1987 3.7L Mercruiser is overheating

[YJ]

So we went out on the lake this weekend, hoping to do some serious tubing. About 5 minutes after we pulled away from the dock, I hear a loud hissing sound and a cloud of white steam going from every crack in the doghouse - turned out the antifreeze boiled out rather violently and came out of the upper filler cap. I let the engine to cool off, filled out the reservoirs with lake water and went back. By the time we got back to the dock, the engine was running between 175 to 200F. I have never had such a problem before - last time went on the lake a month ago without any issues.

Q: any ideas what might have happened and caused the engine to overheat so rapidly? where do I start to troubleshoot this? I have some experience working on car/truck engines/tranys, but never on the marine stuff - I don't even have a repair manual for this boat. Do the marine systems even have such things as a thermostat? Any help would be appreciated. It's an '87 Searay with 3.7l I/O Mercruiser, 165hp, Alpha One 1st Gen., I believe.

 

[Don S]

Re: 1987 3.7L Mercruiser is overheating

Q: any ideas what might have happened and caused the engine to overheat so rapidly?

When was the last time the raw water pump impeller in the outdrive was replaced. You didn't by chance start the engine without putting water to the drive did you? If so, then it doesn't matter how old the impeller is, it's junk.

Being an overheated 3.7L (aka 470) you may have further problem with a blown head gasket.
Yes, there are service manuals, owners manuals, thermostats, and all sorts of stuff for boat engines and drives.
Here is a thread (actually a sticky) with lots of information.
http://forums.iboats.com/showthread.php?t=288430

You can also download the OEM service manual for your engine HERE, and for your outdrive HERE

 

[YJ]

Re: 1987 3.7L Mercruiser is overheating

Thank you, Don, for your quick response and input.

It sounds like I can be put in a general pool of ignorant dummies who learn the hard way - yes, I did start the engine and ran it for a couple of minutes in my driveway without the water supply for the lower unit (to check the charging system). I thought since it was in "throttle only" position, it would not affect anything outside of the engine (which has its own cooling system). It sounds like boy was I wrong.

I'll check the impeller and everything up the line to assess the damage. Really appreciate the links - it will make the work so much more easier.

 

[dirtdiver]

Re: 1987 3.7L Mercruiser is overheating

Yea the block and made out of aluminum and the head is iron, so the chances of blowing the head gasket are high.

 

[stonyloam]

Re: 1987 3.7L Mercruiser is overheating

Plan on replacing the whole pump, impeller, housing everything. When you get it running (on muffs), do a compression check and cooling system pressure check just to make sure you are OK. You can get the tools to do both on free loan from Autozone. Good luck

 

[EddiePetty]

Re: 1987 3.7L Mercruiser is overheating

Plan on replacing the whole pump, impeller, housing everything. When you get it running (on muffs), do a compression check and cooling system pressure check just to make sure you are OK. You can get the tools to do both on free loan from Autozone. Good luck

........and, when you remove the existing pump impeller, and ANY pieces of the vanes are missing, ya' gotta find them!!! The very smallest will have passed through but the larger pieces are probably lodged on the heat exchanger's inlet tube sheet (blocking coolant water). If you severely overheated the engine, in addition to head gasket woes, the flapper (shutter) valve has probably been destroyed and is now lodged in the exhaust passages.

 

[salty3rd]

Re: 1987 3.7L Mercruiser is overheating

Plan on replacing the whole pump, impeller, housing everything. When you get it running (on muffs), do a compression check and cooling system pressure check just to make sure you are OK. You can get the tools to do both on free loan from Autozone. Good luck

You should also pull the end caps of the heat exchanger off to see if any of the rubber impeller pieces are blocking the tubes. I would also check the voltage regulator and power steering cooler for pieces of rubber. When this kinda stuff happens to me I just think of the old saying; "your education always costs a tuition". Good luck and with a little luck you will be back on the lake this weekend.

 

[stonyloam]

Re: 1987 3.7L Mercruiser is overheating

flapper (shutter) valve has probably been destroyed and is now lodged in the exhaust passages.

Excellent observation. Probably a good idea to pull the whole outdrive and take a look. Easier to change the impeller with the outdrive off anyway. Here is where the shutter is: http://www.mercruiserparts.com/Show.../46.png&inbr=908&bnbr=230&bdesc=EXHAUST+ELBOW Good luck.