Diagnosing engine misfires and elevated hydrocarbon emissions using the waveform trace of an oxygen sensor is possible - once you understand what to look for.

Intermittent misfires can be difficult to detect and even more difficult to diagnose because of their unpredictable nature. The misfire may occur only under transient conditions (during acceleration, for example), which may be hard to duplicate in the shop. What's more, the underlying cause may be anything from an ignition glitch to a leaky EGR valve to vacuum leaks or a fuel control/delivery problem that causes the engine, or a cylinder, to go lean.

Regardless of what factor or combination of factors caused it, a misfire allows unburned fuel to enter the exhaust. When the fuel hits the converter, it temporarily overwhelms it. This causes a momentary spike in hydrocarbon (HC) emissions until the converter starts to oxidize the unburned fuel. At the same time, the unburned oxygen that accompanies the fuel causes a momentary spike in the oxygen content of the exhaust. As it passes by the oxygen sensor, it causes a sudden dip in the sensor's output voltage - which shows up as high frequency oscillations or "hash" on the sensor's output signal. If you're watching the O2 sensor's output voltage on a digital storage oscilloscope (DSO) when the misfire occurs, you'll see the normal up-and-down signal pattern suddenly go berserk, like the trace on a lie detector machine when the accused tell a lie.

This kind of hash in the O2 sensor signal may further compound the misfire problem because the feedback fuel-control system isn't designed to react to such sudden changes in the O2 sensor's output voltage. Hash causes the feedback system to momentarily go out of sync with the O2 sensor, resulting in a fuel mixture that's too lean or too rich for what the engine needs. This, in turn, messes up the normal lean-rich-lean-rich rhythm of the exhaust mixture necessary to keep the converter working at peak efficiency. HC and oxides of nitrogen (NOx) emissions go up and the engine may experience additional performance problems.

Misfire hash is hard to miss, but some hash in the O2 sensor signal is considered normal. As a rule, little oscillations or noise between 300 and 600 millivolts in amplitude is nothing to worry about. Bosch and General Motors' O2 sensors tend to produce more of this kind of hash than Japanese O2 sensors because of their increased sensitivity to minute changes in exhaust oxygen levels.

Moderate hash would be spikes of 150 up to 200 millivolts that shoot downward from the top arc of the waveform as it peaks during the rich phase of the cycle. At idle, moderate is usually considered normal and is most apparent on engines with feedback carburetors and throttle body injection. But moderate hash may indicate a misfire problem if it occurs at higher rpms or is visible on the waveform trace of an engine with multipoint injection.

It helps if you have a library of O2 waveforms for reference so you can compare what you're seeing against known "good" waveforms for similar engine applications (see "Waveform Libraries"). If a "good" O2 sensor trace for a particular engine has a lot of hash, then what you're seeing is probably normal for that vehicle. On the other hand, if you're seeing a lot of hash that isn't normally seen on similar applications, it tells you there's a misfire problem.

Severe hash, which indicates engine misfire, typically has an amplitude greater than 200 millivolts. The spikes may shoot down from the top of the sensor's operating range all the way down to its minimum voltage valve. Any engine may misfire occasionally at idle, during a sudden throttle transition or when held at a steady rpm. But if hash suddenly fills a portion of the trace, it indicates trouble.

So how do you turn the hash into cash? Use it to diagnose and fix HC and NOx emissions problems caused by engine misfire. The hash on the O2 sensor's waveform allows you to identify the operating conditions that cause the engine to misfire (only during acceleration, at idle, at steady cruise or all the time, for example). You can then look at the ignition pattern on your scope to rule out ignition-related problems such as bad plugs, wires, cap, weak spark, etc. A compression check will rule out mechanical causes, such as a leaky or burned exhaust valve, flat cam lobe, etc. That leaves vacuum leaks and fuel delivery problems.

Vacuum leaks that cause misfire can be pinpointed using a propane enrichment tool to check the intake manifold gaskets, throttle body and hose connections. If the misfire hash on the O2 sensor signal goes away when you feed propane to a particular point, you've found the leak.

Identifying lean misfire caused by dirty injectors takes a more experienced eye. Most O2 sensors are sensitive enough to pick up a single misfire in an individual cylinder at low rpm. If the hash appears as one or two spikes in an otherwise normal pattern, the engine may have only one or two bad injectors. On the other hand, if the hash is severe (numerous spikes), all the injectors may need cleaning or replacing.

Waveform Libraries

Digital storage oscilloscopes are great for peering into the inner workings of a computerized engine control system, but to understand what you're looking at, you need to know something about waveforms.

A scope displays sensor voltage signals graphically, plotting voltage (vertically) as a function of time (horizontally). The resulting trace draws a line on the screen that shows you the signal's profile, including its minimum and maximum values and how it changes over time. This, in turn, shows you if the sensor is operating normally and/or if it is detecting an abnormal condition in the engine.

But waveforms for certain sensors (like the O2 sensor) vary from application to application, so to aid your diagnosis it often helps to have a reference library of known "good" waveforms for comparison purposes. You can always build your own library by capturing and storing waveforms from various vehicles. But a faster and easier approach is to make use of existing libraries of sensor waveforms.

One source is the new International Automotive Technicians' Network on the World Wide Web. If you have a computer, modem and Internet access, you can reach IATN at http://www.i-atn.com/auto/index.html

The waveform library in the IATN web site currently contains about 85 different waveforms classified by vehicle make, located inside the Member's Only section, which requires you to fill out an electronic submission form. There's no cost involved and membership is open to any qualified repair professional. The IATN web site also has an open technical discussion forum for sharing your questions and comments about waveforms or other topics with fellow technicians.

Another source of waveform information is MAC Tools "WaveFile AutoPro." This Windows based computer program contains a reference library of over 200 waveforms, each of which is fully documented as to the application (year, make/model, engine, fuel system, VIN, vehicle mileage, engine operating conditions, etc.). Included are all types of engine sensors, plus ignition, fuel, starting and charging system, and even ABS waveforms.

More importantly, the program also contains specific diagnostic and repair procedures for troubleshooting sensors and other systems, including background information about each type of sensor, instructions on how to hook up your scope, how to exercise (test) each type of sensor, and how to read and interpret the results. There's even an interactive tutorial section to help you learn how to use the package.

The WaveFile AutoPro program allows you to upload and download waveforms for reference and comparison, to build your own library, to hook up to on-line computer services and to even fax a diagnostic sensor report to your customer. It also works with most popular scopes including MAC ET2020, Fluke 97 and 99, KALSCOPE and Tektronix THM500 series, Snap-on LS2000, and many others.

Created by Progressive Diagnostics Co., the WaveFile AutoPro program is available through MAC Tool distributors.

If you need further information about waveform analysis, try the book "Understanding Automotive Engine Control Signal Waveforms," produced by Edge Diagnostic Systems, a division of Snap-on Inc. This handy diagnostic guide provides valuable information, much of which is vehicle specific. It also includes sample waveforms and how to read them.

Team ZR-1
True Custom Performance Tuning