Coolant leak.

I have a pair of 135 Lehmans , under the port engine pan I noticed a small amount of coolant, less than a pint. I checked the reservoir and it was down a bit so I added enough to reach the dome in the reservoir. Cleaned up the coolant and the next time I ran the boat same thing. Engine is running cool, normal temp and if I don't add the coolant back to reservoir she does not leak. I thought it was coming from the overflow but I am not sure. Any thoughts?? Would appreciate any advice. Thanks

Quit filling it. You are more than likely slightly over filling it and that bit of fluid you keep seeing that has burped out is from the tank finding its happy place. There is a reason its called an expansion tank after all.

In many cases the true full level is an inch or so below the lip of the cap.

Do you have a tube to direct the overflow or does it just spill all over the top of the engine? If you've got a tube, put it in a bucket/jar/ whatever. If it fills that up and doesn't end up in the pan, you're either overfilling like capt bill says or overheating and boiling over with crap gauges.

If it's in the pan, you've got a leak

or it could be something completely different...

Check and tighten all hose clamps. They tend to loosen from vibration.

Most coolant pumps have a weep hole that show drips when failure is imminent. Finding the source of coolant leakage is obviously important, and not always easy. Paper towels can help locate.

"if I don't add the coolant back to reservoir she does not leak."

Almost has to be overfilling. Pretty much anything else that would cause a leak would leak no matter what the coolant level was. At least until the engine ran almost out of coolant.

Just for the heck of it, while you're poking around looking for the problem, I recommend you take the air bleed line between the manifold and the expansion tank apart and make sure the orifices and the tube itself are clean and not plugged up.

Thanks for the suggestions , we on a short cruise this weekend and I am monitoring the coolant carefully. I have a bottle attached to the overflow tube to see if that is where it coming. Also will use paper towels to check. Like Capt. Bill said it may just need its sweet spot for level.
When we run in this afternoon I will watch for signs of the leak. Again thanks , will keep you posted.

I wanted to follow up on the post from a few weeks ago. I finally found the coolant leak. It turned out to be a loose hose clamp on a short coolant hose. What confused the issue was that the leak was intermittent. It took three runs with the boat at different RPM's get it to leak so I could see it. Thanks to a good light, toilet paper, and patience I discovered the loose hose clamp. Many thanks to those who responded , this forum has really been useful since buying the trawler and I appreciate the effort that members make to help fellow trawler owners.