454 Losing coolant in reservoir, but Exchanger is full and no overheat

Hi All,

First post here. I have a 29' Blackfin with Crusader 454's.

On my port motor, I keep having to fill up my coolant reservoir after every 4-6 hours of running. The Motor temp is perfect, no over heating and If I take my cap off of the heat exchanger, it is nice and full. I don't smell coolant, and I don't see any leaks. Motor runs like a champ and at good temps.

Any thoughts?

Thanks

Going by your description the res is losing the AF. or the AF is being sucked back into the HX when the engine cools
Leave the res dry and then see if the HX is going low on coolant

First I would point out that some of these engines (don't know about your model) are difficult to completely fill with coolant. It is not unusual to need several heat/cool cycles to bring in enough coolant from the reservoir to completely fill the system.

But I have a similar issue going on. I am always missing coolant on one engine but the details vary (how much, reservoir full or empty, etc.). I need more data. The boat is at another location right now and I will not return for several months. But for when I do I have obtained what I need to make a simple setup with a small hose leading to a gage leading to a ball valve leading to a fitting into where I can introduce compressed air. I can attach it to a fitting that fits into the hole for one of the temperature senders or splice it with a "T" fitting into a small hose leading from the thermostat housing to a location just under the fill cap (so I can keep both senders active for tests underway). Both will work.

My plan is to first monitor the pressure when underway at temperature to see if the system holds any pressure and for how long.

Then when things are cool, pressurize the system with compressed air (to 7 psi) and see if and how long it holds pressure. When it is quiet I hope to be able to hear the leak. I may have to drain the coolant and do this with the system mostly full of air to get a noise. I will be sure to pull the zinc from the heat exchanger and listen there as well.

I also intend to approach it the other direction to try to find the missing fluid (if needed). I purchased (on the internet) a GM sourced small bottle of "glow juice", a tracing fluid that will glow brightly in dim light with UV illumination (I have small UV flashlight). This GM sourced material is specifically compatible with Dexcool coolant, which my engine uses. Firing up the engine in the dark and looking at the engine, the bilge, the exhaust water, salt water directly from the heat exchanger (put a drain valve in the zinc hole) should enable me to nail the problem. Autoparts stores also sell this stuff. Just be sure what you get is compatible with your coolant.

Hope some of these ideas help.

I had this very same problem on my port 454 engine, turned out the heat exchanger failed. The raw water pushes out the coolant. Need to flush out the
seawater and replace the exchanger. You won't overheat however, your circulating seawater instead of coolant, not good for the engine.