Mercruiser 470 overheating and leaking coolant + raw water in bilge

[Droppin'Lunkers]

Hello all, this is my first post about my first voyage on my first boat.

I recently purchased a beautiful garage kept 1977 Marquis - stingray With less than 150 hours on the engine and routine maintenance kept up and documented.

However, about 30 minutes into my first outing the boat began to overheat and blew the water pump seal and steamed and leaked coolant from the weep hole. Also, the bilge was filling with raw water from
somewhere else and was mixing with the coolant from the weep hole.

luckily I was close to the dock so I let the engine cool down and emptied the bilge, then restarted it and slowly ventured back until it began to overheat again. I would shut it off around 170 and I repeated this process about three times until I got to my trailer.

I am not concerned I blew a head gasket and the engine runs beautifully and starts up first time. I do know I have to replace the water pump and the seal , and possibly the timing cover seal. i also suspect the heat exchanger is at fault but I'm hoping you all can tell me exactly what is going on.

my main question is where in the exhaust system is the raw water coming from in the bilge? Where in the exhaust system is this coming from??? I do know the coolant was leaking from the weep hole at various flows depending on rpms, and also from the reservoir cap.

so where is the raw water coming in from, what is the culprit , and what should my shopping list be? Engine is accessible to replace all of these parts.

I do my own repairs on vehicles and my father is an engineer who fills in the blanks on any issue. This is new territory for me and partly for him. Please help me solve this issue and I can send any pictures you may need.

thank you all

 

[Scott Danforth]

welcome aboard

your heat exchanger discharge goes to your exhaust elbow

you are either leaking raw water before or after, or you have a leak, which is highly probable on a 1977 boat

do some reading on all the issues.....simply google mercruiser 470 problems and you will spend days.

because you over heated, you most likely did blow the head gasket (they blow with as little as a 10 degree overheat)

if you blew water out the water pump vent, you most likely also have water in your oil. you will most likely need a speedi sleeve on the cam snout

 

[Droppin'Lunkers]

Thanks Scott, I am prepared for the worst and am gonna check into all of it. As for the leak , we sat in the in water for a while before each startup and took on no additional water, it would only fill as the engine heated up. What is the normal temp range because it never surpassed 180. Thanks

 

[Scott Danforth]

depends on the thermostat, however should be a 160 degree thermostat

had you come here looking for advise prior to buying, we would have tried to dissuade you from buying a boat with a 470

however whats done is done

before you go out again, make sure you pull the drive, check alignment (annual inspection) check the gimble bearing (annual inspection), check the u-joints (annual inspection) and do a gear oil change (annual maintenance) and replace the raw water pump (should be done about every 5 years.

then do all the engine maintenance.

keep in mind your 44 year old boat only has 150 hours on it. most of which were on the boat between 1977 and 1980 (assume 60). so over the past 40 years, it probably sat for long periods of time

you will find problems.

boats should be enjoyed to the tune of about 50-100 hours per year.

 

[Droppin'Lunkers]

Thank you for the wisdom.

just went around to check a couple things. And noticed a couple irregularities and a couple positives.

positives - pulled the dipstick and no traces of water or brown milky appearance. Fresh black oil that I Topped off before going out. Also, opened the reservoir cap and no sign of oil, just nasty rust clad residue , but it is bone dry after yesterday.

irregularities- ( pictures attached. )

1. The reservoir cap is completely cracked across the inside of it.

2. On the rear of the heat exchanger ( near the stern)it appears that the gasket Between the exchanger and the exchanger-cap is not aligned and Is protruding out.

Could either of these be the reason for raw water leak and/ overheating?