Merc 470 cooling problem (?)

[nola mike]

Noticed that the coolant level in the overflow tank of my 170 was low a bit ago. Had been running fine, no issues with overheating. Coolant in the actual reservoir was fine. So i added coolant to the overflow tank, took it for a spin, saw some smoking in the engine compt. coolant had leaked out the reservoir fill cap. The level in the overflow tank was low again. Again, the coolant level in the reservoir was fine, where it was prior to me adding coolant. At no point did the temp get above 160', even with prolonged high speed running. So...
1.I noticed that the overflow tank has a cap with a nipple on it; it looks like there's supposed to be a hose of some sort on there, but there isn't.
2. When i take off the main reservoir cap and the engine's warm, there is no pressure in the system. Don't know if this is related to point 1.

is this just a problem with me needed either a new overflow/reservoir cap (where can i get one?), or should there be another connection on the overflow cap, or what? as i said, the boat has run just fine for >200 miles since i got it without overheating issues. it's an '86 with a 4" exchanger, if that makes a difference.

 

[stonyloam]

Re: Merc 470 cooling problem (?)

saw some smoking in the engine compt. coolant had leaked out the reservoir fill cap.

Time to do cooling system pressure test and a compression check to see if you have a head gasket problem (you can borrow the tools from AutoZone). These engines can develop head gasket leaks and pressurize the system forcing coolant out. Pay particular attention to cyls. 3 & 4. If either are low it might be time for a new (OEM) gasket. My overflow tank has the same nipple to nowhere, nothing to worry about. Good luck

 

[nola mike]

Re: Merc 470 cooling problem (?)

hmm. haven't done a cooling system test. compression was (i think) 160 or so x 4 since the symptoms started. i haven't ever had it overheated. no coolant loss except to the point i mentioned, after which it stabilizes; i would expect a HG to continue to lose coolant. also no oil/coolant mixing that i can see. lastly, i wouldn't expect the coolant system to lose all pressure unless it's going into the oil/cylinder (and i'd still expect the coolant system to be pressurized somewhat immediately after shutting down). the fact that it's coming out the reservoir cap when "overfilled" but is otherwise not pressurized doesn't make sense to me (again, if it was crankcase pressure, the coolant system should be always overpressurized, right?). any other potential suspects?

 

[stonyloam]

Re: Merc 470 cooling problem (?)

Since your compression looks good, the cooling system check should tell you everything you need to know. You might just have a bad pressure cap. You can check it with the cooling system check kit. According to the book a Mercruiser cap should release pressure between 11 - 17 psi. A bad cap gasket could leak coolant and pressure. If it is just a bad gasket you should be able to pick one up at any auto parts store for a couple of bucks. Just went through all this stuff myself. Good luck

 

[nola mike]

Re: Merc 470 cooling problem (?)

forgot to mention, the gasket on the overflow tank was definitely bad, but i didn't sweat it because i figured the nipple to nowhere would let pressure out anyway...in fact, how does the system stay pressurized with that overflow tank there?

 

[stonyloam]

Re: Merc 470 cooling problem (?)

The pressure cap is the one on the engine reservoir assembly on top of the exhaust manifold. There is a spring in the cap that holds the seal in contact with the bottom of the reservoir neck and a gasket seals the top (take a look at it). As the coolant expands and the pressure builds the excess coolant flows past the seal into the overflow tank (through that little hole in the neck). When the engine cools down the excess coolant is drawn back into the reservoir. the reservoir should be completely full and normal overflow in a warm engine should be about 1/2 pint. You should see the level in the overflow go up and down as the engine warms and cools (just like in your car). If it is leaking from the cap either the hose to the overflow is blocked or the gasket is bad. Might be time for a new cap. Make sure the cap is turned all the way on.