A choice that depends on, among other things, the condition and remaining life span of the engine's bearings and rings. Another occasion for a survey is the purchase of a used boat. Savvy buyers always survey a used boat to minimize the possibility of getting burned. No matter whether you commission a survey, or do it yourself, it's important to know the correct procedure for diagnosing an engine. The more thorough you are, the more information you'll have on which to base your decision whether to reach for your wallet or let it stay in your pocket.
Okay, you've found the used boat of your dreams. Where to begin? Start by checking vital fluids. On stern drives and inboards, pull the dipstick and note the oil level. On closed cooling systems, check the coolant level. On stern drives and outboards, check the trim/tilt oil reservoir level. Note the power steering reservoir oil level, too. Don't rely on your memory - write all this stuff down. Low levels indicate possible leaks. Look further.
Next, start the engine. This immediately reveals the battery's condition and the engine's general state of tune. If the engine won't even crank, its batteries are weak, which is a red-flag signal that the boat has been poorly maintained. Another possibility is that the owner removed the robust batteries and stuck in a pair of junkers. Also note whether the battery bank is hooked up to a charger when you arrive, or whether you can spot a charger in the vicinity. Some folks like to cheat. Caveat emptor.
This might be a good time to ask the owner to see the boat's maintenance records and any receipts he's saved. These records provide clues about an engine's secret past. You may spot a nagging problem area that will warn you away from the boat. Also, you can see what work you won't have to do or have done, because it was recently taken care of.
If the motor is difficult to start, the engine needs a tune-up - another red flag indicating poor maintenance. Obviously, poor starting may also be caused by more serious problems. But these weaknesses will show up later on in the testing sequence when we hook up vacuum and compression gauges - at least they should.
When the engine has run long enough to reach its normal operating temperature, listen to its idle. It should be smooth and stumble-free. Slip the mooring lines and head out past the no-wake zone. Acceleration should be crisp and measured. Hesitation is a red flag. Hold the engine at wide-open throttle (WOT). Watch the temperature gauge for overheating. Have fun with the boat for a while, then return to port.
Recheck all the engine's vitals. If any of the levels have dropped, find out why. If stern drives and inboards have not had the benefit of regular oil changes, they become oil burners. If the owner tells you that burning a little oil is a good thing because the extra oil enhances lubrication, walk away. Also note whether the oil residue on the dipstick is milky, which is a sign that water or coolant has infiltrated the system. This means either blown gaskets or a cracked block and/or heads.
If the engine passes its shakedown cruise, move on to the vacuum and compression tests. Vacuum is always tested with the engine running. Not only can the results reveal a sick engine, but the readings can also pinpoint which components are in need of repair. In other words, if you want the boat at any cost, you should do these tests so that you'll at least know what those cost will be.
There are two varieties of vacuum gauges. The first type, which you sometimes see mounted on dashes, comes emblazoned with red, yellow and green, and is best known for its ability to indicate when an engine is operating at peak fuel economy. When vacuum registers in the green, fuel economy is greatest; if it's in the yellow, back off the throttle a little. If it's in the red, you're guzzling fuel like there's no tomorrow. But these gauges, solely intended to indicate fuel economy, are virtually worthless. The other variety has numeric readouts from zero to 30 inches of vacuum. This is the style you want. Knowledgeable boatmen install such a gauge as a matter of course. Follow the instructions that accompany the unit. Because the installation is temporary, you'll be able to simplify the procedure.
The info pack also details what vacuum gauge results mean. Basically, the higher the vacuum, the more fuel-efficiently the engine is running. For example, a healthy, in-tune engine with properly sealing piston rings, gaskets and valves normally reads between 17 and 21 inches of vacuum and holds steady. Blipping the throttle drops the needle to two inches before it jumps back up higher than normal, and then settles between 17 and 21.
Sticking valves result in a random, momentary drop of four to five points. Sometimes you can see the evidence on the gauge at the same time you hear it from the exhaust. Misfiring spark plugs show up in exactly the same way. Burned valves drop the needle about five points every time that cylinder fires.
Worn valve guides, which also suck oil out of the valve covers, vibrate the needle at idle (it then smoothes out at higher rpm). A slipped timing chain results in a steady low reading that ranges from about eight to 14. A steady, high reading accompanied by the engine's refusal to rev can usually be traced to a clogged or really dirty spark arrestor. Weak valve springs vibrate the needle during acceleration. A really low, constant reading of about five indicates a vacuum leak. You can locate vacuum leaks by spraying a lubricant like WD-40 along intake gasket surfaces. With the engine idling, rpm will climb when you shoot the lubricant on the leak.
Worn compression rings show up on the vaccuum gauge as three to four points below the normal reading. Blip the throttle, and the needle sinks to zero, then soars. Should this happen, you'll want to pay close attention to the compression test results.
A compression test reveals how well, or poorly, an engine's piston rings and valves are sealing the combustion chamber. Consult the engine's service manual to find out exactly what an engine's compression should be. Generally, it's about 150 pounds per cylinder. To run this test, remove all of the spark plugs and fully open the throttle. With the engine warm, crank the engine through two or three revolutions. Read the compression gauge and record its highest reading. Repeat this procedure for each cylinder. Cylinder compression is considered acceptable when registering no less than 80 percent of the manufacturer's specifications. The difference between each cylinder should be no more than 12 to 14 pounds.
A low reading usually indicates worn compression rings and/or leaking valves. Adequate compression on all the cylinders but one pinpoints a weak cylinder. Adjacent low readings can mean the head gasket is leaking between the cylinders. If the compression is low, squirt about an ounce of oil into the offending cylinder. Crank the engine through several revolutions to spread the oil around, then retest. If the compression reading is higher, then the compression rings are worn. If it's not, then it's very likely that the valves are leaking.
Most marine engines die from overheating, so it's important that you make sure that any engine you buy is keeping its cool. The first test should be conducted during your offshore trials. Simply watch the temperature gauge. Dockside, look for signs of overheating around the core plugs (also called freeze plugs). Discoloration often means coolant is escaping and there may have been an overheating problem in the past. Escaping coolant will literally steam-clean the block around its escape path. Also take notice of whether some of the freeze plugs look older than others.
On stern drives and outboard motors, the gearcase is a component prone to problems. Inspect the gearcase for signs of damage or repairs. The skeg should not be bent, be missing metal or show evidence of repair. There should be no welds that indicate past impact damage. Another possible sign of impact damage is a beat-up or welded propeller.
Crack the gearcase's lower drain plug to allow whatever's inside to trickle out. If only a short time has elapsed since you ran the boat, the gear oil will look milky white or cream-colored if there's water in it. If the boat has been sitting long enough for the water to sink below the gear oil, then water will come out first.
When you find water mixed in with the gear oil it means that either the propeller shaft, drive shaft or shift rod seals are leaking. When the engine is running, gearcase pressure forces the oil out through the leak, leaving a vacuum. When the engine stops, the vacuum draws in water. Replacing seals is relatively inexpensive. The real problem is that the leak may have been ignored long enough for the water to attack the gears. Corrosion may have eaten away the teeth. Also, water is a poor lubricant and shortens a gearset's life span.
Physically inspect the trim/tilt unit. Check the hydraulic fluid level, making sure it's topped off. Then run the drive up and down through several cycles. Now check the fluid level to make sure it hasn't lost oil. Then follow the hydraulic lines to the lift rams, looking for leaks. With the drive fully tilted, inspect the lift rams for impact damage. Finally, you have to decide whether the engine is in good enough shape to make the boat agood buy. Hopefully, the above procedures will help you do so.
On four-stroke outboard motors, stern drives and inboards you should have the crankcase oil analyzed. An oil analysis is like an engine X-ray. The procedure is as simple as drawing a sample and sending it to the laboratory for spectrographic analysis. The results reveal bearing, piston and valve particulate, if any. Ask your repair shop for the address of the facility they utilize.
A do-it-yourself method of oil analysis is to dissect the oil filter. Cut it open like a can of beans with tin snips or a hammer and chisel. Be sure to wear leather gloves to protect your hands from sharp edges. Separate the filter element and look between the pleats. Test the debris, if any, with a magnet. This tells you whether it's steel or iron, or conversely, brass, copper or aluminum. You shouldn't find any of these elements. If you do, repairs are in order.