Cracked riser on my 7.4 merc?

[Harvester_1]

I just purchased a 91 carver 370 aftcabin about 2 weeks ago. I had a survey done and he said it was in great shape. He noted a few things in need of repair but nothing major. I havent taken her out other than a sea trial and manuvering to my new slip in the marina. I'm still familiarizing myself with the boat and the systems. This is my first inboard and sealed cooling system but i understand it pretty well or so i thought. Anyway, i started the starboard motor and after warming up it immediately overheated. Checked the obvious first, i put my hand on the risers and noticed that the starboard side riser was at an acceptable temp but the port side riser on the engine was pipeing hot and i could hear the air in the lines bubbling. Air is not good and tells me there's a leak. After looking around and checking fluid capacities i noticed a faint rust drip at the bottom of that hot riser. After looking closer i could see it was cracked and had rusted itself closed to some extent. No water coming out but I'll bet air was getting in. I pulled the riser today to prep for getting a new one and i was surprised to see that it had a gasket that was purposely on there to close off all the 4 outer ports in the exhaust manifold??? I could clearly see antifreeze in one set of the outer ports and i assume the other 2 are for the raw water to exit throught the exhaust because there is a 90 degree hose fitting leading from the thermostat housing that connect to those ports. Why then were they all closed by the gasket? I could see the gasket was made to do that but why...is my question? I need to get a new riser but i need some info b4 i go out to get it. Any info will help...thanks in advance.... Steve

[Harvester_1]

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[Harvester_1]

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The gasket

[Viper]

Looks like you have what I call the "big smiley face" She may not have been drained properly for the winter.

If it's a closed cooled manifold, there will be a blocking gasket; coolant on one side, water on the other.

[Harvester_1]

Well, the exhaust manifold has the 4 slots in it as does the riser. The gasket obviously blocks all 4 slots. How does the raw water exit then. I've done several riser installs b4 just not on a closed system b4. I'm used to doing stern drive raw water cooled systems. So that's why im so puzzled. I just wanted to make sure it was supposed to be that way. The ports on the exhaust manifold look like they have been filled with gasket sealant as well as the gasket itself.

[Harvester_1]

Also, i haven't gotten my hands on a service manual for the engine yet so i was wondering if any one knows what the torque specs are for the riser bolts?

[mjk1040]

The survey I take it didn't include a sea trial? The surveyor didn't even run the engines or was it a dry dock survey? I question the surveyor? Hope this is your only major repair, and you can get out real soon and enjoy her! Mike

[Harvester_1]

Well, mjk 1040....
Yes, we did a sea trial and also, the owner originally drove the boat to my marina from about 8 miles up river. The surveyor was top notch. I was impressed with how maticulouse he was and his professionalism as well. He noted as well as showed me what he thought to be a previously repaired leak on that riser but it looked like it was comming from a loose band at the exhaust hose that had since been tightened. He knew that the boat boat was driven there origibally and that it never overheated at all. I didn't watch the gauges that closely during the sea trial so it may have been running hot. The crack wasn't easy to spot believe me. I sat in the engine compartment after it over heated and went through the whole cooling system visually trying to figure out what i was going to check first. I pride myself on my mechanical skills. I was a mechanical engineer. Of course my mind went to the raw water pump but i found a whole bunch of parts in one of the holds filked ed with cooling system parts like.... new impellers, new pump housings, new heat exchanger caps, new and old water temp sensors...new and old oil coolers.... so, i don't think this over heating problem was new. The surveyor never saw those things and with the fact that it didn't overheat on the sea trial and it was idling in the slip after i parked it there for several minutes with no problems i don't blame him. Believe me, that crack wasn't easy to find or see. It's on the bottom of the riser and i had to pick at it with a razor knife to even see it my self. I have had stern drives do the same thing more than once where the engine would only over heat at idle but not while running up and down the river. i believe it is because of the volume of water being forced into the cooling system due to the higher rpm's. So I'm figuring this is a similar situation. But i wont know if that is the only problem till i get the riser replaced first. The fact that i found all those replacement parts in that hold makes me think that's not the only issue. Keep in mind, this boat is imaculate. The original owner really Took care of this thing. I even have the oriiginal build sheet from carver from 1991 from when he ordered it. I have every manual and spec sheet for everything along with a dated diary of sorts of every repair done on the visssel since it was new so I'm not worried that i bought a lemon. Just about everyboat ive owned...when a mechanical problem arrises it usually has something to do with cooling and the rest of the problems turn out to be electrical. Im just new to inboards so since i do almost all the work myself, i wan't to understand the system as well as repair it correctly. Wich leads me to ask questions to other carver owners. Who better to get info from.....right? I think it was put away for the winter with some water in that riser and it cracked at a weak spot in the casting. I just how tday will fix the problem. Im Going to change the impellers on both motors just to be safe. I don't know when they were last done and since I have new impellers in that hold it isn't costing me anything.

[mjk1040]

Steve; I hope the riser issue solves your overheating problem, just puzzled it didn't overheat during the sea trial. Good Luck hope you get to enjoy her before the season is over. Mike

[Viper]

Let us know how the test run goes after the impellers are replaced. You might want to go an extra step when you remove the pump and backflush the system in case there is debris or old impeller pieces in there blocking flow.