Article > Tech

The Brains Behind the Wheel

By Larry Carley

This is not your fathers Oldsmobile. These days cars are smart, equipped with computers that monitor everything from timing to traction.

Engine management systems do exactly what the name implies they manage all aspects of engine performance including emissions. The powertrain control module (PCM) is the brains of the system. It controls spark timing, fuel delivery, throttle position on some newer engines that have no direct connection between the gas pedal and throttle (such as BMWs "throttle-by-wire" system), and anything else that affects emissions such as the exhaust gas recirculation (EGR) system. The PCM may also interact with the transmission, climate control system, electronic suspension, anti-lock brake/traction control system and anti-theft system, too. It has a lot of things to keep track of and a lot of jobs to do, so it must function flawlessly every time the vehicle is driven.

Since 1996, engine management systems have special software to monitor emissions compliance. It is called "Onboard Diagnostics II" (OBD II), and youll find it on some 1994 and 1995 models too.

The OBD II part of the engine management system runs various self-checks or "monitors" to keep an eye on virtually everything that affects emissions. This includes the operation of the engines sensors, fuel trim, the integrity of the evaporative emission control (EVAP) system, the efficiency of the catalytic converters, the operation of the EGR system, the coolant thermostat on 2000 and newer engines, the positive ventilation system (PVC) on 2002 and up vehicles, the air injection system (if used) and even the A/C system on some 2002 and newer vehicles.

OBD II can also detect misfires. On most vehicles, it uses the crankshaft position sensor to measure small changes in the speed of the crankshaft that indicate a misfire is occurring. It can even pinpoint which cylinder is misfiring. Misfires are bad for emissions because a misfire allows unburned fuel to pass into the exhaust system. This causes a huge increase in hydrocarbon (HC) emissions, which in turn causes the catalytic converter to run hotter than normal as it attempts to burn the pollutants. A steady misfire can create additional problems because the converter may get so hot that the honeycomb inside melts. This may create a partial or complete blockage that increases exhaust backpressure, kills fuel economy and performance and may even cause the engine to stall.

If a problem occurs in anything monitored by the OBD II engine management system, OBD II will record a diagnostic trouble code (DTC) in the PCMs memory and turn on the check engine light or "Malfunction Indicator Lamp" (MIL).

The most important point to remember about the check engine light is that the light itself tells you absolutely nothing except that a problem has occurred, and the vehicle needs to be looked at by someone who knows what theyre doing.

A check engine light that comes on while driving or flashes may indicate a minor problem that is of no real consequence and has no detectable effect on emissions or performance, or it may signal a serious problem that may result in expensive damage if ignored. It all depends on the nature of the problem.

Editors note: For more information on pulling and interpreting OBD codes, see this months feature Check Engine Light: Opportunity or Obstacle?.

The OBD II system isnt perfect, but it does keep watch over a lot of things that affect emissions. One of these is converter efficiency.

The catalytic converter cleans up with the engine management system misses. Traces of unburned fuel (HC), carbon monoxide (CO) and oxides of nitrogen in the exhaust are dealt with in the converter. A good converter working at high efficiency will reduce these pollutants to almost nothing.

OBD II monitors the operation of the catalytic converter by comparing the signal from the oxygen sensor in the exhaust manifold (the "upstream" O2 sensor) to a second O2 sensor located just behind the converter (the "downstream" O2 sensor).

When the converter is working properly, the downstream O2 sensor shows little switching activity. But if the downstream O2 sensor starts switching back and form from high to low like the upstream sensor, it tells the OBD II system that the converter isnt doing its job. The OBD II system will set a trouble code and turn on the Check Engine light.

The converter monitor is a "noncontinuous" monitor that only runs once a drive cycle and then only after certain driving conditions have been met. Getting the converter monitor to run typically requires driving at highway speeds for 15 minutes or more. Even then, it may not always run.

Another important OBD II monitor is the EVAP monitor that checks for fuel vapor leaks. OBD II does this by applying vacuum or pressure to the fuel tank, vapor lines and charcoal canister. A code will be set if it detects no airflow when the EVAP canister purge valve is opened, or it detects a leakage rate that is greater than that which would pass through a .04- to .02-inch pin hole. Finding such tiny leaks is very difficult. Leak detection equipment that uses smoke or dye is often needed to find such leaks.

On older vehicles (pre-OBD II), trouble codes in the computers memory can usually be erased by pulling the computers power fuse or disconnecting the battery (note: this may also cause the loss of "learned" information in the computer as well as radio presets. The best way to clear codes is to use a scan tool or code reader).

On the newer OBD II vehicles, the only way codes can be cleared is with a scan tool or code reader.

In many areas, tailpipe emission tests are being replaced with a simple plug-in OBD II check. Measuring exhaust emissions on a dyno that simulates driving conditions is an expensive and time-consuming method of identifying polluters. The U.S Environmental Protection Agency (EPA) now allows states to certify emissions compliance on 1996 and newer vehicles with a simple plug-in OBD II check. If the check engine light works, and there are no trouble codes in the computers memory, the vehicle passes. This means that sticking a piece of tape over the light isnt going to cut it.

But like everything else, this simple test may result in a failure if the OBD II system isnt ready and has not run all its self-check monitors. As mentioned earlier, the catalyst monitor can sometimes be a stubborn one to set. The EPA realizes the system isnt perfect, so they allow up to two monitors not to be ready prior to testing 1996 to 2000 vehicles, but only one monitor not to be set on 2001 and newer vehicles. The ready status of the OBD II monitors can be shown on a scan tool or some code readers.


PCMs are pretty reliable, but two things they cant stand: water and electrical overloads.

Flood damaged vehicles almost always require a new PCM. A PCM may work for awhile after it has dried out, but sooner or later corrosion will damage the soldered circuit boards and connections causing the unit to misbehave or fail. Reman PCMs are an affordable alternative to new PCMs from a dealer, and most come with 90-day or longer warranties.

PCM failures caused by electrical overloads can often be traced to a shorted fuel injector (which burns out the injector driver circuit in the PCM), shorted power relays, somebody reversing jumper connections when jump starting a dead battery or a charging system thats putting out too much voltage. Arc welding on the vehicle body may also induce stray currents that can damage the PCM or other electronics.

Worn-out or defective oxygen sensors, spark plugs and plug wires also cause a lot of emission and driveability problems. Oxygen sensors in OBD II vehicles should last 100,000 miles or more but may fail sooner if contaminated by silicone from an internal engine coolant leak, or phosphorus if the engine burns oil (worn valve guides, rings or cylinders). On older vehicles, the oxygen sensor may only last 50,000 miles. A bad O2 sensor typically causes a rich fuel mixture, higher emissions and increased fuel consumption.

Spark plugs should be replaced for preventive maintenance according to the service interval recommended in the vehicle owners manual (typically 45,000 miles for standard spark plugs and 100,000 miles for platinum plugs and other long-life plugs).

Plug wires have no recommended replacement interval. But that doesnt mean they last forever. The insulation can be damaged by heat, vibration and mishandling. Any plug wire that is arcing, loose, damaged or exceeds resistance specifications should be replaced. Plug wires on high-mileage vehicles should be replaced as a complete set.

Bad ignition coils can cause misfiring, hesitation and hard starting. Coil failures may be caused by bad plug wires as well as heat, vibration or electrical overload.

Another sensor that can upset the operation of the engine management system and emissions is the coolant sensor. If this sensor is bad, the PCM may not go into "closed loop" and use the O2 sensor signal to control the fuel mixture. The result is a rich fuel mixture, elevated emissions and increased fuel consumption.

Other sensors like the airflow sensor, throttle position sensor (TPS), manifold absolute pressure (MAP) sensor and others can also upset the operation of the engine management system if their signal is lost or they send bogus information to the PCM. Diagnosis requires testing the sensor to see if it responds correctly and reads within normal limits.

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