If the header tank, neck, and cap on the Lehman 4D engine are similar to the same components on a Ford Lehman 120, just sticking an expansion tank on the existing overflow hose won't work. As it was explained to me by Bob Smith at American Diesel, this is because the configuration of the stock neck and cap are such that, while they will let fluid out as it heats up they won't pull it back in as the system cools.
This is why, at least with the FL120, you have to replace the filler neck and the cap if you want to switch to a recovery system. There is a kit available for this, which includes a new neck insert that you install in place of the stock neck insert, a different configuration of cap, and the recovery bottle, holder, hoses, etc.* If your engine doesn't have a recovery system on it now you might want to call American Diesel and talk to Bob or his son Brian to see what you'd need to do to put one on.
We bought two of these kits about ten years ago but have yet to install them as I've been too lazy to do it. As a stopgap we ran the hose from the stock header tank on each engine into a cleaned out oil bottle that sits it the drip pan under the engine. When I do my engine room check before a day's run I simply pour the contents of the bottle back into the header tank. A poor man's recovery system, if you will. This system works so well with zero effort that it's been easy to keep moving the installation of the recovery kits to the bottom of the to-do list all these years.
So as another step to tracking down your coolant loss put a hose extension on the engine's coolant overflow pipe and run it into a bottle to see if that's where the coolant in your bilge is coming from.
However.... a quart of coolant lost seems like way more than what's normally pushed out due to simple expansion. If we top off the header tanks on our engines, once the engines get up to temperature they push a little less than half a pint into the quart oil bottle. After that, no more coolant goes into the bottle no matter how many hours we run, or how many days we run the boat without topping off the header tank.
When you do track down the problem and the solution, let us know what you found.
-- Edited by Marin on Sunday 11th of April 2010 11:51:44 PM