Random over heating

[Crum]

Hi All,

I have a 1970 (ish) Caravelle Tri-hull with a Merc 165 I-6 outdrive. After sitting for a couple seasons I decided to get her up and running this year. After changing the oil and gear oil I put muffs on it and got it running. Needless to say it started overheating. I figured it was the impeller so was going to order one. However, first I decided to check the T-stat and it was stuck open so I removed it and tried again...... it ran nice and cool around 110.

The next day I started it again to check out a water leak I noticed the last time we took it out and it overheated again... still had the T-stat out. there seemed to be plenty of water circulating judging by the amount of water coming from the leak (looks like it's leaking from the manifold gasket close to cylinder 5. I replaced the gaskets with Merc brand gaskets 2 years ago to try and stop the leak, but apparently it might be something else leaking, I hope its not a cracked block or manifold.),

Anyway, could the overheating be caused by an air bubble in the cooling system (it's an open system, no heat exchanger)?
I'm thinking that the water coming in from the lower end is just going straight into the manifold and not getting into the block......

Thanks in advance for any advice. If you need more info let me know and I'll get it.

 

[Fishermark]

Re: Random over heating

Two or three things...

1. Welcome aboard!

2. You most definitely need to replace your impeller AND a new thermostat.

3. Your leak needs to be addressed as well. When you say it is coming close to cylinder 5 what do you mean? There is no water seal between the block and the manifold. So if the leak is near there, then you most likely have a crack.

 

[Crum]

Re: Random over heating

Yeah, I plan on replacing the impeller......

The stream of water flows down the side of the block towards the back, but also flows off the bottom of the manifold, like it's running down from the mating surfaces of the head and manifold in the vicinity of cylinder 5.

So your saying that there are no water passages between head and the manifold, or was my original explanation confusing? :facepalm:

 

[Fishermark]

Re: Random over heating

So your saying that there are no water passages between head and the manifold, or was my original explanation confusing? :facepalm:

That is correct - there are no water passages between the head and the manifold. (I said block and manifold, but should have said between the head and manifold). At least nothing sealed with a gasket. As you can see from this photo, there is a core plug, but other than that, no water passages.

 

[Crum]

Re: Random over heating

Gotcha!
So it's either a crack in the block or head, or maybe a head gasket (though there's no water in the oil).

Although, this leak has never been the cause of overheating (Always been the impeller or t-stat causing the overheating).... but I may just be thinking of this too much like a care engine. Never had the leak until it blew a head gasket a few years ago......

Since I'm going to have to pull the manifold to figure this one out, any tricks on getting the exhaust riser to separate from the manifold? I've tried beating on it with a rubber mallet, but it just won't break loose. It's a real pain to remove/install the manifold with it on there and I risk messing up the exhaust boot every time, not to mention the fact that the manifold is HEAVY!!

 

[Crum]

Re: Random over heating

After looking at that pic again I'm wondering if that rear core plug is the culprit..... It's certainly in the area of the leak.... Could I be so lucky???? Probably not, but it's worth a shot!!!

 

[Fishermark]

Re: Random over heating

but also flows off the bottom of the manifold

Is it possible it is leaking from the manifold itself? Look at this pic - you will see there is a drain plug on the bottom of the manifold in the area you are describing as well:

The riser is held on with four nuts. If you have them off and the rubber boot, it should come off with a little persuasion. A big rubber mallet might break it loose.

 

[Crum]

Re: Random over heating

Well I found the leak.... it was the cast plug on the head..... And a cracked block!!!! It was a small crack so I had a friend stick it and I replaced the cast plug. Runs great now and doesn't leak. I checked the manifold and riser and they are both in surprisingly good condition. It didn't quite overheat but was running warmer than I like. Going to replace the T-stat tomorrow and see if that helps. If not, I'll do the impeller this weekend.

Fishermark, thanks for all your help!!!

 

[Fishermark]

Re: Random over heating

...a cracked block!!!! It was a small crack so I had a friend stick it ...

"Stick it" as in stick welding? Cast iron doesn't like to be welded. It can be done, but someone has to really know what they are doing, (or get lucky), in order to weld cast iron. Keep an eye on the repair just to make sure.

 

[Crum]

Re: Random over heating

I definitely will. I only need it to last me through 1 maybe 2 weekends on the lake this year and then the boat is getting a complete restoration...... that should be fun.....