6 Common Causes of Outboard Motor Vibration
Outboard engine vibration can be caused by a variety of basic issues. While all boat engine types vibrate, outboards are notorious for excessive vibration.
Heavy vibration or jerking from an outboard motor can lead to several problems, so it’s important to identify what’s causing the problem and fix it. Here are 6 common causes of outboard engine vibration.
Outboard Propeller Damage
Damage to a propeller is typically identified as the most common cause of outboard engine vibration. A prop that is bent, broken, worn out or misaligned can create heavy shaking. So can obstructions to the prop like tangled seaweed and fishing line.
One way to tell if the propeller is to blame is intense vibrations while running at higher RPMs. If you suspect the propeller is the problem, inspect it for broken blades, nicks, bends, pitting and obstructions. Another way to identify if the propeller is at fault is by removing it, then running the engine without it to see if it’s still vibrating. You can also try running the engine with a different prop to see if that’s the problem.
Besides the propeller itself, you should also inspect the prop shaft to see if it’s bent or damaged. Make sure to inspect all of the prop parts and hardware (nuts, bolts, bearings, etc.), tighten anything that’s loose, and replace any parts that are worn out. While inspecting the prop shaft, make sure to replace any worn out seals as well.
Loose Outboard Engine Mount
The outboard itself might not be the problem of excessive vibration, but rather what the engine is mounted on. Like everything else, engine mounts also wear out over time.
A poorly mounted outboard is a common cause of engine vibrations. You’ll notice there’s a problem with the engine mount when the vibration worsens as the boat accelerates. Check your outboard's engine mounts and the hull itself for signs of wear or damage that could be causing the problem. Use a torque wrench to tighten up any loose engine mounts per manufacturer recommendations, and replace any hardware that appears to be worn out.
Bad Outboard Fuel Pump
A bad fuel pump can also be the cause of outboard engine vibrations. That’s because when the fuel pump starts to fail, it struggles to do its job.
Engine vibrations that increase at high speeds but dissipate at lower speeds may occur due to the fuel pump overheating, or vice-versa, vibrations occurring at a low idle that go away as the boat accelerates.
Testing the fuel pump is the best way to identify whether it’s the culprit. One way to diagnose fuel pump problems is to perform a compression test. This test is done by removing a spark plug wire boot, then removing the plug itself with a spark plug wrench and attaching a compression tester into the socket. If the compression gauge reading is over or under what the outboard is rated for, you might have a bad fuel pump.
Besides correcting vibrations, you’ll want to rebuild a bad fuel pump or replace it so that the outboard can get the right amount of fuel for the engine to run properly. Otherwise, jerky acceleration and vibrations from the motor will continue to occur, which could lead to other problems down the line. Fortunately, fuel pump repairs and replacements are not too difficult to do yourself if you’re mechanically inclined.
Clogged Outboard Fuel Filter
Another fuel system component that can cause vibrations from the outboard is a clogged or defective fuel filter, which must remain unobstructed to do its job. That job is preventing debris from entering the motor while it’s running.
A clogged fuel filter can eventually lead to corrosion of the entire fuel system, including the fuel lines. When the fuel filter goes bad, the outboard struggles to get the power it needs to operate, making it jerky on acceleration. Fuel consumption also increases. A compromised fuel system due to a bad fuel filter can create engine vibrations, so you’ll want to inspect the fuel filter and replace it if it’s damaged or blocked.
Faulty Outboard Spark Plugs
Perhaps the simplest cause of outboard vibrations are fouled or faulty spark plugs. When a spark plug goes bad, it affects the outboard’s ability to run as it should.
Spark plugs are essential for an outboard to fire on all cylinders, and one bad plug can make vibrations much worse on acceleration due to power loss. In other words, if the engine can’t produce the power needed to accelerate the boat … you guessed it: vibrations and jerky acceleration. Fortunately, testing spark plugs is an incredibly simple task, and if you find one that’s faulty, replacing it is incredibly cheap and easy to do.
Bad Outboard Engine Oil
Another incredibly simple potential cause of outboard vibration is bad engine oil. Oil changes are “Maintenance 101”, so this should never be the cause, but it happens.
When the engine oil is bad or stale, it reduces the outboard’s ability to perform and causes vibrations due to that poor performance. Once again, regularly scheduled oil changes are standard routine maintenance that should never be missed. If all of the above issues have been addressed and your outboard is still vibrating, maybe a simple oil change is all that’s needed.
Speaking of all of the above issues being addressed, some of them are covered in the video above as part of an outboard’s 100-hour maintenance. These include changing the oil and replacing the fuel filter. Watch the video above to learn how to do an outboard 100-hour maintenance service.