antifreeze flows into the the more forward of the two aft tubes and is directed by baffles around the tubes where it is cooled, then exits out of the more aft of the two forward tubes. Here is what a single pass exchanger looks like, we have a 3 pass if a 4": http://www.google.com/imgres?hl=en&...&w=469&h=350&ei=Msz9T-bDHIbE0AGR0ZzRBg&zoom=1

So check for anything that can restrict the flow of water through the exchanger. Pull the hose off of the exhaust elbow and with a garden hose check the flow, there should be little backpressure. Good luck.

Re: 470 Merc over heating issue

can the voltage regulator cause a restriction of flow?

Re: 470 Merc over heating issue

No. Just a metal tube.

Re: 470 Merc over heating issue

ok back flushed heat exchanger with a garden hose today and changed the thermostat. restarted the engine and the temp climbed even faster and higher at an idle. pulled thermostat back out and flipped it over(point up). smae result.....my next step should be?

Re: 470 Merc over heating issue

Maybe a compression check to see if you are getting combustion entering the antifreeze circuit. Does it build up excessive pressure on the hoses or radiator cap?

Re: 470 Merc over heating issue

i do not see an excess build up of pressure on the hoses. infact the hoses are easily squished while the engine is running

Re: 470 Merc over heating issue

Backflushing the heat exchanger probably will do no good at all. You need to take the endcaps off, and clean it properly. Did you check the flow through the exhaust elbow? Put the thermostat back the way it should go. The umbrella (the part that opens) of the thermostat goes UP. Take the t-stat and put it in a pot of water on the stove ans monitor the temperature noting where it starts to open (155-162), and where it is fully open (182). Take a look in the cooling section of the manual here: http://boatinfo.no/lib/mercruiser/manuals/mercruiser8.html#/0

Re: 470 Merc over heating issue

Does the heat exchanger get really hot?

Re: 470 Merc over heating issue

Yes the heat exchanger gets fairly hot except for where the raw waters enters it.

I saw a breakdown of the manifold and it showed the point down on the thermostat. The breakdown was for a 1982 thru 1986 470. I'm not arguing to putting out there what info I am finding.

Re: 470 Merc over heating issue

sparke444 said:

Yes the heat exchanger gets fairly hot except for where the raw waters enters it.

I saw a breakdown of the manifold and it showed the point down on the thermostat. The breakdown was for a 1982 thru 1986 470. I'm not arguing to putting out there what info I am finding.

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you need to get a service manual #8 it will tell you all

Re: 470 Merc over heating issue

a couple things could be going on, either the water flow through the exchanger is being restricted in the exhaust manifold (check for backpressure with a hose), or there is something in the exchanger restricting the flow (remove the endcaps) or it could be that the impeller housing has been damaged and needs to be replaced, along with the impeller again.

Re: 470 Merc over heating issue

When I back flushed the exchanger the water seemed to flow fairly good.

How do I check for back pressure with the boat sitting in the water? Disconnect the hose going to the exhaust manifold and put the hose there?

Trying to do as much before we pull boat out of water to check possible impeller trouble

Re: 470 Merc over heating issue

sparke444 said:

When I back flushed the exchanger the water seemed to flow fairly good.

How do I check for back pressure with the boat sitting in the water? Disconnect the hose going to the exhaust manifold and put the hose there?

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There are a bunch of tubs in there and if there is stiff like impeller pieces in there backflushing would just move them out of the way, and when water started flowing the other way they would just get caught again. There is a good chance there is nothing in there, but you gotta check.

Yes. I would raise the outdrive up as high as I could to try to get it out of the water as much as I could, just stick the hose end on the fitting, should not be much backpressure.

Re: 470 Merc over heating issue

stonyloam said:

There are a bunch of tubs in there and if there is stiff like impeller pieces in there backflushing would just move them out of the way, and when water started flowing the other way they would just get caught again. There is a good chance there is nothing in there, but you gotta check.

Yes. I would raise the outdrive up as high as I could to try to get it out of the water as much as I could, just stick the hose end on the fitting, should not be much backpressure.

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i will have to agree you got something restricting the flow i wonder if you have a 3" or 4" exchanger and i will also say your impeller housing could be shot did you give it a good inspection when you changed the impeller. when the housing never gets changed it also wears out from not being changed and when you changed impeller the housing being worn out does not create enough suction for proper cooling. yes the pointed end of the thermostat points down
P.S. are the impeller fins pointed in the right direction?

Re: 470 Merc over heating issue

Its a 4 inch exchanger.

Used a garden hose to check flow out elbow and flow seemed ok.

Pulled exchanger and found tubes clear and a few small pieces of impeller.

Installed thermostat in proper orientation.

Started engine and temp climbed under idle to a point and stopped.

Exchanger was still on the warm side.

Any other ideas or wait to check it under load?

Re: 470 Merc over heating issue

Upper impeller housing was changed and impeller fins were pointed in same direction as old one.