En­gine over­heat­ing? There's the usual and un­usual sus­pects.



While myr­iad of cir­cum­stances can cause an en­gine to over­heat, it is highly pre­ventable in most cases, start­ing with am­ple wa­ter drawn into the in­takes. In­board-pow­ered boats typ­i­cally have gril­lage guard­ing the in­take pas­sages to pre­vent eel grass and other de­bris from be­ing sucked in. Prob­lems oc­cur when the grill slits or holes are cov­ered with marine growth or too much bot­tom paint, as the flow to the wa­ter pump can be restricted. This can be fur­ther com­pounded when the in­take through-hull fit­tings serve as home to bar­na­cles, which cre­ate more con­stric­tion.

If you rely on the yard to bot­tom paint your boat, in­sist they re­move the screens, clean out the through­hulls and dab some paint on the in­sides. Once marine growth calls these sur­faces home, you own it, alive or dead, and it crowds out the wa­ter avail­able to your en­gine and other wa­ter-de­pen­dent ac­ces­sories.

Some boats also have in­ter­nal en­gine and ac­ces­sory strain­ers. Marine fo­liage can wick its way in­side the metal or ny­lon bas­kets, and I’ve found shrimp liv­ing in mine be­tween reg­u­lar clean­ings. But the best find was aboard a boat in Kil­lar­ney, On­tario. Most in­ter­nal raw-wa­ter strain­ers use a Lexan globe that makes it easy to see when it needs clean­ing. This strainer, how­ever, had a stain­less steel case that re­quired dis­as­sem­bly to re­veal what it con­tained. In ad­di­tion to Ge­or­gian Bay weeds, wedged tightly in­side was a hap­less fin­ger­ling wall­eye.

Ob­vi­ously, this for­eign-built boat never had, or, had lost its ex­te­rior grill on the hull bot­tom. Con­firm any ex­te­rior in­take pro­tec­tion is re­placed prop­erly be­fore launch­ing your boat. On my boat I al­ways use new fas­ten­ers each spring.

Churned up sand or mud from the prop wash that oc­curs while op­er­at­ing in shal­low wa­ter will also find its way into the strain­ers. To save wear and tear on the wa­ter pumps, my rule is to shut off the gen­er­a­tor, air con­di­tioner, and live well when back­ing into the slip if such con­di­tions ex­ist. Sim­i­larly, if it’s low tide when I leave, I will not start up these sys­tems un­til I am in good wa­ter.

Over­heat­ing can also be caused by a worn or dam­aged wa­ter pump im­peller. If you don’t re­call the last time you re­placed the im­peller for your en­gine or other ac­ces­sories, con­sider adding this task to your list of chores, and don’t forget to keep spares in your tool box. If the im­peller is dam­aged and is miss­ing a vane, make sure any dis­placed pieces are ac­counted for and re­moved. Any de­bris left in the wa­ter pas­sages may re­duce flow, which leads to per­sis­tent over­heat­ing.

When a new im­peller is in­stalled, run the en­gine or re­lated sys­tem as soon as pos­si­ble to en­sure proper op­er­a­tion, as I learned from ex­pe­ri­ence.

The en­gine folks who re­placed the im­peller on my star­board diesel in­sisted all of the old pieces had been found and the boat was ready to go. The fol­low­ing week­end, when I had guests aboard and pow­ered up to cruis­ing speed, the en­gine alarm sud­denly went off again. I pulled back the throt­tle and the alarm si­lenced. With both en­gines in neu­tral I checked the wa­ter com­ing out the ex­hausts and no­ticed plenty of flow on each side. Af­ter limp­ing back to the dock with no alarms ring­ing, I climbed in the en­gine room. Af­ter un­cou­pling the raw-wa­ter hoses at the heat ex­changer I found a hand­ful of loose par­ti­cles from the old im­peller clog­ging the bun­dle. Once all the de­bris was cleared and the heat ex­changer hoses re­assem­bled, the en­gine ran fine once again.

Over­heat­ing en­gine? Your strain­ers may be to blame.

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