There’s nothing more alarming than realizing your four-stroke powered pride and joy is burning and consuming abnormal amounts of oil. Whether it’s a first-hand account or lending a sympathetic ear to a buddy dealing with the problem, we’ve all been there to some degree. I recently had a customer detail his machine’s oil consumption problems and logically explained away all the possible reasons why the engine could be burning oil. He had always changed his filter routinely and oiled it properly, he changed his oil every 10hrs, and he always kept up with his maintenance. So what the heck causes our four-stroke engines to consume oil and how do we figure out what we’re dealing with on a case by case basis?
Points of entry
The power cylinder of a four-stroke engine isn’t all that complicated. We have intake and exhaust valves in the cylinder head, and we have a piston assembly that features compression rings and an oil control ring. Knowing this oil can only migrate into the combustion chamber one of two ways, either past the valves or past the oil control ring.
Causation is another story, and the list of possibilities is a bit longer and depends on the unique circumstances of the situation. Contributing factors can range from the person who performed work on the engine, to the environment (dusty conditions and type of dust), to the general design of the engine. Here’s a list of some common causes which ultimately lead to engines that inevitably consume oil:
Improper air filter maintenance - Your engine’s air filter is the primary device that separates out the dirt from the air and ensures your engine consumes highly filtered air. Filters that are improperly oiled or maintained will compromise your engine, especially if you are operating in dirty, dusty environments. Even the tiniest of dirt particles that breach your filter can have quick and adverse effects on the durability of your engine. Dirt ingestion will lead to accelerated wear of piston rings and valves, ultimately driving an increase in oil consumption. The oil control rings and cylinder bore become compromised when the dirt is ingested into the engine.
- Poor air filter seal - An air filter that does not fully seal to the airbox provides an unfiltered leak path for dirt saturated air to make its way into the engine leading to the previously mentioned durability problems.
- A leaking or poor sealing intake system - From filter to the engine’s intake manifold, all the joints and connections must be airtight. An improperly seated throttle body, for example, will allow dirt ingestion to occur.
Bad maintenance habits - Any time you open up an engine, you create a rich opportunity for self-inflicted dirt ingestion. For example, swapping out a dirty air filter for a clean one without first cleaning the entirety of the airbox is a common way engines become saturated with dirt. Our machines are tightly packaged, and it’s incredibly easy to rub your hand against a dirty component, causing dirt to fall into the intake tract or engine itself if performing more extensive maintenance.
- Improper assembly of engine components - For a select few, installing parts such as oil control rings incorrectly can be the reason the engine consumes oil. Similarly, valve stem seals can be installed wrong or more frequently be damaged during a hasty installation. On some engines, where the engine and transmission oil are separate, directional crank seals can be installed backward, leading to migration issues from one cavity to another.
Infrequent oil changes - An engine’s oil has a threshold for the amount of dirt and other contaminants it can suspend before accelerated wear occurs. Operating an engine for too long without changing its oil significantly increases the likelihood of the oil becoming an abrasive slurry causing its internals to wear out.
- Unrealistic expectations - High-performance four-stroke engines have a finite life to them before they require a rebuild. The amount of life any engine has depends on numerous factors, including how it is maintained, what it is used for, and the environment it’s operated in, to name a few. If you have a high time engine that is starting to consume oil, consider being grateful for the trouble-free hours you’ve gotten out of it and take on the rebuild to return it to its trouble-free days.
- Bad rebuilds - Uneducated builders can cause their own problems by attempting to cut corners during their overhauls. A common way to do this is by installing new piston rings to a worn-out cylinder bore. A worn cylinder bore can make proper ring sealing impossible due to both inadequate cross-hatch and out of tolerances in bore taper and roundness.
There are a handful of ways to try and pinpoint where problems are originating. No method is entirely foolproof, and a bit of judgment is always required when making determinations. Here are a handful of diagnostic options you have at your disposal:
Leak down testing - Leak down testing permits a quantifiable way to determine how well the cylinder is sealing and, if it is not, where leaks are originating. When diagnosing oil consumption problems leak down testing can overlook a faulty oil control ring if the compression rings are still sealing well.
- Compression testing - Performing a compression test is one of my least favorite ways of diagnosing four-stroke problems, but I am mentioning it because it can be used in a pinch. The main problem with compression tests stems from the incorporation of decompression systems, which significantly reduce peak pressure readings.
- Oil sampling - having your engine oil sampled by a lab can provide a great deal of information in regards to what is going on in your engine. For example, an oil sample containing high amounts of silicon is indicative of dirt ingestion. An excess amount of other elements can also suggest ring or bearing wear.
- Bore Scope - Checking the cylinder bore’s condition by using a borescope can be an excellent way to evaluate the health of the bore. Visible scuffing or deep scratches can be paths oil can sneak past the rings and into the combustion chamber.
I hope you’ve enjoyed this write-up on contributing factors leading to oil consumption and how to pinpoint them. While the problem is no doubt frustrating, there is no reason it has to be a reoccurring one, nor should you have to stew for days trying to determine the root issue. If you’re interested in more engine building knowledge, check out my Engine Building Handbooks, which arm you with a comprehensive knowledge base for overhauling your two and four-stroke engines. The Four Stroke Engine Building Handbook contains over 300 pages of highly detailed practical knowledge and over 250 high-quality color pictures. The books’ consistent five-star reviews on Amazon are a testament to how beneficial they are to our customers. To learn more, you can check them out on our website or on Amazon.
Paul Olesen is an powertrain engineer, author & long-time rider living in Wisconsin.