First airboat cooling question

[Andrew511]

First time airboat owner here. SBC DD 350 somewhere around a 1982. Boat runs really good doesn’t even usually get to 160 good oil pressure no smoke.. I’m completely new to this so here is my best explanation. There is a metal tank high on my cage that has a radiator cap on it with a hole drilled in it. From there a single hose goes directly to the top of the engine. I filled that container about 3/4 full and went out fishing when I got home I could not feel the water with my finger so I got a stick and it was barley wet at the bottom. Oil on the dipstick looks normal and the oil fill cap doesn’t have any milk look to it. Does any one have any ideas? Is my cap at the top of the cafe supposed to have a hole drilled in it? Does it seem like a blown HG?

 

[ak375hh]

The cap on my expansion tank doesn’t have a hole. But there is an overflow hose from just below that cap to a reservoir mounted on my cage as well. If you get enough expansion it could come out of the hole in the cap and then when it cools down and contracts it’s size it would have less coolant in it...

 

[Andrew511]

The cap on my expansion tank doesn’t have a hole. But there is an overflow hose from just below that cap to a reservoir mounted on my cage as well. If you get enough expansion it could come out of the hole in the cap and then when it cools down and contracts it’s size it would have less coolant in it...

So you have two tanks mounted on your cage?

There is a hose on mine below the cap with a hole in it but the hose doesn’t go anywhere

 

[southern safaris]

Most sbc cooling systems i build are set up as follows , from thermostat housing you can either have an inline filler neck with pressurized cap and then a overflow line to expansion tank or like yours sounds to be rigged which is to have your coolant reservoir with pressurized cap up high and then i would reccomend having an expansion tank to allow the coolant to flow back n forth as needed and what this does is allow the engine to act just as they do in autos and keeps them functioning properly, basically your tank with the pressure cap is just like the top of a radiator just located high to get all the air out of your system then the overflow tank to catch the expansion of coolant and when it cools back down will be sucked back into the first tank, if your tank up high lost coolant there could be several factors , my questions would be is

1 do you have a thermostat and
2 can you post a picture of the hole you mentioned

 

[Andrew511]

Most sbc cooling systems i build are set up as follows , from thermostat housing you can either have an inline filler neck with pressurized cap and then a overflow line to expansion tank or like yours sounds to be rigged which is to have your coolant reservoir with pressurized cap up high and then i would reccomend having an expansion tank to allow the coolant to flow back n forth as needed and what this does is allow the engine to act just as they do in autos and keeps them functioning properly, basically your tank with the pressure cap is just like the top of a radiator just located high to get all the air out of your system then the overflow tank to catch the expansion of coolant and when it cools back down will be sucked back into the first tank, if your tank up high lost coolant there could be several factors , my questions would be is

1 do you have a thermostat and
2 can you post a picture of the hole you mentioned

I’m not 100% sure there is a T stat I’ve never taken anything apart to check. Check out the pics I’ve attached. The hole im speaking of is in the center of the cap on the tank on the cage. From that tank it goes directly to the next picture located on the engine

 

[ak375hh]

The cap on my expansion tank doesn’t have a hole. But there is an overflow hose from just below that cap to a reservoir mounted on my cage as well. If you get enough expansion it could come out of the hole in the cap and then when it cools down and contracts it’s size it would have less coolant in it...

So you have two tanks mounted on your cage?

There is a hose on mine below the cap with a hole in it but the hose doesn’t go anywhere

When I got my boat it was like that. Builder just let the overflow hose dangle getting coolant everywhere. Maybe your missing the recovery tank...

 

[southern safaris]

Ok , i see what they were trying to accomplish now, the top tank is their way of making an overflow/expansion tank and the hole would allow the pressure to bleed off since there is already a cap on your block filler neck, if your losing coolant out of the overflow make sure leaks arnt the culprit, i dont see any clamps on those hoses going from the block filler to tank? I would rule out easy stuff first , install clamps were they may be missing would be first start and i would check the water level at the block filler to make sure engine is full, just be aware that any fluid in upper tank will be lost unless you clamp off the line going up to tank with line clamps or such,

 

[Andrew511]

Ok , i see what they were trying to accomplish now, the top tank is their way of making an overflow/expansion tank and the hole would allow the pressure to bleed off since there is already a cap on your block filler neck, if your losing coolant out of the overflow make sure leaks arnt the culprit, i dont see any clamps on those hoses going from the block filler to tank? I would rule out easy stuff first , install clamps were they may be missing would be first start and i would check the water level at the block filler to make sure engine is full, just be aware that any fluid in upper tank will be lost unless you clamp off the line going up to tank with line clamps or such,

Ok Ill Add the clamps and check the levels in the morning. I’ll take it out and report back what I find after running it tomorrow.

 

[southern safaris]

:thumbleft:

 

[SWAMPHUNTER45]

If you or the prior owner has recently had the cooling system open it could be burping itself. This is where trapped air finds its way to the elevated overflow and the coolant refills the system.

 

[Grizzily2]

From what I'm looking at, you have 3 radiator caps. The one on the thermostat housing and the one on the radiator have open overflow lines on them. The expansion tank mounted on the cage is plugged off. This presents two problems. The first one is with three caps, the weakest cap will release your cooling fluid through it first, this may be at the radiator or at the thermostat housing. Your expansion tank vent is plugged off and can't release. The purpose of the top expansion tank is to allow any air bubbles in the cooling system to flow to the highest point of the system and purge itself by the cap releasing air and fluid as the motor reaches operating temperature. The line should be open on the top tank a routed into the bottom of an overflow container. The overflow container is not sealed and at normal pressure all the time. It will vent away the air that was in your motor and retain the excess coolant that will be sucked back into the system as the motor cools. Check all your caps and ensure your highest pressure caps are at the bottom with the lowest pressure at the on the over flow tank. This will ensure your cooling fluid will only come out of and go back in through the expansion tank, keeping your fluid level constantly above the motor.

 

[Slidin Gator]

From what I'm looking at, you have 3 radiator caps. The one on the thermostat housing and the one on the radiator have open overflow lines on them. The expansion tank mounted on the cage is plugged off.

Grizzly,
Double check the pics, it took me a few views to figure out the set up. Two pics show the expansion tank and one shows the thermostat housing. Radiator is not shown. Overflow from the thermostat housing runs up to the bottom of the expansion tank. The expansion tank is vented via the hole drilled in the cap. There is probably a cap on the radiator too, just not shown. I would run a 15 psi cap on the radiator and then a lower pressure cap (~5-7 PSI) on the thermostat housing to make that the weak point. Keep the hole in the expansion tank cap.

I agree with SS & Swamp, it is likely that the system had some air in it that burped out. Get everything tight and Keep refilling the expansion tank (3/4 full max) and it should stabilize at about 1/2 tank when cool. Wherever it stabilizes, that is the max level to keep in the expansion tank. If it does not stabilize, then it's leaking somewhere.

First time airboat owner here. SBC DD 350 somewhere around a 1982. Boat runs really good doesn’t even usually get to 160

It probably does not have a thermostat in it, you need to get the motor to temperature for best performance. You should run a 180-190 thermostat. For good measure, drill 1-2 - 1/8" holes in the thermostat to vent air and insure some bypass.

 

[Andrew511]

From what I'm looking at, you have 3 radiator caps. The one on the thermostat housing and the one on the radiator have open overflow lines on them. The expansion tank mounted on the cage is plugged off.

Grizzly,
Double check the pics, it took me a few views to figure out the set up. Two pics show the expansion tank and one shows the thermostat housing. Radiator is not shown. Overflow from the thermostat housing runs up to the bottom of the expansion tank. The expansion tank is vented via the hole drilled in the cap. There is probably a cap on the radiator too, just not shown. I would run a 15 psi cap on the radiator and then a lower pressure cap (~5-7 PSI) on the thermostat housing to make that the weak point. Keep the hole in the expansion tank cap.

I agree with SS & Swamp, it is likely that the system had some air in it that burped out. Get everything tight and Keep refilling the expansion tank (3/4 full max) and it should stabilize at about 1/2 tank when cool. Wherever it stabilizes, that is the max level to keep in the expansion tank. If it does not stabilize, then it's leaking somewhere.

First time airboat owner here. SBC DD 350 somewhere around a 1982. Boat runs really good doesn’t even usually get to 160

It probably does not have a thermostat in it, you need to get the motor to temperature for best performance. You should run a 180-190 thermostat. For good measure, drill 1-2 - 1/8" holes in the thermostat to vent air and insure some bypass.

So yesterday morning I removed the cap on the thermostat housing and it was low on coolant I couldn’t see any so I topped it off and filled the tank on the cage about 3/4 full. I then noticed the bottom hose on the radiator is at an angle not fully seated flush, I barley loosened the hose clamp and fluid begin pouring out I tried to force it on straight but I just couldn’t get it. I’ve heard of air getting trapped so I got nervous and tightened it back up (still not fully seated). I topped it off at the thermastat again this time with the engine running. It was bubbling out pretty bad even more so if I hit the gas. I was always told that if it bubbles out when running it’s a blown HG but I’m not sure if it’s air bubbles releasing from when I loosened the hose?
On to the next question, yes there is another cap on the radiator.
So after that I took it out and rode it, I noticed after a long run it was dripping out of the cap on the Tstat. I ran around from spot to spot all day fishing in between and it never once got hotter than usual (140-150) and eventually stopped dripping out of the Tstat I’m guessing when the top tank emptied finally.

 

[Slidin Gator]

So after that I took it out and rode it, I noticed after a long run it was dripping out of the cap on the Tstat. I ran around from spot to spot all day fishing in between and it never once got hotter than usual (140-150) and eventually stopped dripping out of the Tstat I’m guessing when the top tank emptied finally.

Dripping out of the cap itself says the cap is bad, or too low of pressure. It would have drained the expansion tank first, but will continue to leak until there is little fluid left in the engine and it overheats (which would fail the head gasket, or worse). It sounds like fluid is just low due to leaks and it will bubble until all the air is worked out. If it is not getting hot and seems to be running well, I would set concerns over the head gasket aside for the moment and focus on the cooling system.

You should follow through on the previous recommendations, give all the plumbing a wrench check. Make sure all hoses are sound and tight (or replace). Put on new radiator caps and thermostat. Then fill it all up and make sure the level stabilizes and is not leaking.

One minor item, from the picture of the intake it looks like an old school SBC vs. a Vortec, but please confirm that the intake manifold has a total of 12 bolts, 6 on each side, holding the intake to the heads. Vortecs would have 8 bolts total and they have some unique cooling system requirements.

 

[Andrew511]

So after that I took it out and rode it, I noticed after a long run it was dripping out of the cap on the Tstat. I ran around from spot to spot all day fishing in between and it never once got hotter than usual (140-150) and eventually stopped dripping out of the Tstat I’m guessing when the top tank emptied finally.

Dripping out of the cap itself says the cap is bad, or too low of pressure. It would have drained the expansion tank first, but will continue to leak until there is little fluid left in the engine and it overheats (which would fail the head gasket, or worse). It sounds like fluid is just low due to leaks and it will bubble until all the air is worked out. If it is not getting hot and seems to be running well, I would set concerns over the head gasket aside for the moment and focus on the cooling system.

Ok sounds good. Thanks for all the help so far guys. I’ll do all that. I have a couple questions though.
1.My buddy told me to buy a plug for the radiator so it could never be a problem of leaking there. Is that a good thing to do?

2. I’m thinking about just changing all hoses for peace of mind because they don’t seem to be the newest hoses. If I do this I would obviously lose most if not all coolant. So if I replace hoses how do I fill it up with least air possible? Start with filling radiator once that’s full then place my plug in it. Then fill from the tStat housing till it’s full replace with new cap, then fill top tank. The. Run it and check it several times. Is that the right procedure?

3. If I plug radiator what pressure cap should I run at the Tstat?

 

[Slidin Gator]

2. I’m thinking about just changing all hoses for peace of mind because they don’t seem to be the newest hoses. If I do this I would obviously lose most if not all coolant. So if I replace hoses how do I fill it up with least air possible? Start with filling radiator once that’s full then place my plug in it. Then fill from the tStat housing till it’s full replace with new cap, then fill top tank. The. Run it and check it several times. Is that the right procedure?

Yah, 10-4, that's the right procedure. Good plan to replace everything, as long as you are going that far, I recommend you flush the cooling system before replacing anything. Get Radiator flush from auto part store and follow directions. Then drain system and replace all hoses. Then fill with fresh.

3. If I plug radiator what pressure cap should I run at the Tstat?

Plugging the radiator is fine, I recommended a 15 PSI cap on the radiator to effectively do the same thing, either way works fine. Then put a 5-6 PSI cap on the Tstat housing so that is the cap that lifts and blows excess up to the expansion tank.