Let us say that you are going to look at a used boat and are now ready to check out the engine. This is what I would recommend that you do; Wait until you have a nice sunny day. Do not check out anything that you want to buy at night or in the rain. Remove as many covers as possible and take a general look at the engine compartment. Ask yourself, do the lines look fresh and not cracked? Is it clean? Are the battery terminals corroded? Is there oil, soot, or corrosion on any particular parts of the engine? Take a screwdriver and see if the hose clamps are ready to pop off when they should not. Make sure nothing is laying across the exhaust manifold.
Is there oil in the bilge water? If so, toss in some diapers and sop the oil out before you start the engine. Pull the dipstick and rub the oil on a clean white cloth or piece of porous paper. It will look black, even if it has had just a few hours of running time. If it is clean and clear, it is new and doesn't tell you much. Hold the cloth or paper up to the light. The oil should have a transparent effect. If it is solid or spotted with black specks, it needs to be changed, along with the filters. The old oil can be analyzed with a kit, or by a professional, later.
Look at the cooling water. If it is clear, or better yet green, that's good. If it is brown and rusty, dump it and change it. Use radiator flush (with thermostats removed) the second time you change it. But for now, let's just concentrate on checking this motor out. By now, you should be getting a feel for the type of maintenance this motor has had.
Drain the fuel sumps to check for water. Most diesel systems will have some condensation. That's OK, just drain it off. If the water is all black and cruddy, the fuel tanks may need to be cleaned, later. Diesel comes in a variety of colors, which the distributor can explain. If the motor has been run out of fuel, you will need to re-prime the fuel system. Open the line before the filters and at the return line. Pump or blow fuel through everything until no bubbles come out at the return line.
Make sure to boat is secured to the dock. Start the motor. Watch for sparks and electrical problems in the ignition system. Let the engine warm up. Look for leaks and listen for bad sounds. Look over the side and confirm that the raw water is being pumped through the system and overboard at the proper rate. If not, you will need to change the impeller (of which there should always be a spare). In a few minutes you should be satisfied that you can run her up. Do this at the dock, so as to avoid the distractions of being under weigh.
Put her in gear and give her full throttle. Stand feet flat on the deck and feel for vibrations. If there is a vibration, check the engine and transmission mounts. They OK? Take her out of gear and see if the vibration remains. You may have excessive growth and barnacles on the propeller, worn cutlass bearings, or worse yet, a bent shaft.
Look at the color of the exhaust. It should be clear. If it is black, there is a shortage of air at the intake. Clean the filter or clear the blower. White smoke means there is unburned fuel coming through the system. You may have a bad valve, bad injector nozzel, or low compression in one or more cylinders. Blue smoke is the worst. It indicates that oil is getting through the system. You may have worn valve stems and guides or piston rings, or oil could be getting into the intake system through a leaking gasket.
Now that the engine is at operating temperature, wet a finger and lightly touch each of the exhaust ports. That is the plumbing from the engine to the manifold. Right where the iron part is bolted on to the engine, the temperature should be the same at each port. If one is noticeably cooler than the others, you may have valve out of adjustment or a injector nozzle malfunction. You need a tune up. Detroit diesels are notorious for having loose bolts here. Check for black soot exhaust leaks. Check for proper oil pressure, voltage, amperage and other gage indications before shutting down.
If this is the motor for you, will need to periodically check that the zincs in the raw water system are replenished. The zinc prevents electrolysis, which is to your boat, like termites are to your house. During the maintenance, you may run into a stubborn bolt or two. Heating, tapping and slightly tightening a bolt after a liberal dose of magic spray will usually work. If the bolt is on an expensive component, you may want to remove the entire piece and take it to a machine shop. If you break the head or shaft of the bolt, you may be able to drill, tap and helecoil the threads. You may need to take the part to the machine shop. To prevent that from happening in the future; use never-seize or a like product liberally, especially on aluminum parts with steel bolts.
Take good care of that motor and she will take good care of you.

Detroit diesel
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