7.4 Mercruiser overheat

94SD370 said:
I do not know much about thermostats. The guy at the boat store said I need a 140 thermostat for my motor, so I went with that. Both motors typically run at 160, not sure if that makes sense for a 140 stat? My temp gauge gradually climbs to 160, and never moves or fluctuates beyond that. I did not consider that the brand new thermostat could be bad.
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140F thermostats are typically used for raw water cooled motors in saltwater. The salt comes out of solution over 140, so a 140 thermostat minimized buildup due to salt. In freshwater raw water cooled motors they typically go to 160. In a closed-cooled system you have a lot more flexibility and they typically run even higher temperatures because the engines are more efficient. You should be fine running ANY temp thermostat and not having an overheat unless the thermostat is faulty or there are other issues in the cooling system - it won't be because the thermostat is the 'wrong temp'.
 
I had a similar experience and it was corrected by replacing the the engine coolant pump, (mounted on the block). The metal centrifugal impelled was loose in the shaft. I found by removing it but I have others say a slight rattled can be detected with a stethoscope.
 
Wanderer 290 said:
I had a similar experience and it was corrected by replacing the the engine coolant pump, (mounted on the block). The metal centrifugal impelled was loose in the shaft. I found by removing it but I have others say a slight rattled can be detected with a stethoscope.
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When cracked the bleeder did you still have coolant pump out, like a small geyser? I figured the circ pump was good because of that. I also have small air bubbles that appear at the bleeder and in the heat exchanger.
 
94SD370 said:
When cracked the bleeder did you still have coolant pump out, like a small geyser? I figured the circ pump was good because of that. I also have small air bubbles that appear at the bleeder and in the heat exchanger.
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I have only seen a loose water pump impeller twice in 40 years of working on boats and cars. It can happen and it is frustrating to find. That's why if I am going to change a thermostat I will also change the water pump since you are opening up the entire closed cooling system. The water pump cost is $30-40 and just not worth the headache on a 30 year old engine.

As to your exhaust manifolds and risers.....are they original?
 
PlayDate said:
Are you having cocktails for lunch? ;) He has a 1994 370 Sundancer with V drives. The last thing he needs is Bravoitis.
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upload_2023-7-23_10-39-10.jpeg
 
Exactly right. Buy boat with known issue. Lol
 
Wanderer 290 said:
Known / known, known / unknown and the best of all, unknown / unknown.

All boats have these.
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LOL. Pick your poison.
Bellows, steering pin, Bravoitis, upper lower drive problems vs: Cutless bearings, stuffing boxes (dripless seals), rudder stuffing tubes and transmission problems.

Same same on either. they are all a PITA at times.
 
PlayDate said:
I have only seen a loose water pump impeller twice in 40 years of working on boats and cars. It can happen and it is frustrating to find. That's why if I am going to change a thermostat I will also change the water pump since you are opening up the entire closed cooling system. The water pump cost is $30-40 and just not worth the headache on a 30 year old engine.

As to your exhaust manifolds and risers.....are they original?
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I am replacing the recirculating pump this weekend. I am curious about the “$30-40” comment. The one I bought is a Quicksilver and they all run around $250. Are you just replacing the internal impeller or the whole pump?

I believe the manifolds and riser elbows to be original, my elbows are stainless and no longer manufactured. To my knowledge this has always been a freshwater boat. I am told that is not uncommon. That being said, like I mentioned above, I do plan to change the manifolds late August/September. I do run it for about 10 mins in and out of the marina without overheating. If I run into a bigger issue while changing the manifolds, it will probably end my season. It’s a short season where I am, just trying to enjoy it with the kids while I can.
 
I replaced the circulation pump and I have the same issue. I ran it without the thermostat, and it still over heated.

I also still have the air bubbles in the coolant. I’m leaning more towards this being my issue. Could these bubbles be causing the circ pump to cavitate? I did not run the motor while pressure testing the heat exchanger. I found a procedure for this in the service manual.

When replacing the coolant, I crack the bleeder and fill from the heat exchanger until I get a steady flow of coolant through the bleeder (As per service manual).
But when I crack the bleeder when running, there is air escaping the bleeder.

It seams like air is being introduced to the system, but only when running. There was no exhaust gas detected in the coolant, but I don’t have a ton of confidence in that test. If it is a head gasket, I would think the heat exchanger would not hold pressure with the engine not running. I do produce some steam from the exhaust, but both engines do that. How much is normal, I have no idea.
 
Something is going on with the raw water side. You have eliminated the coolant side of the system with the thermostat and the water pump. The problem is on the raw water side not cooling the heat exchanger and probably the exhaust.

There is no easy way to solve it. You have to go step by step from the raw water intake valve through the connected system. I would start with opening the strainers and then opening the valves to see if the water overflow matches between the strainers. If you have a clog on the thru hull .......it will show up as reduced water flow. Clogs on the thru hulls happen all the time. Debris too long never gets to the strainer gets hung up and cuts water flow. I have seen plastic bags sticks and all sorts of junk clog the thru hull to the strainer.

The next step in the supply hose to the raw water pump. Yes....it can collapse especially if it is 30 years old. Then you go from there.......


It is a pain but just be glad you are not working on a 340. o_O
 
I know you mentioned the oil cooler, but have you back flushed the trans cooler? Mine seems to get restricted with algae in the screen. It was restricting my flow a bit from the exhaust and that took care of it.
 
I didn’t even realize this existed! (Newbie). I see it in the diagram, right after the sea water pickup and before the oil cooler. I will chase that hose and try to find it. Thank you for the tip!
 
E
PlayDate said:
Something is going on with the raw water side. You have eliminated the coolant side of the system with the thermostat and the water pump. The problem is on the raw water side not cooling the heat exchanger and probably the exhaust.

There is no easy way to solve it. You have to go step by step from the raw water intake valve through the connected system. I would start with opening the strainers and then opening the valves to see if the water overflow matches between the strainers. If you have a clog on the thru hull .......it will show up as reduced water flow. Clogs on the thru hulls happen all the time. Debris too long never gets to the strainer gets hung up and cuts water flow. I have seen plastic bags sticks and all sorts of junk clog the thru hull to the strainer.

The next step in the supply hose to the raw water pump. Yes....it can collapse especially if it is 30 years old. Then you go from there.......


It is a pain but just be glad you are not working on a 340. o_O
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Easy now! I'll be following this path Saturday AM.
 
Hopefully it's a collapsed hose seen it happen more than once and it's very common actually more than I thought it would be
 
I finally got around to chasing this overheat issue. The results of the compression test is not good. Especially, cylinder 6 at 65 PSI. The other results are 8=120, 6=65, 4=112, 2=94, 7=118, 5=95, 3=110,1=122.

Could this be a valve, or head gasket issue? Or is it time to start looking for a new motor. FWIW, the heat exchanger held 19PSI indefinitely, which leads me to believe, at least the exterior portion of the gasket is good. But with 8 & 4 numbers good, I’m not sure if it could be a leak between cylinders. Thoughts and ideas welcome, thank you in advance.

EDIT: I realized I did the compression test incorrectly with the throttle closed. I redid the test on the low cylinders and found cylinders 2&5 jumped to 110 PSI. I put a teaspoon of oil in cylinder 6, and it jumped to 85 PSI. The result on 6 is the same with the throttle open or closed.

Both test performed on a cold engine.
 

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