77 mercruiser 470 overheating

[Jamesdfw123]

Hello all, new to the forms but been on Iboats for a while. Just wondering if somebody could point me in the right direction on finding a water clog.

Couple of recent things done to the boat... by mechanic.

Fixed stringers

New impeller

New fiberglass work for motor mounts

New bilge pump and a few other miscellaneous items.

Things I have recently replaced or repaired.

New thermostat and gasket ( Due to recent overheating problems)

New steering wheel

New stereo, speakers, amplifier (not installed yet)



So I pick up the boat from the mechanic a couple of weeks ago and take it out on a test run the following day. Runs great, temperature never exceeds a 120. About a week later I get around to taking it to my slip and a couple of minutes after I launch it, starts overheating. Replaced thermostat. Only actions taken so far. I did do a coolant flush and while I would pour the coolant in as it was draining out the heat exchange it would drop back down to 120 or 130ﾟ but Eventually it started bubbling back up on me. Think I'm gonna start at the back of the boat where the water 1st enters and work my way to the other side of the engine.


My experience with boats is minimal but I'm pretty mechanic savvy when it comes to general engine knowledge. I'm just having a hard time finding a good schematic on the hoses.


Thanks to any help anyone can maybe give me.

 

[GA_Boater]

Welcome aboard.

Are you boating in salt or fresh water? Salt buildup can cause clogs.

When the impeller was replaced, Do you know if any of the vanes were missing? If so, the vanes can block water flow and all the missing chunks need to found.

I would take the heat exchanger to a radiator shop for pressure/leak testing. Also after a 470 overheats, compression and leak down tests are almost a necessity because the aluminum block and iron head expansion rate differences can cause a head gasket to fail. If you sniff the filler of the exchanger do you smell exhaust odor? Exhaust smell or milky oil are signs of a blown head gasket.

Hope some of the more knowledgeable 470 guys will offer more and/or better advice.

 

[Scott Danforth]

I agree with GA on the HX being tested

once you over-heated, you need to change the head gasket as it is toast.

 

[Bondo]

and while I would pour the coolant in as it was draining out the heat exchange it would drop back down to 120 or 130ﾟ but Eventually it started bubbling back up on me.

Ayuh,.... Welcome Aboard,..... Sounds like a compression leak into the coolant passages,....

Probably the blown head gasket Scott mentions,....

 

[Jamesdfw123]

Thank you all for your responses. So the culprit could be a faulty heat exchanger which in turn Has most likely messed up my gaskets?

 

[Jamesdfw123]

Oh, by the way it is fresh water.

 

[Jamesdfw123]

If I have a 3" heat exchanger, can I go to a 4"?

 

[Scott Danforth]

you can, however the initial issue was most likely your raw water impeller. the original 470 with the 3" HX did fine, until the very moment a leaf or bug or air-bubble blocked the smallest amount of water to the HX. once the temperature of the motor rose by 10 degrees, the head gasket was toast. the system was well balanced with the 3" HX, the 4" HX just gave a bit of head-room prior to needing to change the head gasket.

the head gasket issue is because the block is aluminum and the head is steel. compound this with the floating bore design and head gaskets get replaced often on the 470. use the OEM mercruiser gasket. it appears to be the only gasket that lasts a few seasons.

back to your motor. yes you can upgrade to a 4" HX. however unless you get a smoking good deal, the HX upgrade and other system upgrades will cost you more than a 4.3 replacement.

when you replaced the raw water impeller, if there was any scoring of the pump cover, replace it. if there was a missing vane from the impeller, find it. anything that is not 100% perfect will take out the head gasket.

you changed the thermostat, however that probably was not the root cause issue of your over-heating.

 

[QBhoy]

Have you checked the obvious overheating cause on this engine ? The closed cooling water pump seal ?
as mentioned...if she has overheated badly enough..likely the head sealing interface may be damaged. Two dissimilar metals from memory. Doesn’t end well usually.

 

[Jamesdfw123]

I have not checked that yet. I did notice while I was trying to diagnose the problem, the hoses around the back of the engine had extremely high pressure and were very hot. All of the other hoses seemed a bit flexible or squeezable. But coming in from the Stern of the boat into the Stern of the engine side were extremely hot and again so much pressure I could not squeeze at all.. Don't know if that helps at all. I'm going to take it to the house this weekend , put the muffs on it and see if that helps me with anything. If a gasket is blown can I do some kind of pressure check on the cylinders or see water running down side of engine? I have ran across a few 4" heat exchangers for a couple hundred dollars which is not bad. Have not spoken with my mechanic yet to see how much head gasket replacement would cost. Considering I just put $2400 on it last month, I'm not in a hurry to ask him. Haha

 

[Jamesdfw123]

There's also been some confusion on exactly what Engine is in the boat. When I got the thermostat the parts guy said it was a 165 due to its location. 4624738 is the serial number. Has a 470 Stern drive. I've contacted a few parts people and other companies but they really can't seem to narrow it down. Anybody know of a way to tell the difference? I would post a few pictures but file sizes too big.

 

[Scott Danforth]

no, you wont see water streaming down the outside of the motor.

with the 470, you really need to learn to turn your own wrenches. taking it to a mechanic will cost you more than the boat is worth in a very short time. nothing against buying shop labor, however it can get pricey very quickly.

if you turn the wrenches yourself, your head gasket replacment is about $100 for gaskets and coolant and about 4 hours of your time.

if you buy a used HX, you need to have it tested.