22 Ways to Screw Up Your Engine

Ignorance isn't bliss, it's expensive.

Think you know how to keep your engine going? Well, you might. You probably filter the fuel, change the oil, and make sure the cooling water is flowing. And of course you’ve read the engine manual. Or have you? I’m betting there are lots of things you’ve missed in that little bible, and that once you got past Page 7, you only looked at the pictures. Too bad, because even though you think you know what you’re doing, you might be screwing up your engine by doing dumb things such as…

Economic Indicators

Ignore an idiot light or warning buzzer once it comes on and risk a whopping repair bill. But sunlight can overwhelm LED or low-watt bulbs in indicators, making them hard to see. Solution? Provide shade, or move them. Even better are warning buzzers. But can you hear yours over the engine, stereo, and wind? At high speeds, noise levels can be in the 95+ dB-A range (like standing in a New York City subway station). Can your audible alarm top that? To be sure, install a Screamer 110 dB-A sounder for $8 ( But keep in mind that by the time a warning goes off, the problem has already occurred. We like items such as Caterpillar’s in-line water flow sensor that triggers an alarm when cooling water slows-well before overheating begins.

In Hot Water

Install your thermostat backward so the heat-sensing part is located on the wrong side. This is a surefire way to guarantee your engine overheats. It’s such an easy mistake, even the pros do it.


You Never Listen to Me

Turn a deaf ear to your boat. A sound that imitates a squealing pig probably means a belt is loose. A metallic tapping that keeps time with engine rpm means your valves, lifters, or rocker arms are in trouble. Grinding and grating metal could mean a water pump bearing is shot. If your exhaust gets louder and has a higher pitch, there’s probably less cooling water running through it-check the water pump. The cacophony is music to a mechanic’s banker.

Don’t Look Down

Disregard engine gauges at your own expense. Airplane pilots are taught to regularly scan their gauges. They start at one side of the panel and pan slowly across, looking at everything, and then pan back the other way. With only a few gauges on a boat, this pan and scan shouldn’t be a big deal. Some owners turn their gauges so the needles point up, to high noon, when running normal. It looks odd, but it’s easier to recognize when a change occurs.

Antifreeze Meltdown

Use straight antifreeze instead of topping off your sealed cooling system with a mixture of antifreeze and water. More is better, right? Wrong! The closer the mixture in your cooling system gets to pure antifreeze, the hotter the engine will run. Undiluted antifreeze boils at a higher temperature (about 230 degrees Fahrenheit) than water (212 degrees Fahrenheit). It therefore retains more heat, allowing the engine to run hotter or even overheat.


Turn Up Your Nose

Smell the fresh sea air and ignore the engine room’s funk. A healthy engine has a distinctive odor. Take a whiff and remember it. Then when an odorous messenger shows up, you’ll know something’s wrong. The acrid stench of burnt rubber could come from a dry-running water pump impeller (look for a clogged water intake), not enough cooling water reaching the exhaust, a slipping V-belt, or a failed coupler in the stern drive. A hint of burnt oil might mean the engine was run hard, or if it’s a strong odor, there’s an oil leak dripping on the hot engine. A cloying sweet aroma indicates leaking antifreeze. And a smell like burnt hair could be an electrical short.

Running Dry

Fire up the engine while it’s on the trailer to make sure it’ll start. You figure that in those few seconds the engine won’t overheat. You’re right, too. But the water pump’s impeller can get damaged. Impellers need water for lubrication. Without it, they wear away or just fail completely.

Cheap Oil

Feed your two-stroke outboard bargain oil and ensure rapid aging. Don’t think you’re saving money when you buy some off-brand oil. To meet industry standards, oil has to pass the test only once, meaning it’s not monitored by the batch. Better oils are uniform, consistently meeting the standard case after case. During the average summer, a 100-hp engine may burn only a gallon or two of oil. The difference between cheap oils and the ones offered by engine manufacturers may be about $10. Remember: The most expensive thing you can put in an engine is cheap oil.


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First Coarse

Assume new filter elements are doing their job. Diesel engines, and gasoline engines running far offshore, should have two water separator/filters per engine. But don't put the same type of element in both filters, or worse, put the finer element in the first filter. Doing so makes that filter do all the work, with little to no benefit from the other. Put a coarse (30-micron) element in the first and a finer (2-micron) in the second. This way both share the load, do a better job, and give you longer element (and engine) life.


Engines will always vibrate, so you learn to ignore it. If your bowrail starts to shake each time the boat climbs on plane, but you don't feel it at the helm, it's likely that one or more of your prop's blades are bent. A vibration only at certain rpm could mean your prop needs balancing. When the whole boat rumbles and shakes at all speeds, the engine and shaft are out of alignment. If you want to see how and when your boat vibrates, watch the surface of a bucket of water placed in the cockpit.


Jets Suck

Water jets let you run in shallow water, so you go in freely. Unfortunately, you may not come back out. Jets use some of the water they suck in for cooling. If that water has sand, muck, or rocks mixed in, your cooling system may get clogged. One jet drive manufacturer goes so far as to warn: "Avoid shallow-water conditions." Continuing with, "Always be in at least two to three feet of water, especially when accelerating from idle speeds." If you must run in skinny water, go fast. This way the jet sucks in cleaner surface water as the boat rides high and well above the bottom.

Forget to Flush

Put your jet drive away after each use without thoroughly flushing the engine and you may have to put it away permanently. Although it may not ingest something large enough to cripple it while underway, it can develop a slow buildup of crud. Clean out internal passages by flushing your jet with freshwater for 10 minutes, and use plenty of pressure. All current models have a flushing port. Older Mercury jets can be updated with an accessory flushing attachment that costs about $25.

Last Gasp

Rev your engine before turning it off. It was probably a mechanic needing work who started this myth. The idea is that it's supposed to make the engine easier to restart. But all it does is leave unburned fuel coating the cylinders, which forms a gummy deposit.

Flick the Switch

Turn the battery selector switch incorrectly while the engine is running and, bam! No more alternator. When switching from 1 to 2 or Both, never pass through the Off position. For the microsecond the switch is completely closed, the alternator's output has nowhere to go. This will blow out the diode and kill the alternator. Prevent this with a $13 gadget called Zap-Stop. It automatically directs any charge over 16 volts to ground.

Anode Ignorance

Sacrificial zinc alloy blocks go only on your drive or propshaft, right? Wrong. Your engine has an internal one, too. You'd better find out where it is and learn to replace it every time you change your oil. If left in place too long, it will start to crumble and give off particles that may clog your cooling system.

Stainless Steal

Assume that if the ring of a hose clamp is stainless, the spiral screw that tightens is made of the same stuff. Often the screw is mild steel, which rusts if you put a salt shaker near it. A broken hose clamp is a sure way to lose water, fuel, or an exhaust hose-or to sink a boat.

Dangle a Line

Leave lines hanging off the stern. Wrap some 3?8" nylon around your prop at a few thousand rpm and bearings get shot, drip glands rip up, transmission gears strip, and if the engine mounts are weak, the engine pulls out of alignment or loosens from its bed.

Neglect Corrosion

Ignore your engine's need for a dry environment. Keep electrical connectors coated with WD-40 or Vaseline to repel moisture. If boating in saltwater, occasionally wipe down the engine with a rag damp from freshwater. Make sure you get rid of all dried-on salt crystals, which attract moisture. Wipe dry, spray on a light coating of oil, and spread that oil around to make sure all parts (even undersides) get protection.

Don't Change

Don't change your oil-just dilute what's been left behind. Your goal is to completely replace the old oil with new. Compare how much oil comes out with what goes in. The difference is the amount of foul oil left behind. If you can't suck out all the old oil from the top, install an oil draining kit under the sump. Just make sure the engine's mounting angle allows oil to collect over the drain plug. Or, change your oil more frequently so that the oil left behind never has a chance to get bad.

With This Ring

Purchase the cheapest fuel-'round here we call it "frog wizz." You will not only get the worst possible performance but you'll likely destroy pistons, rings, and cylinders. Your engine's manufacturer specifies a minimum octane rating. Go lower and your engine may knock or ping under load. It can put up with this for a while, but do it for too many tankfuls and carbon can build up in the piston ring's grooves and behind the rings. This forces ("jacks") the rings outward, scoring the cylinder or, worse, locking up the engine. While almost anything will run some of the smaller outboards (rated for as low as 84 octane), most engines require at least an honest 87 octane, with 89 preferred. For high-performance models, use 91 octane.

In Bearings We Thrust

Replace a prop on an outboard or stern drive and forget to put the thrust bearing back on first. Or if you have a few lying around, put on the wrong one. Without this washer-like bearing, the prop's hub rides hard against the gearcase each time you shift into forward. Check the price of a gearcase housing and you'll never forget your thrust bearing again.

Ultimate Screw-Up

Don't read the instructions. I used to write these neglected pieces of literature and accepted that most people ignore them. Too bad, because our lives would be a lot easier, and our engines would last a lot longer, if we only took the time to refer to them every once in a while. As one manufacture put it, "The only thing we like to see wear out is the owner's manual."