The owner of a twin-engine trawler was concerned because one of his engines consistently had 10 pounds less oil pressure than the other. The engines were identical and had about the same hours. Why would there be a difference, and what was the correct pressure?
Let’s see if we can shed some light on the engines’ circulatory system. (And hopefully, it won’t just be the glow from the oil pressure warning light.)
Pump it up
The process of efficiently converting fuel into mechanical energy requires precisely machined parts in close proximity, lubricated with a thin film of oil. This oil slows wear and helps cool hot parts.
A geared oil pump gets that oil flowing from the sump throughout the engine the same way our hearts move blood throughout our bodies. As the oil moves through the parts and passages in the engine, pressure builds (measured in pounds per square inch, or psi, in the United States).
If the pressure is too low, those moving parts create friction that can heat them up, swelling them to the point that no oil can pass through. This condition can cause moving parts to fail or the engine to seize. If the pressure is too high, there is likely a clog somewhere that may be starving a critical component of lubricant; or, perhaps, too thick an oil has been used. Oil pressure that is too high can reduce horsepower, blow out oil filters and seals, and damage bearings.
What might cause low oil pressure? On most marine engines, oil is collected at the bottom of the engine in a pan called the sump. A gear-driven oil pump draws oil up from the sump through a small tube. This tube usually has a mesh screen around the bottom that collects larger debris, typically anything larger than 1/32 of an inch. It is possible that especially dirty oil could clog this screen.
The oil pump consists of two specially shaped gears that create suction on the input side and pressure on the output side. Your oil filter cleans the oil coming out of the pump. Oil coming into the pump has only the pickup tube screen to clean it. As a result, the oil pump encounters more gunk and subsequent wear than the rest of the engine. A worn, clogged or leaking pump will reduce oil pressure.
After leaving the pump, the oil passes into the oil filter. The filter collects particles and contaminants and keeps them from traveling with the oil throughout the engine. Engineers plan for some degree of restriction in the oil filter, but also include an overpressure valve in the filter in case it becomes too dirty; it’s better to allow dirty oil into the engine rather than not enough oil. If that valve gets stuck fully or partially open, it can reduce oil pressure.
Some manufacturers specify that their filters have an additional screen installed around the overpressure valve to filter the bypassed oil, but not all filters are built this way. The lack of a screen means that a clogged filter may allow completely unfiltered oil to travel throughout the engine. Since the primary reason for the filter is to collect bits of metal and debris from reaching the precisely machined pistons and bearings, this is not a good scenario. It is easily preventable by regularly changing your oil and filter, by the book.
Check your dipstick
When checking the dipstick, there are three possibilities: the level is between the marks, the level is too low or the level is too high.
What if the oil level is particularly low? If the oil pump starts to suck air, the oil can fill with bubbles, causing it to foam. Foaming can deteriorate the oil’s lubricity through oxidation. The air will also lower oil pressure further. There will be less oil to carry heat to the sump, further enhancing the cycle of failure. Some engines are known to have pickup tubes that are angled to one side, and certain maneuvers or rough weather can exacerbate a slightly low oil situation. If you notice your oil pressure dropping during a high-speed turn or during a particularly rough day, this could be the culprit.
From the oil filter, the oil travels to a gallery inside the engine and is distributed to the valves, camshaft, cylinders, pushrods or timing chain, and main crankshaft bearings. The main crankshaft has oil passages inside the journals where the bearings and piston connecting rods are attached. Turbos, if fitted, also spin on a very thin film of oil. Your engine has numerous narrow passages and tubes that direct the oil flow into tightly fitted, machined parts that need lubrication. The flow of the oil through all these parts creates resistance and builds the oil pressure.
An oil pressure switch and (usually) an oil pressure sender installed in the oil path monitor this pressure. The switch will turn on a light or buzzer, while a sender will send pressure information to a gauge or computer. When an engine is new, all the parts fit as intended. With wear, however, bearings, pistons and sealing surfaces can create gaps that allow the oil to move much more freely. This lack of resistance lowers the pressure. A trained ear can hear especially worn parts slapping and clunking.
As the oil moves throughout the engine, not only is it lubricating, but it is also picking up heat and moving it back down into the sump, where it can cool before being picked back up again for another circuit. Some engines include a seawater-based oil cooler to facilitate the heat removal. If the cooler were to develop a leak while running, the oil pressure would be higher than the water pressure, and oil could be pushed out into the seawater. If you consistently have low oil but cannot find any leaks, check the oil cooler for leaks.
Conversely, once the engine is shut off and the oil pressure drops, seawater could be drawn back into the engine oil. If the oil level is high and seems to be rising, it might be caused by seawater leaking into the oil because of a cooler leak.
What’s in your oil?
The viscosity of the oil plays an important part in determining the oil pressure within the engine.
Oil gets thicker when it is cold and thinner when it is hot. Lower numbered weights are thinner, and higher numbers are thicker. In the old days of motoring, when monograde engine oils like 30W were the only choice, engine manufacturers would recommend different summer- and winter-weight oils, requiring owners to change oil based on expected outside temperatures. If the oil is too thick, the engine might not even start, let alone be able to pump the oil through all the necessary passages. If the oil is too thin, it can break down and fail to provide necessary lubrication.
As oil engineers gained understanding about what is going on inside the engine and where the lubrication is most needed, they have sought to solve problems of lost viscosity due to high-temperature shear, oil oxidation and aeration, filter blocking, soot-related viscosity increases, low-temperature viscosity increases, and all sorts of sludge and varnish-reducing issues. Solutions have been made through additives to oil.
Multigrade oils were developed to work well through a much wider temperature range and to prevent getting caught by a cold snap with the wrong oil in the engine. The number 5, 10, 15 and so forth, followed by the letter W, indicates the oil’s winter grade. The second number indicates the summer weight. A popular diesel grade is 15W-40, although that is changing as new formulations are brought online.
The American Petroleum Institute also has a service rating for engine oils. Every oil container has an API donut-shaped label that describes the service rating, a practice that began in the 1920s with the designation SA for gasoline engines. Oil rated SA met the car engine requirements of that era. As advancements have been made, the designation has changed numerous times. Current gasoline-rated oils are designated as SP. (For diesels, the rating started with CA and progressed to CK-4. The C stands for compression, the S for spark.)
Be sure that the rating on each bottle matches your manufacturer’s recommendation. Ratings are backward compatible, but not forward. This means you don’t want to put an old, dusty bottle of CB-rated oil in your new 2021 engine.
One of the most likely causes of low oil pressure—short of massively high running hours—is what we call an event. That’s when something goes seriously wrong and causes premature internal wear.
Perhaps a careless mechanic changed the oil and didn’t realize the O-ring for the old filter was still stuck to the block when the new filter (with its new O-ring intact) was installed. The double O-rings worked until the engine got hot, and then blew out, allowing all the oil to leave the engine into the catchment pan. If no one noticed the low-pressure alarm, the engine could stop by seizing.
Occasionally, after an event and when the engine has cooled, it can be freed. The oil filter can be fixed and the oil replaced, and the engine started. That engine may run for many years after an event, but there may be serious damage to bearings and other moving parts.
As the next owner of a boat, you won’t necessarily know if there have been any events. That is why it is so important to have a mechanic who can access stored data on computer-controlled engines for prepurchase surveys.
Short of engine oil loss, overheating is another potential cause of damage. Blocked seawater intakes and strainers, failing impellers or poorly maintained coolant systems can all cause the engine to run hot, straining the oil’s ability to lubricate the metal surfaces moving against one another. The event might be an explanation for why one engine has lower oil pressure than the other.
Other potential events might not be catastrophic failures, but rather maintenance failures such as chronic low oil, cheap filters, the wrong oil, or oil and filters that rarely get changed. Since some combustion by-products and moisture end up in the engine oil and tend to be acidic, there can be damage to the metals inside the engine. While it would be unusual for someone to maintain one engine better than the one next to it, it is conceivable that one engine might mistakenly get the wrong oil or filter.
Alarms and gauges
Best practice calls for checking the alarms before heading out. Most engines have this check built in: When you turn the key but before you press start, the alarms should sound and/or illuminate. A malfunctioning alarm system can allow a problem with the engine to get out of hand before damage is done.
Most modern engines will display flashing hazard alarms if critical parameters are out of range. Simpler engines may rely on a warning light, a buzzer and an attentive captain.
Gauges add to the information gathered as long as they are monitored, and as long as the acceptable parameters are known. Gauges can also be notoriously inaccurate. One explanation for differing oil pressure values on twin engines can be that the oil pressure sender, wiring or gauge on one or both engines is simply wrong. Since electric gauges work in part on resistance, a particularly long wiring run or cruddy connections can throw off a gauge. Good mechanics have mechanical pressure gauges that can be used to confirm whether the boat’s gauges are accurate.
On our trawler, we confirmed oil pressures on both engines with a shop mechanical oil pressure gauge and then did troubleshooting on the oil pressure sender, wiring and gauge on one engine. They all needed replacement.
The good news was that the engines themselves still showed oil pressures within specifications. There is no generic proper oil pressure; the specific answer depends on your engine. But assuming that you have the recommended oil and filter, most diesels will settle out to somewhere around 20 psi at idle warm and will run under load between 30 psi and 65 psi. Pressures exceeding 80 psi are too high and may indicate a blockage and potentially oil-starved areas of your engine. Less than 20 psi when the engine is running requires further investigation. Oil pressure warning lights can come on with as little as 7 psi.
Most engines’ shop manuals will list these specifications tailored to their engines. Just like your own blood pressure, it is wise to be aware of your engine’s oil pressure parameters and take steps to lead a healthy life.