Starting a couple weeks ago I rarely get cooling water out the tailpipe on the first startup of the engine. If I shut down the engine, then restart it, it often solves the problem. More recently, I figured out that with the engine running, if I simply crack the lid of the Vetus strainer, then screw it back down, it gets the water flowing. I changed the impeller (the old one looked fine), checked all the hose clamps, and also lowered the Vetus strainer from above waterline to waterline level. I might try it lower still so most of the basket is below waterline, but would have to get some new hose and re-route the lines.

I did add a transmission cooler this year which adds marginally to the burden on the SW pump (it's a simple box that water is pulled through). The Vetus strainer is empty, in fact I have caught exactly one blade of eel grass in the entire 12 years with this boat (there's an exterior grill on the SW intake through-hull, the Vetus is almost unnecessary).

Any suggestions what might be causing this? Wouldn't an airlock in the transmission cooler work itself out? Once the flow starts, it is never a problem and there is plenty of seawater flowing through.

For now it's just an annoyance, but I'm not comfortable until I solve whatever is causing this. Thanks for any suggestions.

Max