Trou­bleshooter Take Your Tem­per­a­ture

An In­ex­pen­sive Piece of Gear Can Di­ag­nose an En­gine’s Ill­ness.

Passage Maker - - Troubleshooter - BY BY STEVE ZIM­MER­MAN

We can of­ten get a quick read on some­one’s health by tak­ing their tem­per­a­ture. The same can be said for a power­boat’s en­gine space. Tem­per­a­ture can be a re­li­able in­di­ca­tor of ma­chin­ery con­di­tion—es­pe­cially when com­pared to a known nor­mal or base­line tem­per­a­ture. For­tu­nately, a com­monly avail­able tool can be pur­chased for this ap­pli­ca­tion: an in­frared py­rom­e­ter. A de­cent py­rom­e­ter can be pur­chased for less than $50 and one should be kept aboard ev­ery cruis­ing power­boat and in­cor­po­rated into your reg­u­lar rit­ual of en­gine room in­spec­tion. For the pur­poses of this col­umn, all tem­per­a­tures are given in Fahren­heit.

To use a py­rom­e­ter, sim­ply aim the de­vice’s laser beam at an ob­ject. The beam does not play a role in sens­ing tem­per­a­ture; it sim­ply lets you know where you are aim­ing. Although it might seem like you are “shoot­ing” the tem­per­a­ture, in fact, sen­sors inside the py­rom­e­ter are re­ceiv­ing and dig­i­tiz­ing the en­ergy it re­ceives. Your en­gine room con­tains a rich sup­ply of mean­ing­ful tar­gets.

Let’s start by con­firm­ing the ac­cu­racy of the tem­per­a­ture gauge at the helm sta­tion. Note the tem­per­a­ture shown on the in­stru­ment panel and then aim the py­rom­e­ter at the top of the ther­mo­stat hous­ing and record the tem­per­a­ture. Keep in mind that a py­rom­e­ter reads sur­face tem­per­a­ture ra­di­a­tion and it may take longer for the metal hous­ing to reach the coolant tem­per­a­ture. It is not un­com­mon for the py­rom­e­ter to read slightly lower than the gauge, but a vari­ance of more than about 5 per­cent in­di­cates a prob­lem with the gauge.

Heat in the en­gine’s fresh­wa­ter cool­ing sys­tem is main­tained by a heat ex­changer that’s cooled by sea wa­ter. Over time, a heat ex­changer can lose its ef­fi­ciency due to ac­cu­mu­la­tion of de­bris (in­ter­nal zincs that break up, bro­ken im­peller vanes) or from a coat­ing buildup on the in­ter­nal cop­per tubes. The amount of heat ex­changed varies de­pend­ing upon the sea­wa­ter tem­per­a­ture: colder wa­ter will more ef­fec­tively re­move heat from the en­gine. At a min­i­mum, for most con­di­tions you should see about a 10-de­gree dif­fer­ence be­tween the tem­per­a­ture of the fresh­wa­ter in­let on one end of the heat ex­changer and the fresh­wa­ter out­let on the other end.

The same prin­ci­ple ap­plies to the trans­mis­sion oil cooler (the tem­per­a­ture drop will oc­cur be­tween the oil in­let and out­let ports on the cooler). Since the oil tem­per­a­ture will vary ac­cord­ing to the load on the trans­mis­sion, test­ing un­der sim­i­lar loads each time is im­por­tant. If you make it a habit to per­form this check you will grad­u­ally learn what to ex­pect and will be more likely to no­tice a grad­ual re­duc­tion in per­for­mance.

The ex­haust hose of­ten fails where it at­taches to the riser. The high­est tem­per­a­tures oc­cur here as sea wa­ter is in­jected into the flow of ex­haust gas, which might be as high as 700°F. The raw wa­ter dis­perses through small pas­sages de­signed to cre­ate a con­trolled spray pat­tern to en­sure there are no hot spots on the hose.

Over time th­ese pas­sages can be­come clogged, chang­ing the cool­ing pat­tern and al­low­ing an area of hose to over­heat and fail. You can use your py­rom­e­ter to check all the way around the ex­haust hose just abaft where it at­taches to the riser. The al­low­able tem­per­a­ture varies ac­cord­ing to the type of hose used, but in gen­eral the hose should not ex­ceed 200°F.

Com­par­ing cylin­der tem­per­a­tures can also pro­vide valu­able data. Find­ing a suit­able tar­get will vary from en­gine to en­gine—the goal is to hit the top of the cylin­der. For most en­gines, the vari­a­tion should not be more than

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Both cheap and easy to use, a hand­held py­rom­e­ter like this one could be one of the most use­ful tools in your col­lec­tion.

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