Skills | Engine Troubleshooting


What If Your Temperature Gauge Spikes?

By Tom Neale
Published: October/November 2013

When your engine's got a fever, quick action will often save the patient. Here's what to do when things start to heat up.

If Things Are More Complicated ...

If one of the above is the problem, you should be able to deal with it and proceed with your temperature back in the normal range. If not, you're probably not going to be able to remedy the problem on the water, but you may still be able to limp home without doing permanent damage. Exactly how you limp home will depend upon what you think the problem is.

A slow but steady rise in temperature could mean you have a deteriorating impeller in your raw-water pump. Eventually, the impeller may completely strip out, resulting in temperatures rising quickly to a very high level, leaving you stranded. Also, parts of the impeller could lodge downstream, causing blockage and exacerbating the problem. Changing an impeller on an inboard can be difficult, even if you have a spare. If you have an outboard, you don't want to even think about changing the impeller at sea, as the job requires dropping the lower unit. Rather than tackle this job on the water, head home very slowly, keeping a close eye on that temperature gauge, and stop if it starts to spike — or call for a tow. Changing the impeller at least every two years goes a long way toward ensuring you never find yourself in this situation.

If you've seen a gradual increase in running temperature over time, your cooling passages may be restricted. Scaling in your engine's cooling passages — including heat exchanger(s) in inboards — will occur with age unless you regularly clean them. Outboards may be more likely to suffer from corrosion or scaling within their cooling passages because these passages are typically aluminum. This will be less of an issue with outboards used in fresh water or flushed with fresh water after use. This can seldom be fixed at sea. It develops slowly, but if it's causing your engine to slightly overheat, running slowly back to the dock may be fine, watching the gauge to be sure that the temperature doesn't get too high.

Photo of a thru-hull strainer that had clam growing inside
Photo: Mel Neale
Thru-hull strainer that had clam growing inside, causing overheating.

Finally, the gauge may be going up because your thermostat has failed. While occasionally a thermostat gets stuck in the partially open position, it's more typical that it fails in the closed position, blocking water flow completely. Usually the location of the thermostat on outboards and inboards is obvious, and it's relatively easy to remove the housing, taking care to preserve the gasket. If you have an outboard, the cooling water is probably salt or brackish (unless you operate in a freshwater lake). Corrosion and debris around the thermostat suggest it may have failed. If you have an inboard with freshwater cooling, the thermostat should be clean, but it may still fail. If you suspect thermostat failure and don't have a spare, pull out the bad one and replace the cover, taking care to reinstall the gasket. Watching your gauge, proceed directly in. The engine will probably run too cool, but this shouldn't hurt for the short run back to dock. If the problem is elsewhere, sometimes removing the thermostat will let just enough cooling water through to allow you to get in, although this is risky depending on what's actually causing the problem. Opening the thermostat housing on an inboard with freshwater cooling will often mean that you must add more fresh water and antifreeze to the system to replace what has spilled out.

If you can't diagnose the problem but there's some water coming out, you may be able to get back home by running slow and watching the gauge. But, you may risk additional damage if the water flow decreases as you run. This means the problem is worsening and the gauge will probably read higher. If you reach the point where there's little or no water coming out, call TowBoatUS Note that with some outboards, your forward movement may force a little water through the cooling system, but this may not be enough to sufficiently cool the engine, even when running slowly.

After any overheating situation on any engine, it is prudent to replace the impeller, which may well have been damaged. Whenever you have overheating in an engine with contained coolant, check to see that you haven't lost coolant through heat expansion. When the engine overheated, you probably noticed the smell of antifreeze. This coolant must be replaced and the cause investigated. To avoid getting burned, wait until the engine cools before opening the closed raw-water system.

If your engine catches a fever, don't despair. In most cases, the patient needs a little examination and some minor treatment, and then you'll be on your way again.End of story marker

Tom Neale, who leads our BoatUS Magazine Ask The Experts team, lives aboard his 53-footer with his wife, Mel.

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