Engine cooling system air lock

[Metterdvm]

Hello all,
Wondering if anyone has had this problem before. Launched a new to me 2003 356 with 8.1 Mercruiser Horizon engines this weekend. Both engines started right up. Let them idle and after a few minutes, the smart craft gauges started going off with an engine fault warning. Shut em down to investigate- seacocks open, oil level ok, plenty of antifreeze in the closed system, but could see that there was still glycol in the sea strainers. Thought perhaps the thermostats failed to open. Started engines back up again. Same thing happened once the engines got up to temp around 160 degrees F. Looked at strainers— still blue colored glycol. So, thought bad impellers, but both going bad at once?? Checked the port engine impeller and it looked perfect. So, while I was reassembling the port water pump, my buddy tried the starboard engine again. This time instead of just letting it idle, he revved the engine a few times. Water started coming out of the exhaust and no warning signal when the engine got up to temperature. Finished reassembling the port engine, fired her up, revved a few times and she worked perfectly. All I can figure is that there was an air lock that would not clear just at idle and needed some RPM’s to clear. Anyone had this problem before? Frustrating to figure out, but glad it was something simple.

[tomschauer]

Its not unusal to have to "goose" the throttle a bit to get the initial vacuum high enough to lift the water to the impeller / raw water pump, at least with mercs.

[buster53]

It does happen, especially the first time the boat is put back in the water. Not unusual for the engine cooling system to loose it's prime when the boat is taken out of the water. Happened to my boat when we did a short haul during survey.
ALWAYS A good idea to have someone watching the exhaust when starting the engines to make sure you are pumping water. Don't wait on an alarm to let you know. By the time the alarm goes off, you have already burnt up your impeller.

[Metterdvm]

Thanks! Now I know!

[Gag_Halfront]

Would this same behavior / advice apply to an older (1990-ish) open cooling system 350cid?

[tomschauer]

Yes it would unless you have alpha or any out drive that has the pump inside the drive. Their pumps are always below the waterline.