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Removing/Bypassing Heat Exchanger

GAS engine, transmission and generator repair and maintenance discussion forum.
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390Express
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Re: Removing/Bypassing Heat Exchanger

Postby 390Express » August 8th, 2022, 10:32 pm

km1125 wrote:When I say " taken an IR gun and compared readings across both motors", I mean comparing very specific points on both motors. For example, taking the IR readings on the input and output of the heat exchanger on both the raw and fresh water flows (4 readings on each engine), and other parts like below and above the thermostat housing, and before and after the manifolds, etc. Things like this would help narrow down why and where that engine temp is getting out of control.

With the history on that motor, I would certainly take the hot water tank out of the picture. I don't know if those are 3/4 or 5/8 hoses offhand, but I would bypass that and eliminate it from the equation totally. If you do this when it's been a while since the engine has run, you can skip the "remove the radiator cap step" as the system shouldn't have any pressure in it.

Until you solve the problem, you don't NEED to run a coolant mix in there. The engine will run fine and should not overheat with a 180F thermostat, especially if you have pressure in the system (~15PSI is all you need). You could drain whatever coolant you have now and save it and just run tests with just water, so if you spill anything or need to refill it's not a big deal. Of course ultimately you'd want to have the coolant mix back in there (unless you do actually just convert to a raw water system). Once you get the actual problems solved (both overheating and losing coolant) then you could drain and refill with the proper coolant mix so you don't have to worry about winterization of that part, and it would protect the internal metals from corrosion like it's supposed to do.

It's going to be a lot of parts and time to convert to raw. You might still end up wanting to do that if you find you have a bad heat exchanger but that should be a decision for later.


When I say that an “IR temp gun confirms that the port engine is running 20-30 degrees warmer than the stbd engine. IR temp gun confirms that raw water side remains cold, and similar comparative temps to stbd, entire fw side runs hot, no discernible culprit.”

This is exactly what I did: “I mean comparing very specific points on both motors. For example, taking the IR readings on the input and output of the heat exchanger on both the raw and fresh water flows (4 readings on each engine), and other parts like below and above the thermostat housing, and before and after the manifolds, etc.”

Again, the raw water side is cool and only moderately warmer than the stbd side. The entirety of the FW side of the port engine is hot. There is no discernible point on the FW side of port engine that is hotter than the rest; the entirety of the FW side is 20-30 degrees hotter than the stbd.

I’m going to try eliminating the HW heater first, with your clamp and cut section of the lines that feed the tank. I figure I have nothing to lose. If the HW tank is bad, I’ll remove it, and cap it at the intake. Guessing I’ll be able to scavenge a gallon or so out of the tank. I’ll add that back to the system to top it off and see if it still leaks. I’ll also pressure test the Hw heater while it’s out. If that isn’t the problem, I’ll dismantle the heat exchanger.

The system is ridiculously difficult to drain. I’m not draining it and refilling in its entirety again. The manifold drains are right above the stringer, the block drains are impossible to get anything under. There is no way to get it out from the top. Last time I drained it, I stated with siphoning from the heat exchanger down, and still lost almost two gallons of antifreeze all over the bilge.

We had a 79 scarab 377 that ran lake water for 40 years and never had an issue with corrosion.


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