Sailboat Diesel Engine Analysis

Make a thorough engine inspection part of your fitting out routine and identify problems ahead of time.
performing a load test
If your boat is tied to a dock with stout cleats, performing a load test will allow you to measure a wide range of functions, from cooling water flow to alternator output. Scott Fratcher

Working as a marine engineer I get called to inspect a lot of boats and have to make quick determinations of whether the engine is ready for sea or, in the case of a pending sale, to check the condition of the motor before purchase. To do this, I have devised what I call a fast Engine Technical Analysis to help find problems before they become a sea story.

In essence, an ETA is an advanced system of inspecting a boat’s drive train. I perform these tests to help catch pending mechanical issues before leaving the dock. An analysis is much more comprehensive than a typical pre-departure peek at the engine. I visually inspect the motor, of course, but also take a series of measurements that become a base line for future tests. The goal is to use this analysis to help make easily informed decisions about the boat’s drive system now and in the future.

The steps to performing an engine analysis include a visual inspection, a more detailed evaluation of fluids and equipment, a load test, and analyzing the numbers. Here’s how to perform one:


Visual Inspection

To get started, give the engine a general once-over, looking for blatant flaws and obvious irregularities. Open the engine room door (or compartment) and look, feel and smell.

Look for oil drips or other fluid under the engine. Inspect for obvious cracked hoses. Give a gentle bend where a hose attaches to the engine and look for cracks in the rubber cover or for loose hose clamps. Check the fan belts for cracks. Grab the alternator pulley and see if it will spin free on the belt. If it does, the belt is worn or loose.

Feel the inside of the engine room for any oil film. Touch the areas of the engine you can’t see, feeling for rust or salt trails.


Smell for any burned chemical type odor. Smells can be difficult to locate, but in general any burnt, oily or diesel aromas should be searched out. A good nose can tell the difference between a hot belt and a hot wire.

Look over the complete engine for oil leaks. Pay particular attention to the front seal and the area between the gearbox and engine. Leaks in the front or rear seal can mean a coming engine rebuild. Inspect the head-to-block seam, looking for oil or water trails.

Take photos from all angles as a base line for later use.


This is the point where many an engine inspection ends, but in an analysis, we want better scrutiny to help build confidence in the propulsion system.

An infrared-thermometer scan can pinpoint cooling and lubrication blockages. Scott Fratcher

Details, Details

The next series of checks focus on fluid inspections. With the engine off and cold, check the following:

Oil: Oil is the lifeblood of any engine. It lubricates, cools and quiets the diesel. But how do you know if the oil is doing its job? Begin by pulling the dipstick. Smell for signs of diesel odor. Diesel could mean a leaking fuel pump or an injection pump about to go bad. Pinch a small dab of oil between your index finger and thumb, and expand slowly to see how far you can spread the oil before the gap opens. Take a photo and compare this gap to new oil. Write down the difference in your notes. This is a crude method of checking viscosity and diesel intrusion.


Open the oil fill and look inside the cap for water droplets, condensation or worse, a gray gooey substance indicating water penetration into the oil. Put a small drop of oil on a paper towel and compare it to a new oil drop. Note the amount of black carbon and any shiny deposits.

A more careful inspection can include a look inside the oil filter. Next time you change the oil keep the old filter. Leave it to drain upside down, then cut open the canister and look at the inside of the element. It’s the inside of the paper filter that will contain any heavy deposits or metal shavings. Take photos for later comparison.

cable inspection
Inspect throttle and shift cables to make sure they move smoothly and that their housings are securely attached. Scott Fratcher

Later, when you start the engine, note the oil pressure. Do so again during the load test, and a third time immediately upon reducing throttle after the test. Does the gauge needle dip? It should remain steady from start up and through all the following tests. A dipping gauge can indicate a faulty oil pressure release valve or worn engine bearings.

Coolant: A typical yacht engine has two cooling-water loops: fresh water and raw water. Let’s look at the freshwater loop first. With the engine shut off and cool, open the expansion-tank cover or the heat-exchanger cover — or where you check the coolant water. The reservoir should be filled almost to the top with green or red coolant. A low coolant level can mean a small air leak in a hose.

Check the coolant for oil. It should look new and fresh. The underside of the cap should be clean. No brown, gooey oil or rust should be present under the cap. Any strange colors can mean a failing heat exchanger or failing head gasket.

Next, find the saltwater pump. Inspect for leaks, green drips and rust streaks. Pay particular attention to the area between the pump and the engine. Any leaks mean the internal seals are failing; or maybe a clogged heat exchanger or exhaust elbow is causing back-pressure on the pump.

Diesel Filter: Locate your diesel filters. Most boats have two sets, one on the engine supplied by the manufacturer and a pre-filter or set of filters, often marked with the Racor brand. On the bottom of most fuel filters, there will be a drain valve. Drain off a small amount of fuel into a container and check it for water, dirt and any long, stringy algae. If you find more than a couple teaspoons of water or much dirt, then it’s probably time to change the filters. If your filter system has a vacuum gauge, note the readings in the log for future comparison.

Once you’re done with the engine’s fluids, move on to other critical elements.

Control system: The engine control system, which includes the shift and throttle levers at the helm, directs a skipper’s intentions to the motor. Inspecting the components is critical. For example, if the engine fails to engage in reverse during docking, the results can cause boat damage or worse. Begin by feeling for play in the shifting linkage. Move the controls through all possible motions, feeling for hard spots. Does neutral have a detent or click to signal the transmission is in neutral?

Inspect where the cables attach inside the shifting housing. Look at each split pin for wear. Wiggle and physically inspect each connection looking for cracks, breaks or anything that looks amiss. Pay particular attention to where the cable cover end attaches to the shift housing. If this point slips, you can lose engine control immediately.

volt meter
A volt meter will identify possible cable, battery or starter problems. Scott Fratcher

Inspect the cable at the engine, and look for chafe from engine vibration. An area where the plastic cable cover has chafed through can let in small amounts of water that will corrode the cable in areas where it can’t be seen, causing an unpredictable loss of engine control.

Gearbox and coupling: Inspect the gearbox-to-prop-shaft flange bolts. Attempt to tighten the Allen setscrews in the coupling. Loose bolts or setscrews indicate a vibration or misalignment. Pull the gear-oil dipstick. Check the level and perform the viscosity check again. Look for water under the cap and smell for any burned odor. Most yacht gearboxes don’t have a filter so any bits of crud or metal keep getting recirculated. It is imperative to keep an eye on the oil.

Starter load test: Place the jaw of a clamp-amp electrical meter over the positive cable leading to the starter. Clip your voltmeter to the starter’s positive and negative. Hold the stop button or manual engine shutdown while turning the engine over for 10 seconds. (The engine should not start.) Note the starter amp draw. On a typical 40- to 75-hp engine the starter draw reading should be 225 to 275 amps. Look at the starter for a rated amperage or wattage (watts/volts=predicted amps). They should match what you see on your meter. Drawing more amperage could indicate poor cables, a bad armature in the starter or failing batteries. The voltage during this test should remain above 9.5 volts at the starter. The results of this test should be placed in your notebook and the vessel’s log for future reference. This is an important test as it indicates if the starter is about to fail, and helps troubleshoot later problems.

inspecting hoses
Bend hoses to find cracks before they become a headache. Scott Fratcher

Air inlet: In order for a diesel to run efficiently, it needs a large amount of good, clean, cool air. Here are some tests to help ensure the quality of the air feed to the engine: With the engine off, pull the air filter and inspect for cleanliness. Reach a finger down the air inlet. It should be lightly covered in dirty, but not gritty, oil. Any loose oil or splatter indicates a possible faulty intake valve, or maybe a turbo failing. Check for grit, indicating a leaking air filter. If the engine has a turbo charger, you might be able to reach inside and give the blades a spin. They should move easily with no restrictions or any bumps.

Load It Up

The next series of tests duplicate the engine working under load, such as when motoring hard into a head sea. If the engine is relatively small, the idea is to pull against the dock lines while safely tied in the marina. This test can also be performed at anchor, if the boat is run hard in reverse. Larger yachts with more horsepower will need to conduct these tests while underway. Note: It is imperative to inspect the dock lines, anchors and cleats for suitability for such loads. Double up your spring lines and inspect the dock cleats before beginning. Be sure to locate the manual shutdown on the injection pump, and be ready to shut the engine off if you suspect any problems.

Belt inspection
Inspect engine belts for tightness, and roll them to look for cracks. Scott Fratcher

Start up check: Start the engine; wait a couple seconds for oil pressure to build. First off, before the batteries have had time to charge, test to see if the alternator is operating at full output. With the engine in neutral, increase the throttle to 1,000 rpm. Set your clamp amp to DC and place the jaws around the positive/red cable at the back of the alternator. Check the output and note the DC reading. Change the meter to AC and again note the reading. The AC reading should be around 3 amps. If the AC reading is near half of the DC reading, this indicates a faulty alternator diode. Return at the end of your testing and verify the voltage has stabilized around 14.2 volts.

Seawater flow test: While the engine continues to warm up, increase the throttle to about 2,000 rpm in neutral. Take a bucket to the engine exhaust and time how long it takes to fill the bucket with exhaust water. This number will come in handy if you ever suspect a failing impeller, clogged raw-water loop or clogged exhaust riser.

RELATED: Quieting Your Boat’s Engine

Load tests: Wait for the engine to warm up (maybe 5-10 minutes). Put the engine in forward (reverse if this is a test at anchor). Slowly bring the engine up to half throttle. Check all the dock lines one more time. If all the lines and cleats appear strong then slowly increase the engine to full throttle.

From wide open, reduce the engine speed by 200 rpm for testing. For example, a Yanmar might have a max rpm of 3,600, but would only reach 3,400 pulling against the dock lines, and thus should be run at 3,200 rpm for the remaining tests. In other words, run the engine hard, but not overloaded. A motor in good condition, properly installed, should be able to run under this type of load for hours without overheating or causing other problems.

check the oil level
Before starting the engine, pull the oil dipstick to check the oil level and sniff for any odor of diesel fuel that could indicate a leaking fuel or injector pump. Scott Fratcher

Before proceeding, recheck the dock lines and cleats.

Look at the engine mounts. They will be taking the thrust of the engine and should be compressed forward. Inspect them for any indication they are “rolling out,” indicating a coming failure. Even under strain, the rubber-mounted studs should still sit vertically. Look for metal-to-metal contact in the base. At the end of the load test, run the engine hard in reverse and recheck the mounts. Take photos of the mounts under load for later comparison.

Check for smoke: As the engine comes up to full temperature, you might see signs of smoke. Determine if it is escaping from the engine, or simply gassing-off an old film of oil and dirt. Pay particular attention for smoke escaping from between the engine and exhaust manifold. This may be combustion gas that could contain carbon monoxide, which is a dangerous, colorless, odorless gas that can kill when released into confined spaces.

Inspect the back of the engine. Take a good look at the shaft-to-gearbox coupling. It should be sitting almost perfectly smooth. Any pumping, fore-and-aft motion could indicate failing engine mounts. A circular motion could indicate a bent shaft, out-of-alignment engine or possibly a damaged prop.

Look at the packing gland for the amount of water dripping past the shaft into the boat. Compare this to the manufacturer’s recommendation. Be sure to note the amount of drips over a 60-second period and write this in your notebook and the ship’s log, as it’s a common point of worry for crew. Remember, when motoring at speed there will normally be less dripping at the packing gland due to the forward motion of the boat causing a low pressure or suction at aft end of the hull. (This is why some skiffs will self-drain when they get up to speed.)

Temperature tests: By now the complete engine should have stabilized in temperature. It’s time to do a few tests with an infrared thermometer. If you have not used a laser temperature gauge before, you’ll be amazed at how much information this simple tool will show you about an engine. They can be purchased from most tool shops for about $30.

Begin by scanning the ­gearbox looking for hot spots. Pay careful attention to the gearbox output bearing and the area of the case around the clutch packs. If any part of the gearbox is going to build up heat, it will be during this pull test. The complete gearbox temperature should be under 170 degrees F; 120 degrees F is typical.

Continue checking with the laser gauge along the cylinder head. It should read about 180 to 190 degrees F everywhere. There should be no hot spots. A hot spot would be more than a 10-degree change in the head from one end to the other.

Note: Testing the area ­directly around the exhaust ­manifold does not count as a hot spot, as this will be hot due to the high temperature of the exhaust. Concentrate on the area around the injectors.

Check the oil temperature by shooting the oil filter and oil pan. The oil temp should be about 7 to 15 degrees above the head temperature, but not above 220 degrees F.

Check the inlet ­temperature to the saltwater pump and the outlet of the final heat ­exchanger. The difference should be less than 15 degrees. A higher difference can indicate an engine producing too much heat (perhaps because of a failing head gasket) or too little water flow (a clogged saltwater system).

Next, check the freshwater temperature inlet and outlet of the heat exchanger. The difference should be about 15 to 20 degrees F and stable. You should be able to move the thermometer beam along the body of the heat exchanger and show the cooling effect of the heat exchanger. By this method you can see how much reserve cooling you have left in the heat exchanger.

If your engine is turbocharged, the pre-turbo exhaust temperature should be about 550 to 750 degrees F.; the after-turbo should be between 150 and 250 degrees F lower. Check the exhaust gas temp between the cylinders and exhaust manifold for each cylinder. Normal for a loaded non-turbo engine is ­between 450 and 550 degrees F. A ­cylinder temp lower than the rest shows a clogged injector or perhaps a dead cylinder. A high cylinder temp possibly shows a leaking exhaust valve or poor injector spray pattern.

Turbo pressure test: If the engine has a turbo, then before the load exam, find a test port on the intake manifold and install a pressure gauge to check the turbo boost. Compare this to the manual. Many turbochargers boost up pressure to 30 psi. Most Yanmar engines boost to around 18 psi. The pressure should be stable. Any cycling of pressure, or coughing, can indicate after burning, blow-by or a clogged air inlet.

Reduce throttle tests: Take one last look for any signs of problems and slow the ­engine down to an idle. With the ­infrared thermometer, check to see if the engine cools quickly, in less than five minutes.

Verify the alternator is ­producing 14.2 volts. Less means the voltage regulator needs to be inspected, and more may mean the batteries could fail early.

Shut the engine down and be sure to take careful notes on ­every reading you took.

For most yacht owners, an in-depth inspection of the ­vessel’s drive system can mean more confidence, safer travel and a much easier time of finding trouble spots in the ­future. Remember to keep careful records and photos of all the tests performed so down the road when something has changed, you can repeat the tests, make short work of the ­troubleshooting and get back to enjoying time on the water.

Scott Fratcher is a licensed marine engineer who has acquired over 300,000 sea miles. He was a critical member of Earthrace that currently holds the Union International Motonautique Round The World speedboat record. Scott now lives in New Zealand with his wife, Allison.

Gear Up

  • Here is the equipment needed to perform a thorough engine analysis:
  • Camera
  • Notepad
  • Clamp amp meter; ­electrical multi meter
  • A 0- to 45-psi pressure gauge (if your engine has a turbo)
  • An infrared thermometer
  • Any other special tools the engine might require
  • A few rags and spray cleaner