Five Key Engine Checks Before Pulling the Trigger on a Used Boat
July 06, 2022 9 min read
Five Key Engine Checks Before Buying a Used Boat
Almost without exception, when you buy a used boat, you’ll discover issues the first few times you run it that you missed before purchase — a float switch that sticks on, a bow light that doesn’t work, a leak in the livewell plumbing, a glitchy trim tab switch. It happens, and it’s not a big deal. In fact, you should assume going in that there will be a “punch list” phase, during which you find and fix assorted issues on your new-to-you boat.
On the other hand, discovering issues with the engine(s) and/or hull can be potentially catastrophic. Major engine repairs, rebuilds and replacements are expensive, as are structural repairs on fiberglass hulls. As a point of reference, MSRP on a new Yamaha F150 with mechanical controls is upwards of $15,500 in 2022. For an F250, it’s around $25,000. And significant hull repairs like replacing a transom or stringers can easily run well into five figures.
The point is that when evaluating a used boat, you should concentrate on the big stuff. A chartplotter that cuts out intermittently, a blown speaker, a torn leaning post cushion, aging trolling motor batteries — those are just opportunities for negotiation. It’s problems with the hull and/or the engine(s) that should make you think hard about walking away.
Fortunately, even with little or no mechanical know-how, you can perform a few simple checks that will tell you a lot about the condition of the engine(s) and hull. Let’s look first at five key things to check on used outboard engines. Remember, this is not an exhaustive checklist!
This is the only one of these tests that requires any technical ability. That being said, it’s well within the abilities of the average do-it-yourselfer. You’ll need a spark plug wrench, a compression gauge ($20 - $40), and possibly another simple hand tool or two, depending on the engine’s make and model. There are dozens of YouTube videos showing the procedure, like this one from BoatU.S. If you don’t feel comfortable performing your own compression test, hire somebody to do it for you.
With a little internet searching, you can find the manufacturer’s minimum compression specification for your engine, but in most cases you won’t even need that. Rather, you simply need to verify that compression is consistent across all cylinders, meaning compression in each cylinder is within around 10% of compression in the other cylinders.
"you simply need to verify that compression is consistent across all cylinders, meaning compression in each cylinder is within around 10% of compression in the other cylinders."
Problems usually reveal themselves in one cylinder that shows markedly lower compression than the rest, indicating a problem with the piston, rings, valves or the cylinder itself. You can’t diagnose the issue from a compression test alone, but a cylinder with low compression is a big red flag suggesting the engine will need at least a major teardown if not an entirely new powerhead — both of which are going to cost easily in the thousands of dollars. Either walk away or get a professional diagnosis.
2. Wide-open Throttle RPM
You should never buy a boat — used or new — without a sea trial, and perhaps the most important thing you can do on your sea trial to evaluate engine health is to run the boat at wide-open throttle (WOT) for a few minutes and check the tachometer.
Every outboard has a WOT RPM range specified by the manufacturer. For a typical modern four-stroke, it will be something like 5,000 to 6,000 RPM, meaning that with an appropriate propeller, engine RPM should be somewhere between those two numbers at full throttle. Two strokes generally have lower ranges, typically topping out around 5,500 RPM. Before your sea trial, use the internet to find the range for the year, make and model engine(s) on the boat. Once on the water, verify that when you open the throttle all the way, RPMs fall within the range.
RPMs higher than the specified range most often indicate that the boat is “underpropped,” meaning the propeller doesn’t have enough pitch. That’s a fairly minor problem in itself — a new stainless propeller runs $400 to $700 — but it could suggest that the engine has been run a lot at, near or even above its maximum recommended RPMs. You may want to consider having a technician or shop hook it up to a diagnostic system, which often will indicate how long the engine has spent operating at various RPMs.
"There’s nothing harmful about running a healthy outboard at WOT, even for extended periods of time, provided RPMs don’t exceed the manufacturer’s recommended range. If a seller won’t let you run a boat at WOT for at least a few minutes, that’s an issue in itself."
RPMs lower than the specified WOT range, on the other hand, can indicate major problems. Yes, it could be simply that the boat is overpropped. But it could also mean that because of some issue the engine isn’t capable of producing its full power.
As with a compression test, it’s difficult to make a diagnosis based only on the fact that an engine isn’t reaching the recommended WOT RPM range. It could be due to a fuel delivery problem, an electrical or ignition problem, an internal mechanical problem, or a host of other things. Even if nothing is wrong with the engine, low RPMs at WOT could indicate that the hull is waterlogged and too heavy for the engine to push. Regardless, an engine that can’t reach its recommended RPM range is another big red flag. Have it professionally diagnosed, or look for another boat.
It’s worth noting here that some sellers — both individuals and dealers — won’t want you to run a boat at WOT. That’s not realistic. There’s nothing harmful about running a healthy outboard at WOT, even for extended periods of time, provided RPMs don’t exceed the manufacturer’s recommended range. If a seller won’t let you run a boat at WOT for at least a few minutes, that’s an issue in itself.
Running at WOT is also a good way to verify that the cooling system is functioning properly. Engines with failing water pumps or impellers, or with cooling system corrosion, tend to overheat at WOT. That shouldn’t happen, and don’t let a seller tell you otherwise. Again, every outboard is engineered to provide sufficient cooling at WOT.
3. Idle RPM
While not as big a telltale as WOT RPM, idle RPM should also be checked during your sea trial. Most outboards are designed to idle in neutral at less than 1,000 RPM — typically in the 600 to 800 range.
Idle RPM can be manually adjusted, so a fast idle doesn’t necessarily mean something is wrong. It could, though, suggest that the engine doesn’t idle properly at the normal RPM so the owner increased the idle speed to smooth it out. (Note that outboards tend to idle faster on muffs, so check idle speed with the boat actually in the water.
"An engine that won’t idle smoothly under 900 RPM or so, or even stalls intermittently, may have electrical, air flow or fuel system issues. Have it diagnosed by a professional or walk away."
An engine that won’t idle smoothly under 900 RPM or so, or even stalls intermittently, may have electrical, air flow or fuel system issues. Have it diagnosed by a professional or walk away.
While you’re checking idle RPM, you should also shift between neutral, forward and reverse a few times to verify that the engine goes smoothly in and out of gear. Shifting problems could be caused by something as simple as a cable that needs lubrication or adjustment or as costly and complicated as damaged gears in the lower unit. A fast idle can also make shifting hard.
4. Engine Hours
This isn’t something to check so much as something to analyze. Virtually all boats with modern outboards have hour meters, making it easy to see total hours. But it’s not that simple.
First, look at the total hours in comparison to the number of years the engine has been in service to get a picture of how frequently the engine is used. Typical annual use for a recreational boater runs from say 25 or 30 hours for a holiday boater, to 75 or 100 hours for an active family boater, to 300 or even more for a diehard who spends every spare minute on the water.
If average usage per year is under 30 hours or so — especially on an engine older than a few years — that’s a good indication that a boat has sat without running for extended periods of time. That’s another red flag. If you’re looking at a 15-year-old boat and engine with 100 hours on the meter, you need to learn why that is. Did it sit unused for years? Is the meter wrong? Are the hours since a rebuild or powerhead replacement?
Likewise, high annual usage and/or high total hours should be considered in context. Well maintained four-stroke outboards — usually those that are used daily in commercial service and carefully maintained — can and do provide 4,000 to 8,000 hours of service. So high hours alone aren’t necessarily a problem. In fact, knowledgeable boaters often say they’d be less concerned about buying a 10-year-old engine with 1,500 hours than a 10-year-old engine with 150 hours.
"If you’re looking at a 15-year-old boat and engine with 100 hours on the meter, you need to learn why that is. Did it sit unused for years? Is the meter wrong? Are the hours since a rebuild or powerhead replacement?... knowledgeable boaters often say they’d be less concerned about buying a 10-year-old engine with 1,500 hours than a 10-year-old engine with 150 hours."
Very high hours — say upwards of 500 per year or 2,000 total — usually indicate that the boat has been used by a guide or in some other kind of commercial service. Again, that doesn’t have to be a deal breaker, but it does warrant more investigation. With heavy use, you’ll certainly want to see evidence of good maintenance and any repairs.
Because there are so many other variables, it’s difficult to say how many total hours is “too many.” Despite the fact that guide and commercial engines regularly run for 4,000+ hours, 2,000 hours is probably a more reasonable estimate of life expectancy for a well-maintained outboard engine in typical recreational use.
Finally, most modern engines can be connected to diagnostic software to generate a report of how many hours they’ve been operated at various RPM ranges. All else being equal, a 500-hour engine that has spent 70% of its hours below 2,000 RPM, most of the rest cruising between 3,000 and 4,000 RPM, and only a few in excess of 5,000 RPM likely has more life left than the same engine that has spent much of its time running around at 5,000+ RPM.
If you’re concerned about high hours, try to obtain a report to learn more about how the engine has been used. Likewise, if hours seem unrealistically low, get a report to make sure the hour meter is correct.
5. Gear Lube
You can learn a lot simply by looking at the lower unit gear lube on an outboard or sterndrive, which you can easily extract a small sample of. Make sure the seller is okay with it, then, with the engine tilted all the way down, slightly crack the lower drain plug screw on the lower unit and then the upper plug to let a small amount of lube drain into a cup or dish.
Lower unit gear oil should be a uniform translucent or opaque dark brown or black. A burnt smell is typical. A few small metal shavings in the sample are also not something to worry about. Rather, what you’re looking for is lighter colors. Any sort of milky gray, light brown, yellow or whitish coloration indicates that water has mixed with the oil, usually from a bad seal on the propshaft or elsewhere. This shouldn’t necessarily be a deal-breaker — it could be from something as minor as a leaky drain plug — but it should certainly be positively diagnosed by a professional.