Soot: How Much is too Much?

Blackstone offers percent soot testing as an optional test above and beyond what we do in the standard analysis, which is checking for soot with the insolubles and viscosity tests. It’s something that a lot of our diesel customers have shown interest in. It can be challenging for people to understand how much soot is a problem and how much is normal, so in this article we’ll shed some light on our testing process and what it can tell you about the health of your engine.

Here’s a brief run-down of how it works. We use an FTIR (Fourier Transform Infrared) spectrometer to measure the  percentage of soot present in an oil sample. Essentially, soot raises the absorption rate of the infrared light spectrum and when an infrared beam is shot through the sample, the rate of absorption is measured and quantified. The lab operator ensures the machine is running properly by performing a series of check standards, including calibrating against a known 2.0% soot sample to ensure accuracy.

Okay, nap-time is over. So what is soot and what impact can it have on an engine? Soot is a natural by-product of internal combustion. Diesel engines tend to produce more of it than gas engines do simply due to a generally less efficient air/fuel ratio and the manner in which the air and fuel mix to create combustion. Soot is why diesel engine oil turns black, sometimes only after a few miles. When it becomes excessive it can thicken up the viscosity, leave deposits on wearing components, and ultimately clog a filter (or perhaps worse, an oil passage). Soot also tends to stick to itself in a process called agglomeration. Essentially, that happens when the oil can no longer disperse the soot effectively, further compounding the problem. Excess soot can have an abrasive component and has the ability to stick to wearing surfaces, potentially increasing oil consumption.

A used sample from a Volkswagen 2.0L TDI. Soot reads at 0.4%.

We consider soot at 1.0% to be high and anything higher than 2.0% to be cautionary. So what exactly does that tell you? In layman’s terms, excess soot indicates a combustion problem. Pinpointing that problem (or problems) can be a bit more difficult, but there are a couple fairly simple things to check if you think you’re seeing excess, or just more than normal, soot in the oil. Make sure that the fuel system is maintained and properly calibrated so that the injectors are operating at peak efficiency and with a proper air/fuel ratio. Also check for intake leaks and make sure that the air filter is clean and serviceable. Make sure injection timing is set correctly as well. Change the oil and filter regularly to prevent soot build-up.

It’s always possible that excess soot is due to a mechanical problem too, and that’s obviously a bit more involved than just changing the oil or swapping out a dirty air filter. Excessive ring clearance is the most common wear factor that can result in excess soot. Keep an eye on oil consumption as increases in that area can also show a developing ring problem.

Several of our customers have installed by-pass filtration systems in an effort to keep soot lower and the oil cleaner in general, and there’s nothing wrong with that. Soot particles are cylindrical (think bead-like) so good filtration should, in theory, lower the overall amount of soot present. Obviously, employing an oil analysis regimen can be helpful in assessing wear metals and other contamination like soot, but you, as a valued Blackstone customer, already know that.