How To Perform A DIY Diesel Engine Test

Let’s say you have an older diesel engine and want to gauge the state of its health. Before you spend a lot of money on an engine survey, it may be worthwhile to carry out a preliminary survey of your own.

The following non-intrusive set of procedures can be undertaken by just about anybody without specialized tools, and involves no disassembly beyond loosening a hose clamp or two and undoing a few screws. Most times, this survey will give you a pretty good sense of impending and potentially expensive problems. For a more detailed survey, you will need to hire a professional with specialized tools and knowledge, in which case you should seek out someone who has specific experience with the engine in question.


If you are buying a used boat, the first thing to do is look for an hour meter and a maintenance log book, which you hope show regular oil and filter changes and other periodic maintenance undertaken by the previous owner. Without this, there is no way of knowing if the engine has been reasonably cared for.

This diesel sample shows some seriously fouled fuel.

This diesel sample shows some seriously fouled fuel.

The fuel system is the single most expensive set of components on a diesel engine. It is typically off limits to amateur mechanics, especially if this is a high-pressure common-rail diesel, because loosening any of the high-pressure connections can be lethal. For all engines, at a minimum, there should be a primary filter (mounted off the engine) and a secondary filter (mounted on the engine). Maintaining these filters and keeping the fuel clean is the most important aspect of engine maintenance. You want to know that this has been done diligently.

checking the boat's Racors is often a good practice.

checking the boat's Racors is often a good practice.

If possible, I try to draw a sample of fuel from the lowest point in the fuel tank, using a small manual pump and a jelly jar. If the tank is clean, then most likely the fuel system has been seeing clean fuel. All too often, there is a significant amount of water and sediment in the tank, in which case I want to investigate further. As an aside, if you proceed with the purchase of the boat, the tank should be cleaned at the earliest opportunity. Oftentimes, access to the bottom of the tank requires removing an access plate or pulling out a fuel-level sending unit, in which case the suction line may be as close as you can get. However, this is typically set an inch or two off the bottom of the tank and can miss a lot of crud that will get stirred up and enter the fuel system when the boat is out in rough conditions.

I like to check the primary filter for signs of water and dirt and, if present, check the secondary filter. If this is seriously fouled, there is a risk that the contamination has entered the injection system and caused expensive damage. If you check the filters, you will need to know how to prime the system to get the engine running. If this is not your engine, you will also need permission to do this work.


Pull out the dipstick and wipe the oil onto your fingertips. If the oil has not been changed recently, it will be black. Up to 50 hours or more after an oil change, it should still have an element of translucency to it. Very heavily sooted oil may be indicative of blow-by past the piston rings, lowering the engine compression (more on this below). An oil sample should be sent to a laboratory for more serious analysis. At a cost of around $30, it will detect any major contamination and also pinpoint its source.

From there, I remove the oil filler cap, and also any oil filler cap(s) on top of the valve cover, and inspect the underside of the cap(s). Run your finger around the inside of the openings; if there are sludgy black deposits, oil change procedures have almost certainly been seriously neglected. If there is emulsified oil (creamy in color and texture), there is water in the oil. Bad news in both cases.

Sampling the oil from the transmission is also a good test to perform. Transmission oil is not contaminated by any combustion products and as such should remain clean and translucent. Black transmission oil indicates that the clutch has been slipping andfriction surfaces burning, which is relatively expensive to repair.

This sequence of photos shows how to access a fuel tank through the tank level sender and fuel lines access plate to clean it out.

This sequence of photos shows how to access a fuel tank through the tank level sender and fuel lines access plate to clean it out.


I check the raw-water circuit from the inlet seacock through the strainer and pump to the heat exchanger and the discharge point into the exhaust elbow. Depending on how far I want to go with the survey, I may remove the pump cover and check the impeller for missing vanes, cracks, or excessive wear. If this is happening, the impeller tips will be worn flat.

I may also remove any sacrificial anodes from heat exchangers. If anodes are corroded to the point of nonexistence, or nearly so, they have been neglected and expensive corrosion problems are possible. Note that larger engines with multiple heat exchangers may have up to six sacrificial anodes at various points in the raw-water circuit.

Small propeller shaft damaged through over-tightening conventional flax packing.

Small propeller shaft damaged through over-tightening conventional flax packing.

If the engine is below the waterline, check for the existence of a vented loop on the raw-water circuit. If not present (it is surprising how often they are missing), the engine may have flooded with salt water at some point. If possible, remove the exhaust hose from the exhaust elbow and inspect the raw-water injection point. This is frequently corroded. On many engines, these injection elbows need replacing every few years. The other thing that may be found is carbon crust buildup in the exhaust system.

Any form of carbon crust indicates poor operating practices—typically, running the engine at light loads, such as when battery charging at anchor, or running a generator with light loads—which will have fouled the exhaust valves, and probably pistons and piston rings. At best the engine will lose power at high loads because of the obstructed exhaust system, and at worst, there will be a substantial loss of compression requiring an expensive overhaul. Carbon crust in the exhaust is another piece of bad news.

On larger and more expensive engines, it is worthwhile sending a sample of the coolant from the freshwater (engine) side of the cooling system to a laboratory for analysis to make sure that corrosion inhibitors (included in all antifreeze) have been maintained. Typically, the entire system should be drained and the antifreeze replaced every two years.


While working around the cooling and exhaust systems, inspect all hoses and squeeze them to find soft spots or cracking. This indicates whether or not the hoses need replacing.

This hose is well beyond its lifespan.

This hose is well beyond its lifespan.

When undoing the hose clamps, examine the screws for rusting, as well as the bands where they are in contact with the screws. If rust is present, inferior hose clamps have been used, in which case the screws are not 300-series corrosion-resistant stainless steel. You should consider replacing these hose clamps and any others like them, especially those on the exhaust system, with clamps labelled all 300 stainless steel, or, better yet, all 316 stainless steel.

Engine feet should also be checked for severe rusting, signs of oil and diesel spills, and softening of the rubber inserts caused by these spills. During sea trials, the engine should be shifted from slow ahead to reverse, and the boat then backed down at around 50-percent throttle for a few seconds, while observing the engine feet. If the engine moves much, the rubber is shot. Engine feet need to be periodically replaced, and the cost is typically high, because access to them may be difficult, and the engine needs to be realigned after replacement.

Next, check the propeller shaft seal, stuffing box, or other type of shaft seal for signs of leaks, particularly evident by rusting components—engine control brackets and cables, for example—that may have been subjected to salt water thrown out by the propeller shaft. Also, make sure there is adequate access to maintain the shaft seal, as some require the engine to be removed, making routine seal maintenance expensive.

I like to test cable-type engine controls to make sure they are smooth and free from undue resistance and that the gear shift clicks into neutral at the appropriate point in the travel. Stiff or sticking cables may just need lubricating, but probably need replacing.


Note that I have not yet started the engine. Turn it on and check that the alarm sounds. If it is not working, who knows what harmful conditions may have been overlooked.

The engine should be cranked from a cold start–the colder the better–first by activating any cold start devices it may have. A diesel engine has no ignition system. It relies solely on the compression of air to generate enough heat to set fire to injected diesel fuel. The colder the engine and the incoming air, the more compression is needed to get the engine to fire. If a cold engine fires right up, it is almost certainly in pretty good internal mechanical condition. If, on the other hand, it is being fed clean fuel and is reluctant to fire, it may well have a compression problem that is going to be expensive to fix. A professional engine surveyor will run a compression test and be able to confirm this for you.

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The warmer an engine, the easier it is to start even with substandard compression. If you are thinking of buying a boat and the engine has already been warmed up before you get there, you would be well advised to come back another day and to insist on the engine being cold when you test it.

If I suspect a compression problem, with the engine shut down I put a socket wrench on the crankshaft pulley nut and turn over the engine in its normal direction of rotation. As each cylinder comes to compression, it should take a considerable effort to turn the engine through the compression cycle. If this effort is not required,especially if I can hear air blow-by hissing down the sides of the pistons, I know the engine has a serious compression problem.


Next up is to identify the crankcase ventilation hose/breather. It will come off the valve cover at the top of the engine and will run to the air inlet, maybe via an oil-separating canister. Disconnect this hose at the valve cover, take the boat to sea, and throttle up. This applies a significant load to the engine, at which point blow-by will be accentuated.

A healthy engine will register small pressure changes at the crankcase breather, with maybe a gentle outflow of gases, whereas an engine with blow-by will create a substantial breeze. Check the exhaust color when the engine first fires up and during sea trials. It will be one of the following: clear, blue, black, or white (the colors are quite distinct). It should be clear, although there may be an initial thin blue haze until the engine warms up. Continuing blue smoke is generated by burning oil (either coming up the sides of the pistons or down valve guides, or both).

Corroded packing gland.

Corroded packing gland.

Black smoke is from improperly burned fuel, which may be a fuel injection problem, and is likely to be relatively expensive to fix, or, on older engines, the result of an obstructed air inlet, partially plugged exhaust, engine overloading, or sudden acceleration. White smoke may be caused by water vapor in the exhaust—which, if the engine does not overheat at higher loads, is not a problem—or unburned diesel. The unburned diesel scenario is a significant problem, and a simple sniff test will often tell you all you need to know. For this, put your hand briefly over the exhaust and sniff it (your hand, not the exhaust). If the vapor is unburned diesel you will immediately smell it.

During sea trials I have no qualms about running an engine at wide-open throttle for ten minutes or so. It should achieve its rated maximum rpm to within +/- 5 percent and do so without overheating. If it does overheat, this is not necessarily a major concern, especially if the sea trials are being conducted in warm waters and I have not already checked the raw-water system. There may simply be damage to the pump and or partial obstructions in the raw water cooling system, none of which is likely to be particularly expensive to fix.


While at or near full load, an infrared temperature meter can be used to check engine temperatures. The most telling point is at the same spot on the exhaust side of each cylinder. Any substantial divergence from other cylinders indicates that a cylinder is not firing properly (temperature too low) or working too hard (temperature above the other cylinders). You will not be able to do anything about this, and if the engine is otherwise running fine, you may choose to ignore it. On the other hand you may want to consult a professional, which is never a poor choice and can save money in the long run.

Another set of useful temperature readings is at the freshwater inlet and outlet to the heat exchanger. If there is not a significant difference (a minimum of 10˚F once an engine has warmed up and is under load) the heat exchanger is either seeing a lack of flow, or else is scaled up. Most likely the engine will overheat if run at wide-open throttle for any length of time.

The temperature meter can also be used on a traditional stuffing box. If this is running hot (more than 20˚F above the ambient temperature) it has probably been overtightened (to get rid of a drip, for example) with the risk of damaging the propeller shaft, which is likely to be a fairly expensive repair.


The above tests will give you a pretty good idea of whether or not an engine has been properly maintained, and whether or not it is in a decent state of health. Assuming the engine passes these tests, if it is a large, expensive engine in a boat that you are now fairly committed to buying, you should first consider paying for a professional survey just to make sure there are no hidden, expensive problems in the making.

Want to learn more about Maintaining and Troubleshooting Diesel Engines? Check out the Marine Diesel Maintenance & Troubleshooting Course over at Boaters University, our online education platform.