On Board Diagnostics

The workings of OBD-II and its effect on modifications
by Ray T. Bohacz
Reprinted from and with the permission of High-Tech Performance Magazine


The year is 1981, and things are looking pretty dismal for the auto enthusiast and the performance industry in general. Motor Trend tested a new Mustang Cobra with Ford's latest rendition of its small-block, a 255-cubic-inch weakling, that in the same issue was beaten by a Honda Civic in all acceleration tests. A low time indeed. That year GM pioneered an emissions control system called Computer Command Control that is placed on all of its cars. In all actuality, that was OBD-I.

Within a few years, all the other manufacturers had employed some sort of electronic management on their engines, and the death knell was sounded for the performance enthusiast. Or so we thought. The term "computer cars" was coined, and in every hot rodder's garage, bench racing sessions took place, dreaming of the old days of Chevelle 396's, Tri-Powered GTO's and Hemi-powered Mopars. Little did we know that in less than five years the fuel-injected Mustang, TPI Chevys and Pontiacs, and blown Buicks would be born. It was the renaissance of performance.

No longer did horsepower mean hard starting and poor driveablity. The computer car became our friend and a whole new industry was born - electronic tuning. With ever-stricter emissions and fuel economy standards and the need to standardize diagnostics so state inspection stations could have access to fault codes, the performance industry was brought to a standstill once again. But that wasn't for long, thanks to numerous computer-hip silicon heads with an undying love of the performance car who are successfully seeking ways to circumvent the actions of big brother.


The challenges brought forth by the EPA for longer time frames between the deterioration of emission output has caused Detroit to totally rethink its approach to engine management. Prior to 1994, a vehicle manufacturer had to certify that is product would remain emissions compliant for at least 50,000 miles. From that date forward, the window was expanded to 100,000 miles along with stricter standards. This forced attention to be paid to things that were ignored before. Emissions controls were always a sort of a Band-Aid, cleaning up what we couldn't burn in the combustion chamber. Thirty-year-old cylinder head designs with slow-burning chambers and enough crevice volume to hide the Grand Canyon were redesigned for efficiency.
In essence, OBD-II is not a rethinking of how an engine runs, but a very sophisticated data acquisition system that is programmed to notify the driver of a system failure that would cause a rise in emissions output. For most of us, this is a very perplexing scenario. With my love of engineering, I marvel at the technology that's incorporated into today's cars, but I also resent it because it doesn't allow me to modify an engine to meet my particular needs.


OBD-II has more ability than ever before to know what is going on inside an engine. The main reason is the increased emission output when something goes astray, but more important to protect the catalytic converter from being damaged by excessive hydrocarbon or carbon monoxide. Due to this, new areas are monitored, including catalytic converter conversion efficiency and crankshaft speed to determine engine misfires. If you read the article "Emissions Formation" (Jan. '97), you'll recall that any time an engine misfires, hydrocarbon emissions increase drastically. Continually dousing the converter with high hydrocarbons will cause it to either overheat or melt down, which is a problem in the eyes of the EPA.

As can be seen in Fig. 1, most sensors on the car remain the same, regardless of whether the car is equipped with OBD-I or OBD-II. there's no visual way to recognize an OBD-II-equipped car without close inspection of the engine. The complexity of the terms and their length lends itself to using acronyms and abbreviations, Fig. 2 below will make those terms clearer. Since OBD-II is a strategy more than anything else, we will concentrate on those areas that will impact us the most.


To understand OBD-II logic, we will first start with a few basic terms.
a test that is run on a system or component to determine it is operating according to specifications. Main areas of concern include misfire, oxygen sensors, oxygen sensor heaters, EGR and catalytic converter efficiency.
Enable criteria:
an engineering term for the conditions necessary for a given diagnostic test to run. Each test has a certain number of conditions that need to be met before it is executed. Some common enable criteria are : engine speed, vehicle speed, ECT, MAF/MAP, barometric pressure, IAT, TPP, canister purge, fuel trim, TCC enable and A/C on-signal.
a key-on, key-off cycle allowing the vehicle to operate in a manner that satisfies the enable criteria to run a given diagnostic.
Diagnostic executive:
a set of coded instructions designed to process and control other coded instructions.
Passive vs. Active tests:
a passive test is a diagnostic test that monitors a system or component. An active test actually executes an action when performing a diagnostic function.
Warm-up cycle:
the engine temperature must reach a minimum of 70 degrees C (160 degrees F) and rise at least 22 degrees C (40 degrees F) over the course of the trip.
Intrusive diagnostics:
any on-board test run by the diagnostic management system that will have an effect on emissions.
Freeze frame:
stores vehicle information at the moment an emissions-related fault is stored and the MIL is commanded to illuminate.
Inspection/maintenance-ready status:
a signal ECU flag for each emissions system test that has been set in the ECU.
OBD-II drive cycle:
its purpose is to run all of the on-board diagnostics so that the I/M ready flags will set.
the main area of concern when trying to modify an OBD-II car is in choosing modifications that will not flag areas during the drive cycle test. The main concerns are coolant temperature, idle speed, idle variation that would be flagged as a misfire, incoming air volume and catalytic converter efficiency.

If you can pass the driving cycle test, you should be home free. Since modifications like camshafts, rocker arms, cooler thermostats, and superchargers all affect these areas, they are of great concern.
Heated oxygen monitoring (HO2s) (Fig. 3):
consists of three parts: response time, time to activate, and sensor voltage checks.

Response time (Fig. 4):
By studying the cart below, you'll see that the amount of time needed to make the transition from rich to lean cannot surpass 100ms on the pre-catalytic converter O2 sensor.

Time to activate:
the time required for the HO2s to become active during a cold start. This time frame is compared to a calibrated malfunction threshold to judge the capability of the sensor heater.
Sensor voltage checks (Fig. 5):
applied to both pre- and post-catalytic converter HO2s. The ECU looks for a pre-cat sensor to be very active, while a post-cat sensor's output is flat, as seen in the chart (Fig. 5). A good catalytic converter will have a hydrocarbon conversion efficiency of approximately 95%. A degraded converter has a conversion efficiency of 65% or less.

Fuel trim system monitoring (Fig. 6):
the theory for fuel trim monitoring is to look at the average short- or long-term correction needed to bring the air/fuel ratio into line. If these fuel trim values reach and stay at their limits for a period of time, a malfunction is indicated.

EGR system monitoring (Fig. 7):
This uses a change in MAP to determine how effective the EGR system is. The EGR valve will be forced open during closed throttle deceleration and a change in MAP corresponding to the chart below will be required.

Idle Air Control System (Fig. 8):
by comparing actual idle speed to commanded idle speed, the IAC will determine if a diagnostic follow-up test is required.

Misfire Monitor (Fig. 9):
regulations require misfire monitoring under cruise, acceleration and idle conditions. The diagnostics must determine whiter there is single or multiple cylinder misfire, then identify the offenders. One of the major obstacles to program and identify misfire software is the effect of rough roads on misfire detection.

A rough road will cause the torque applied to the drive wheels to vary, and this can easily be misinterpreted as a misfire. Complicated algorithms are applied to differentiate between an actual misfire and a rough road. The companion charts show how crankshaft acceleration is affected by road surfaces (see Fig. 10).


Now that you have a basic understanding of the complexity of OBD-II, let's see how common modifications can evoke an MIL and put the vehicle into a limited operating strategy.

Higher fuel pressure or larger injectors:
A potential problem with a fuel trim diagnostic failure exists when changes are make to the flow rate of the injectors or the fuel pressure. The criteria that are needed or, in other words, the amount of correction that is allowed, will determine the success of this modification. In all fairness, on a totally stock vehicle there's no reason to change either one of the above-mentioned areas. A highly modified engine would probably evoke trouble codes in other areas first.
Cat-back Exhausts:
There should be no problem with cat-back exhaust systems since their improvement to airflow is not monitored. There may be a possible problem area in EGR function if mufflers are not used.
Increased Rocker Arm Geometry:
There is no interference with OBD-II functioning by increasing lift with rocker arms of a different ratio. Even though increased lift through rocker arm geometry has a slight effect on duration, its more dominant are is in valve moment.
This is major area of concern with possible problems all over the map. Valve event timing will have a drastic effect on hydrocarbon generation, which will affect both HO2s time to activate and response time. It may also have an effect on converter efficiency due to the increased hydrocarbon load placed on the converter. Another area of concern is in idle stability and misfire detection. The roughness that we all like in a cammed engine most likely will be interpreted as a misfire, which will be confirmed by the lack of converter conversion efficiency. Camshafts with slightly increased durations and lobe separations angles of at least 112 degrees will most likely be tolerated.
Cooler Thermostats:
Without letting the engine reach normal operating coolant temperatures, the drive cycle will not be completed.
Cylinder Heads:
It appears that increased volumetric efficiency through better-flowing heads and a slight raising of the fuel pressure to keep the fuel trim in check should go totally undetected.
Emissions-legal headers will have no effect on OBD-II.
In theory, since WOT is not monitored, the only possible problem arises with fuel trim under closed loop boost and idle stability with the air being forced into the throttle body. Even though superchargers do not affect idle quality, there will be fewer counts of the IAC to achieve the same idle. This should not pose a problem. The increased volume of air passing through the MAF will most likely be detected and recorded. Since it will only be for a short period of time, the system should respond like Ford's EEC-IV by seeing an uncalculated amount of air and illuminate an MIL.
It looks as if nitrous is the safest be for adding performance on OBD-II vehicles. This is almost a contradiction in itself; since nitrous is only operated at WOT, the ECU will not care (see Fig. 11).


The whole problem lies not with OBD-II strategy, but with the aftermarket's inability to access the parameters and their hysterics to compensate for minor modifications. The Specialty Equipment Manufacturers Association is petitioning the EPA to have the Big Three automakers make available the information needed to change the above-mentioned areas. As expected, there is reluctance from the car makers to divulge this information when the EPA is requiring them to stand behind a vehicle for 100,000 miles. If SEMA wins, the EPA will probably require extensive testing to be done by the aftermarket to prove that their changes do not have a long-term effect on emissions. If they don't require this, Detroit will cry foul, and rightly so.

The real question is, if the aftermarket obtains access to OBD-II, will they be willing to invest the funds needed to certify their parts? If that does happen, it will probably be in the form of a package of modifications to keep the cost of compliance down. On the other hand, Detroit is producing better, more powerful and faster cars than ever before. The new LS1 small-block, Ford's 4-cam Cobra and Triton family of truck engines as well as Chrysler's Viper GTS with an extra 50 horsepower are a few good examples.

To test the waters of OBD-II tolerance, we have just completed the buildup of a 1996 Impala SS that's been modified to the hilt. We built a long-rod 383 and bolted on an ATI intercooled supercharger. So far, the results have been very promising. Watch for future issues, when we will report our findings on the effectiveness that these major modifications have had on OBD-II. Even with these added restraints, the future is brighter than you think.

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Jon N. Steiger / stei0302@cs.fredonia.edu / SUNY College at Fredonia