If something happens to the EVP sensor circuit to disrupt the EVP signal back to the computer, the computer has no way of monitoring or confirming the exact position of the EGR valve. Even so, it will still attempt to regulate the EGR valve based on other inputs such as throttle position and manifold absolute pressure. However, control will not be as accurate and some driveability problems may result.
If the engine is experiencing hesitation during acceleration, rough idle and/or hard starting, the problem is probably the EGR valve itself. Any of these symptoms could be caused by an EGR valve that's stuck in the open position, probably due to a buildup of carbon at the valve's base. Removing the valve and removing the accumulated carbon should solve the problem. When detonation is the primary driveability symptom, it means the EGR valve is failing to open. The loss of exhaust gas recirculation allows combustion temperatures to soar, resulting in spark knock during acceleration or when running under load. In this instance, the problem may be a faulty EGR valve (failed vacuum diaphragm) or a loss of vacuum to the valve (vacuum leak, faulty EGRC or EGRV vacuum solenoid, or an electrical problem in either of the two solenoid's control circuits).
Problems with the EVP sensor itself can be caused by electrical shorts or opens in the VREF, SIG RTN or EVP circuits. Bent or corroded pins in the electrical connectors account for most such faults. Another would be wear or sticking in the EVP sensor that would affect the accuracy of the EVP signal.
TROUBLESHOOTING
Ford lists a number of trouble codes that are related to the EVP sensor. These include:
Code 31 -- EVP out of limits. This code means the computer is not receiving a proper signal from the sensor. Disconnect and plug the vacuum line at the EGR valve and perform an "Engine Running Quick Test" to see if the code repeats. If it does, there's an electrical problem at the sensor. Use a digital ohm meter on the 200,000 scale to measure the sensor resistance between the EVP SIG and VREF terminals and then the SIG RTN and VREF terminals. It should be between 5500 (EGR valve closed) and 100 ohms (EGR valve open), and change smoothly and gradually as the valve is opened with a hand vacuum pump. You can also do this manually by removing the EVP sensor from the EGR valve and pushing the EVP shaft. If the sensor's resistance is out of specs, fails to change smoothly or change at all, replace the EVP sensor.
If the sensor itself checks out, use a digital voltmeter on the 20V scale to measure the VREF to SIG RTN voltage and the VREF to EVP signal voltage at the EVP harness connector (key on, engine off). You should see between 4 to 6V. If not, there's an electrical short or open in the wiring. Turn the key off, unplug the computer and use a breakout box to check wiring continuity. If no wiring faults can be found, replace the computer.
Code 32 -- EGR not controlling.
Code 33 -- EVP not closing
Code 34 -- No EGR flow Any of these three codes indicates a problem with the EGR valve itself, the EGRC or EGRV vacuum solenoids or a vacuum leak. A Code 83 (EGRC circuit fault) or Code 84 (EGRV circuit fault) would indicate an electrical problem in one of the solenoid circuits. The solenoids should have between 30 and 70 ohms resistance.
If the problem is not electrical, you'll have to use a vacuum gauge to figure out why vacuum isn't getting through. You should see the vacuum cycle on and off in less than 2 seconds at the EGR valve when the engine is at normal operating temperature and the throttle is opened and closed.
Always follow the diagnostic procedures outlined in your shop manual to isolate the fault when the EVP sensor appears to be faulty. If it turns out to be a bad sensor, replacing it is all you can do because no adjustments or repairs are possible .
In both cases, simple repairs are possible. The EGR valve can be clogged, and a simple cleaning of a mechanical EGR valve may solve the problem. In the case of an electrically controlled EGR valve or a more complex mechanical problem, replacement of the valve may be needed. The EGR valve is an expensive motor vehicle component, and care should be taken to ensure that it needs to be replaced rather than repaired. A reputable mechanic should advise you appropriately.
Failure to repair or replace the EGR valve will not lead to a life threatening condition, although it will reduce the life of your car and increase the potential emissions. Especially in areas which require smog testing, an automobile with a malfunctioning EGR valve may not pass.
