Dual heat exchanger on Mercruiser 470

[vinodubber]

After doing everything to try to overcome an overheating 81 Mercruiser 470 including unblock heat exchanger, replace blown heat gasket and machine head, replace impeller and housing, replace intake hoses and the list goes on. Anyway it now stays cool at a bit above idle but gets hotter as the revs increase. I assume this is due to still having the original 3 inch heat exchanger.

I was wondering if I could install another 3 inch exchanger to double the cooling capacity? I would just try the 4 inch but they are very expensive and hard to find in Australia. I was thinking of putting a Y piece on the outlet of the raw water pump and again on the outlet of the exchanger.
Just wondering if anyone has had any experience with this?

 

[Don S]

Re: Dual heat exchanger on Mercruiser 470

No, you can't use dual heat exchangers. Have you checked the exhaust riser to make sure it's not plugged up? If raw water doesn't flow through a heat exchanger, it will overheat.

Even a 3" heat exchanger won't overheat just above idle.

Have you checked for full water flow coming from the raw water pump in the drive? New thermostat ????

 

[achris]

Re: Dual heat exchanger on Mercruiser 470

Exhaust elbow would be my first step. If that checks out, then do the test for a leaking water pocket cover. A piece of clear hose will make the job real easy. If it is the WPC, then be prepared for a tough fight.....

Chris....

 

[stonyloam]

Re: Dual heat exchanger on Mercruiser 470

The first thing you need to do is find out how hot it is getting. The temperature range for a 470 is 165 to 200 degrees F. If it stabilizes in that range you should be OK. Make sure the temperature gauge is reading properly.

 

[vinodubber]

Re: Dual heat exchanger on Mercruiser 470

The raw water pump has full flow. Exhaust riser is a bit rusted but not blocked. Thermostat is fine. How do I do the WPC test? I have spent a lot of time and money to get this far and have not got much closer to getting the boat usable. Very frustrating..

 

[achris]

Re: Dual heat exchanger on Mercruiser 470

...How do I do the WPC test? ...

Get a length of clear hose of the right diameter and put it on the outlet from the transom to the (if you still have it) voltage regulator. Put the boat in the water (needs to be in the water to put back pressure on the exhaust system) and run the engine. You should see a solid stream of water, if you have any bubbles in the tube, Water Pocket Cover is leaking....

Chris.....

 

[alame]

Re: Dual heat exchanger on Mercruiser 470

Get a length of clear hose of the right diameter and put it on the outlet from the transom to the (if you still have it) voltage regulator. Put the boat in the water (needs to be in the water to put back pressure on the exhaust system) and run the engine. You should see a solid stream of water, if you have any bubbles in the tube, Water Pocket Cover is leaking....

Chris.....

sorry but I'm a little lost on this test " transom to the voltage regulator " ??

 

[Don S]

Re: Dual heat exchanger on Mercruiser 470

Get a length of clear hose of the right diameter and put it on the outlet from the transom to the (if you still have it) voltage regulator. Put the boat in the water (needs to be in the water to put back pressure on the exhaust system) and run the engine. You should see a solid stream of water, if you have any bubbles in the tube, Water Pocket Cover is leaking....

Chris.....

Don't get me wrong Chris 'cause I've never tried this. What I don't understand is how the water pocket, that's on the pressure side of the raw water pump, can suck in air at the same time?

 

[fmalott]

Re: Dual heat exchanger on Mercruiser 470

sorry but I'm a little lost on this test " transom to the voltage regulator " ??

when water enters the motor it comes from a hose from your outdrive then goes to voltage regulator

 

[achris]

Re: Dual heat exchanger on Mercruiser 470

Don't get me wrong Chris 'cause I've never tried this. What I don't understand is how the water pocket, that's on the pressure side of the raw water pump, can suck in air at the same time?

It doesn't suck air. With back pressure in the exhaust system the exhaust gases are pushed into the water flow through a leaking gasket, usually caused by the cover being softened and distorting from a previous overheat when there was no water in the exhaust gases to cool them. I've replaced quite a few, and getting the screws out can be a mission. 1/4" SS screws into an aluminium housing, with time and lots of nice salty water.. :facepalm:...

 

[Pete104]

Re: Dual heat exchanger on Mercruiser 470

Yea, maybe in salt. OMC loved this trouble shooting method, also.
Definitly worth looking at!

 

[Don S]

Re: Dual heat exchanger on Mercruiser 470

Thanks Chris, now it makes sense. I wasn't even considering the exhaust. I have run into those bolts on a couple of occasions.

 

[alame]

Re: Dual heat exchanger on Mercruiser 470

when water enters the motor it comes from a hose from your outdrive then goes to voltage regulator

ok..got it now....thanks

 

[cdiamond]

Re: Dual heat exchanger on Mercruiser 470

air into the pressure side of the water pump output - highly unlikely

can't imagine any exhaust 'backpressure' forcing itself into the water pocket
against the output pressure of the water pump, especially a small leak

the small leak will have greater pressure

any air in a water pump stream that i have seen ( in over 25 years)
has ALWAYS been on the suction side of the pump

just saying

 

[cdiamond]

Re: Dual heat exchanger on Mercruiser 470

to explain further:

air HAS to be DRAWN into a water pump flow
there aint' no way free float air is going to overcome any pressure in a water flow

as far as exhaust 'backpressure'
that's what the exhaust relief ports on the transom assy. are there for

sorry there just isn't any significant 'backpressure' around the water pocket cover

we're not talking about air being introduced - like a combustion leak

 

[achris]

Re: Dual heat exchanger on Mercruiser 470

to explain further:

air HAS to be DRAWN into a water pump flow
there aint' no way free float air is going to overcome any pressure in a water flow

as far as exhaust 'backpressure'
that's what the exhaust relief ports on the transom assy. are there for

sorry there just isn't any significant 'backpressure' around the water pocket cover

we're not talking about air being introduced - like a combustion leak

I have fixed many an overheat problem, that other shops had given up on, by checking (and finding) a leaky WPC gasket. When you find an engine that's got a leaky WPC gasket you'll also find a light coating of exhaust 'soot' in the cooling passages of the engine. If you don't want to use a clear hose, just pull the leg, split it and pressure test the WPC... If it leaks, you'll get exhaust gases in the cooling system. EVERY TIME!!! I know it sounds unlikely, as unlikely as water being drawn back into the cylinders on a 2 stroke with a leaking exhaust cover gasket (old in-line 6 Mercs were notorious for it), but it happens. The dynamics of what goes on in the exhaust passages are very interesting. Just look at the current V6 Mercruisers. If you idle them on the flushers they suck water into the cylinders (it's called 'reversion'). You need to have them running at around 1400rpm (it's in the owners manual). What really twists your nipple-nuts is that they need pressure in the exhaust system (created by being in the water) to stop drawing water in... Go figure...

Just saying!

 

[Bamaman1]

Re: Dual heat exchanger on Mercruiser 470

Good explanation of a potential problem.

 

[cdiamond]

Re: Dual heat exchanger on Mercruiser 470

well chris

i guess some strange things are happening 'down under'

if you think about it - the water pocket cover is not even in the water when the boat is on plane
how in the world is exhaust gases getting in a pressurized system - makes no sense to me

as far as checking it, with the clear hose, idling at the dock

those results are dubious -at best
too many exhaust bubbles around the intake area of the lower unit

i have found the clear hose test
best done while under way

your examples of reversion
i believe are a result of valve overlap
as i understand it

an increase in rpms
forces the exhaust out faster
again - as i understand it

i know some older big blocks suffered the same condition, years ago

enjoyed the discussion

 

[Technorunner]

Re: Dual heat exchanger on Mercruiser 470

achris Will it be the same with a 4.3 with a hullthru system? You wont get that pressure in the exhaust system. So will it suck water back in the cylinders on idle ?

 

[ziggy]

Re: Dual heat exchanger on Mercruiser 470

fwiw
the way i found to find a leaky wpc gasket was while flush water through it backwards. i had a overheat from a bum (dry run) impeller. while flushing back and forth to try to rid the cooling system of impeller pieces, i discovered my wpc cover leak while observing the impeller pieces that i was flushing out. i was introducing water at the hose that comes from the transom and flushing it aft. i had the lower removed. the water was coming out of the wpc and dumping into a bucket. while flushing, i looked at the wpc and ya could see the water coming out of the gasket. it was easy to see the leak. a real bummer to extract 3 broke ss screws in cast alum.