Mercruiser 470 cooling system

"I bought a 79 mercruiser 470

"I bought a 79 mercruiser 470 boat last summer, had troubles with it running hot. Replaced impeller, cleaned out heat exchanger and installed new radiator cap and housing. Pressure tested system and noticed coolant leaking out of lower unit from water intake area. Wondering where this is coming from? It seems to me it can only be mixing in the heat exchanger or exhaust mainfold. Any help would be appreciated."

Sounds like the heat exchanger

Sounds like the heat exchanger may have a leak since you "Pressure tested system and noticed coolant leaking out of lower unit." You could have antifreeze leaking from the exh. man./riser gasket too. It sounds more like the exchanger.

"You should be able to take yo

"You should be able to take your heat exchanger off the boat and into a radiator shop for pressure testing. Most places don't even charge for it. At least you will know it's the culprit with minimal cost. If it isn't the problem then move on the the really expensive part, the ex manifold. Let's hope for a bad heat exchanger!

BTW my 470 runs at around 140 degrees F. I think they run these motors pretty cool relatively since they are aluminum blocks."

"Since yours is a 79, you prob

"Since yours is a 79, you probably have a 3 inch. If you need a new HE, consider up-grade to a 4 inch. My 470 ran at around 160-170. The old log style exhaust is not availabe, you will need a used one or upgrade to newer style. Since the older style are FWC, there are plenty still around.
Chris"

"Thanks for the responses, I

"Thanks for the responses, I took apart my heat exchanger and pressure tested it. Found three tubes had holes in them. Brazed the ends of the bad tubes shut and reassembled. Took the boat out today at it never got over 150. I'm still thinking of upgrading to the 4 inch exchanger, not sure if my band aid fix is a long term fix. Thanks for the help everyone."

"I would change it out if your

"I would change it out if your current HE has a lot of corrosion and has the possibility for new holes in the future.

But, if it aint broke don't fix it. I have the stock 3" HE and it works fine as long as there are no holes in it. If you have confidence in the braze job and the rest of your HE looks good then spend the money elsewhere. Just keep flushing it out regularly. They are easy to service, just pull the end cap off and lower your bow.

A OEM 4" HE may not be as expensive as an Ex manifold but they are not cheap either. A few hundred bucks anyway."