Frustrating Cooling System Issues

[SteamboatWillie]

Hello fellow gluttons for punishment,

I have a 1976 MerCruiser 165 I6 with a closed cooling system that just doesn't want to cool. We have replaced the impeller, all the hoses, the t-stat, the exhaust riser, the exhaust manifold, de-scaled and cleaned out the heat exchanger, pulled the hose off at throttle to ensure that the impeller is pumping... but the damn thing keeps overheating. The t-stat is a 160 and it cycles properly at idle or at low RPM, but once we get going it rips up to 180 and if sustained starts creeping up to 200. The circulation pump is the only thing we haven't checked but it passes the sound test and the water appears to be moving through the system. The only thing has made us go "hm" is that when we pulled the hose that runs from the riser to the heat exchanger there was water in the line that got pushed out before we saw any exhaust - not sure if that is supposed to happen. Here's a pic of my setup:


ANY help or advice would be welcome at this point.

The more I think about it the more I think that there is an issue with the circulation of water through the heat exchanger. I think for some reason the impeller is fighting pressure from the hose that's coming from the exhaust riser and it's not cycling through the exchanger fast enough to cool the coolant efficiently. Happy to be told I'm wrong.

 

[Scott06]

Hello fellow gluttons for punishment,

I have a 1976 MerCruiser 165 I6 with a closed cooling system that just doesn't want to cool. We have replaced the impeller, all the hoses, the t-stat, the exhaust riser, the exhaust manifold, de-scaled and cleaned out the heat exchanger, pulled the hose off at throttle to ensure that the impeller is pumping... but the damn thing keeps overheating. The t-stat is a 160 and it cycles properly at idle or at low RPM, but once we get going it rips up to 180 and if sustained starts creeping up to 200. The circulation pump is the only thing we haven't checked but it passes the sound test and the water appears to be moving through the system. The only thing has made us go "hm" is that when we pulled the hose that runs from the riser to the heat exchanger there was water in the line that got pushed out before we saw any exhaust - not sure if that is supposed to happen. Here's a pic of my setup:


ANY help or advice would be welcome at this point.

The more I think about it the more I think that there is an issue with the circulation of water through the heat exchanger. I think for some reason the impeller is fighting pressure from the hose that's coming from the exhaust riser and it's not cycling through the exchanger fast enough to cool the coolant efficiently. Happy to be told I'm wrong.

the hose from the heat exchanger to the riser (elbow) should have water in it not exhaust. this is how the spent cooling water goes over board.

there should not be any back pressure from exhaust pushing against thre cooling water discharge - is the exhaust flapper blocking exhaust flow down the pipe to gimbal housing ?

 

[SteamboatWillie]

the hose from the heat exchanger to the riser (elbow) should have water in it not exhaust. this is how the spent cooling water goes over board.

there should not be any back pressure from exhaust pushing against thre cooling water discharge - is the exhaust flapper blocking exhaust flow down the pipe to gimbal housing ?

thanks for your response! when i saw all that exhaust pumping out of that hose I was like "this doesn't seem right". Going to inspect the exhaust flapper next.

 

[Scott06]

thanks for your response! when i saw all that exhaust pumping out of that hose I was like "this doesn't seem right". Going to inspect the exhaust flapper next.

so it is pushing back to the heat exchanger? is so either exhaust outlet is blocked or elbow is rotted through. Elbow is raw water so if in salt should be a wear item.

would be leaning towards blocked exhaust outlet as rotted elbow would probably not create back flow of exhaust.

if the flapper drops it can make it all the way to outdrive

 

[SteamboatWillie]

so it is pushing back to the heat exchanger? is so either exhaust outlet is blocked or elbow is rotted through. Elbow is raw water so if in salt should be a wear item.

would be leaning towards blocked exhaust outlet as rotted elbow would probably not create back flow of exhaust.

if the flapper drops it can make it all the way to outdrive

we just replaced the outdrive because the gears on our old one got all chewed up. i have to wonder now if the flapper was the culprit. Do you know if it's possible to inspect/replace the flapper without removing the exhaust riser?

 

[Lou C]

If you don’t have raw water flow from your heat exchanger to the exhaust system you can overheat and that water should be going out the exhaust ports on the outdrive or else you will overheat. The raw water flow is what picks up the heat from the antifreeze in the heat exchanger. So in a closed system you need good cool raw water flow in to the exchanger & good water flow out & to the exhaust; and good circulation of AF between the engine & H/E.
Your closed system depends on both good raw water flow in & out and good AF circulation.
Depending on access you might be able to slide down the rubber exhaust hose and check the flapper. On an in-line engine there is only 1.
It’s also possible to have air trapped in the closed side which can prevent AF from circulation. I would get a IR temp gun & take readings on the H/E, exhaust manifold & elbow…

 

[Scott Danforth]

Is this a half system or a full system? A separate line to the elbow says full system

I'f there is no water flow to the exhaust elbow, there is no raw water coming in the system heat exchanger. Go to the raw water pump and find out why it's not moving water

Since you wan the system without water. Inspect the flapper and the rubber couplings because without water flow, those parts burned up

 

[alldodge]

Looking at manual 2 and closed cooling doesn't list a full system, but as Scott mentioned does raw water exit via the elbow?