EGR function and test

[Q45tech]

"OBD I diagnostics and trouble codes were added in to flag opens, shorts and sticking solenoids.

OBD II EGR systems OBD II requires that the EGR system be monitored for abnormally low or high flow rate malfunctions. The EGR is considered malfunctioning when an EGR component fails or a fault in the flow rate results in the vehicle exceeding the Federal Test Procedure (FTP) by 1.5 times. FTP is the government-mandated drive cycle smog test that all new cars must pass and adhere to.

The diagnostic executive, controls the EGR monitor. The executive is an OBD II software agent given the task of managing all the onboard monitors and the scan tool interface. The executive coordinates the sequencing and actuation of all the monitor's test routines. There are eight main monitors whose sole function is to directly monitor and test the components assigned to them to ensure they meet FTP standards for life. These monitors are:

Catalyst monitor EGR monitor EVAP monitor Fuel system monitor Misfire monitor Oxygen monitor Oxygen heater monitor

A closer look at the EGR monitor Monitor tests are both intrusive and non-intrusive. An example of an intrusive test is when the EGR monitor cycles the EGR valve during a condition when it normally would be closed. In some cases, the customer may feel an intrusive test as a slight miss. The method of testing used by the EGR monitor varies according to the manufacturer, but there are three main types.

One method includes looking for a change in manifold pressure as the EGR valve is actuated on and off.

A second method involves cycling the EGR valve and looking for a change in short-term fuel trim. When the EGR valve is opened, it displaces some of the air fuel mixture. When the EGR valve is closed, more oxygen enters the combustion chamber, which then leans the mixture somewhat. The O2 sensor will respond with a lean signal to the PCM, which in turn increases pulse width. This is called short-term fuel trim compensation. The EGR monitor looks to see that all these things are occurring as they should. It repeats the tests and averages the results. Before the EGR monitor can begin its testing, it must first receive clearance from the diagnostic executive. The executive ensures that there are no conflicting conditions that would invalidate the EGR monitor's tests. For example, if the car had a lazy O2 sensor, fuel trim compensation to the EGR opening and closing would be inaccurate. Therefore, there are many safeguards built into OBD II to prevent this type of occurrence from happening. OBD II also has rationality checks. In other words, it uses deductive logic and constantly compares its inputs against each other to make sure all are in sync with one another. After the EGR monitor gets the OK to run its tests, it uses strict enabling criteria to ensure accurate testing such as:

Engine temperature more than 170 F. Ambient air temperature more than 20 F. Engine run time more than three minutes since 170 F. Engine speed 1848-2688 (auto. trans.). 55-82 mphManifold absolute pressure from 5-20 hg. Short Term Adaptive Fuel Trim is adjusting pulse width by less than +7 percent and more than -8 percent. TP sensor from 0.6 to 1.8 volts. Vehicle speed sensor more than 40 mph.

The above is used for illustrative purposes only. Refer to your manual or CD-ROM information system for specifics to the car you are working on.

The third type of EGR monitoring design includes monitoring an EGR position sensor and a back-pressure sensor. Some Fords use a differential pressure feedback sensor that reads exhaust back-pressure upstream and downstream of the EGR valve to determine its flow rate and operation.

While OBD I systems would usually flag an inoperative EGR system, OBD II systems are given the task of determining the correct amount of EGR flow to keep the car running clean.

Besides emission EGR percentage is critical in reducing cruise pumping losses and improving highway mpg!