Two Reasons for Rough Idling on a Ford Focus

Vacuum leaks or a faulty DFBE sensor might be to blame

When a Ford Focus exhibits problems with running roughly at idle speeds, auto mechanics generally look first to either a vacuum problem, or more often, a problem with water getting into the Differential Pressure Feedback Sensor (DPFE), a part of the EGR (exhaust gas recirculation system). This is a notorious problem with Focus models built between 2000 and 2003. This is so common, in fact, that it is generally the first place a mechanic will look.

Possibility 1: Water in the DPFE Sensor

Like most modern vehicles, the Ford Focus is features an EGR system designed to lower the exhaust emissions. The system works by recirculating exhaust gases back into the engine in order to lower cylinder temperatures and emissions. The EGR system has several components that work together to do this. One of these components EGR's differential pressure feedback sensor, commonly known as the DPFE. When the pressure feedback senses that pressure is low, it opens the EGR valve to increase the flow of recirculating exhaust gases, and shuts down the flow when it senses that pressure is high. 

When the DPFE sensor is failing or goes bad, it causes rough idling, a reduce in power, and it may cause the "check engine" light to come on. If you live in a state with vehicle emissions testing, this may be the reason why your car fails the test. 

With the Ford Focus in particular, the problem may be caused by water getting into the DPFE sensor, interfering with its ability to accurately gauge pressure changes in the EGR system.

The fix is to seal the DPFE sensor so water cannot get in, but the way you do this will vary depending on whether the sensor is mounted on the firewall or is a tube-mounted DPFE. 

For a firewall-mounted DPFE sensor: 

  1. Remove the DPFE.
  2. Fold the insulation on the partition wall downward so it lays over the top of the EVR.
  1. Reinstall the DPFE in such a way that the insulation is trapped in place between the bottom of the DPFE and the top of the EVR. Tighten it to 36 +/- 6 lb.-in. (4.1 +/- 0.7 N.m)
  2. Verify that the DPFE and EVR hoses are fully seated.

For tube-mounted DPFE sensor: 

  1. Remove the EVR solenoid.
  2. Trace out a 2.5" wide x 3" tall rectangle in the insulation, starting from the bottom, and just outside of the EVR mounting lugs.
  3. Cut vertically upward from the bottom along each of the two vertical lines, stopping at the line drawn horizontally.
  4. Fold the section of insulation upward.
  5. With the insulation held up, reinstall the EVR solenoid. Tighten to 36 +/- 6 lb.-in. (4.1 +/- 0.7 N.m) 

Possibility 2:  Vacuum Leaks

Another possibility common to all 2000 to 2004 Ford, Lincoln, and Mercury products is a vacuum leak. So, a thorough check of all the vacuum lines and hoses in the EGR system is a good idea.