Other Articles - September - 2018

What’s Causing That Shudder? A Deep Look at a Misfire Issue

As today’s vehicles continue to become more complex, the problems we face also tend to increase in complexity. Several years ago I wrote an article about the “new direct-injected gasoline engines” that were just being introduced. Since then, most direct injected engines have added turbochargers, superchargers, variable cam timing, and now variable lift camshaft technology.

Volumetric efficiency is the target and the manufacturers have done a good job getting more efficiency from these smaller engines. They’re getting more horsepower and torque from smaller and smaller engine displacements.

Another area that’s seen a lot of changes in the past few years is engine oil. Oil companies are under pressure to eliminate old-time chemical packages from their oil to make it more environmentally friendly and more compatible with the emission control systems on today’s vehicles. Additives such as zinc, phosphorous, and ash are gone from today’s oils, for the most part.

You can go into most dealerships today and you’ll find a lot of engine failures, such as damaged camshafts, bent valves, bent rods, burned pistons, and cracked heads and blocks, just to name a few. While there are many reasons for these issues, such as lack of maintenance, some issues may be due to an entirely different situation.

One phenomenon we’re starting to see is a miss that develops at mileages as low as 40k. By the time the miss starts to develop, internal engine damage has likely occurred. The problem is generally associated with turbocharged/supercharged direct-injected engines, although it can occur on other applications.

The issue is known as LSPI, or low speed pre-ignition. A lot of terms are used to describe LSPI, including super knock, mega knock and stochastic preignition, but no matter what you call it, it’s bad news for your engine… and your wallet. Assuming that your misfire is something simple, before you can thoroughly check out the issue, it may result in an upset customer.

We normally think of pre-ignition and detonation issues as being related to the quality of fuel, carbon accumulations in the combustion chamber, and spark timing. In this instance, there’s more than meets the eye.

Pre-ignition occurs when the combustion process starts before it should; in other words, prior to the spark plug firing. This leads to excessive cylinder pressure and possible engine damage.

LSPI typically occurs during low-speed city driving with engine speeds between 1200 and 2700 RPM. Moderate-to-heavy acceleration, such as when you’re accelerating from a stoplight or in traffic, may lead to LSPI.

A tremendous amount of study has gone into this issue as the number of engine failures continued to pile up. As you well know, engine spark-related knock has been all but eliminated with the introduction of knock sensors. This issue is different as it isn’t related to detonation but rather pre-ignition, so retarding the timing won’t correct it.

Engineers believe the issue is partly due to current engine designs, but it’s also due to today’s engine oils. During operation, when the connecting rod/piston changes direction, a few particles of engine oil are thrown into the combustion chamber. A direct injected engine injects fuel at around 2000 PSI as a highly atomized vapor.

Engineers believe that the fuel and oil mix, leading to tiny, highly combustible droplets. Cylinder pressure increases dramatically during acceleration, as the turbo/supercharger increases intake pressure. The combination of the droplets and additional intake pressure leads to pre-ignition.

During normal combustion, cylinder pressure is typically in the 600 PSI range. During LSPI events, cylinder pressures reach nearly 1600 PSI. And, if those high cylinder pressures continue, piston, valve, connecting rod, and gasket failures will likely occur. Misfire, engine noise, and finally, engine failure, are likely to occur.

In most instances, if you check compression on an engine damaged by LSPI, it isn’t uncommon to see 0-30 PSI on the suspect cylinder (figure 1).

The belief is that calcium-based oil detergents increase the oil droplet combustion tendency. Oil manufacturers are developing hybrid detergent packages to address the issue. The new detergent lowers the calcium and typically replaces it with a magnesium-based or other compound.

Some oil companies and auto manufacturers recognized the issue early on, and developed an LSPI standard for their oil. An example is the Dexos oils developed by GM.

A new standard has been introduced to address this issue across the board. In 2017, ILSAC (International Lubricants Standardization and Approval Committee), and a group of automotive original equipment manufacturers (OEMs), petitioned the API (American Petroleum Institute). A supplement to the GF-5 API SN standard was developed and released for production, effective May 2018.

Oils are currently labeled GF-5 SN, but the new labeling will be GF-5 SN PLUS (figure 2), which is how you identify the updated products. New oils like 0w-8, 0w-12, 0w-16, and 5W-16 are designed to meet the SN PLUS standard. The vast majority of synthetics, such as Mobil 1, Pennzoil, Quaker State, and AMSOIL, already meet the standard.

As we look back, your job has gotten more and more complex. You need to understand that a lot of transmission-related issues aren’t actually in the transmission. Driveability, body, and chassis control systems should be part of your transmission diagnosis strategy, and now we see that even maintenance items, such as engine oil, can have an impact on your ability to diagnose vehicle issues properly. S

o what does the future hold? A new generation of engines is slated for release by most manufacturers in the next couple years. A new engine oil standard is also in the works for these new engines. GF-6 will replace the GF-5 oil, hopefully in 2019 or 2020.

So the next time someone tells you that all oils are the same, you’ll be able to share a little valuable information with them. Your customers are depending on you to keep their vehicle in good working order and, in our industry, knowledge is key to meeting that need.

Until next time, remember, “When all is said and done, usually more is said than done.”