Description of the EGR and DPFE system:There's a vacuum line coming off the intake manifold to the EGR actuator. The actuator is on the firewall and it's an electric device controlled by the PCM that modulates how much vacuum pressure gets past it. From there, another hose goes to the EGR Valve. Vacuum pressure opens the EGR Valve, allowing exhaust gases to be pulled from the exhaust manifold, past the DPFE tubes, then back into the intake manifold.
When gas flows through the EGR tube, the two tubes leading to the DPFE Sensor get pressurized. There's a differential in the pressure, one tube has more pressure than the other one. The DPFE registers the amount of pressure difference and can tell how much exhaust gas is flowing through the EGR system. It tells the PCM how much is flowing, and The PCM uses this reading to control the actuator.
DPFE Sensor FailureWhen the DPFE Sensor starts failing, it gives the PCM bad information. The DPFE Sensor is less sensitive, and the PCM thinks less recirculated gas is being burned than is actually so. Trying to compensate, the PCM opens up the actuator too much. This excessive amount of EGR combined with the air coming in via the throttle body leans out the fuel mixture, causing a lean misfire, which is the "stumble" or "hesitation" that is common as the DPFE Sensor fails. The DPFE sensor in the Focus is not very robust, requiring frequent (every 30k miles or sometimes less) replacement or a work around. Note that complete DPFE sensor failure is a very slow process, and the check engine light won't come on until the DPFE sensor is pretty much done for.
If you're experencing stumble or hesitation while cruising, and think that it might be related to DPFE sensor failure, try one of the workarounds listed toward the end of this article. If the stumble goes away, then your DPFE sensor is more than likely failing. Of course, replacement of the bad DPFE sensor is the official recommended fix for this problem. However, workaround #2 listed in this article takes a while longer to perform but makes for a nice semi-permanent solution that won't cause any damage or check engine light warnings if the DPFE Sensor hasn't completely died. Workaround #1 only takes a few seconds but is good for making a quick diagnosis to see if the EGR system is leaning out and causing problems.
Identification of EGR system componentsOn most Foci, the DPFE Sensor is a "Winged" Black box bolted to the firewall pretty much in the top middle, with 2 rubber hoses coming down off of it, and a 3-wire plug going into it perpendicular to the firewall. The 2 rubber hoses go down to the EGR Pipe, which is the 1/2" or so diameter tube you see coming off the driver's side of the exhaust manifold and going around the driver's side of the engine under the coilpack.
Photo of the more common style DPFE Sensor:
On some '01 foci, the DPFE sensor is a more squarish black box with a 3-wire plug in the top and 2 hoses coming off the bottom, it sits almost on top of the EGR pipe, and is located somewhat under the air intake tube. It also connects to the EGR pipe via rubber hoses, but the hoses just stand straight up and they're short. There are actually 2 different DPFE sensors for this location, but they are both located in the same place, and look similar. Yours may not look exactly like the one in this photo, though.
Photo of the '01 replacement DPFE Sensor (thanks to Jaspo from the FocalJet forums):
Basically, follow the EGR pipe until you see 2 metal tubes next to each other split off from it, then follow those to the rubber hoses and those hoses to the DPFE sensor.
The EGR Actuator is an electric device that allows small amounts of vacuum pressure to pass on to the EGR Valve.
Photo of EGR Actuator (DPFE Sensor wiring and a hose removed for clarity):
The EGR Valve is a vacuum-operated gate that opens up to allow exhaust gases to travel from the exhaust manifold to the intake manifold for recycling.
Photo of EGR Valve with vacuum hose attached to the top:
Replacement of DPFE Sensor
You replace the DPFE sensor by un-bolting it from the firewall with a 10mm socket (unless you have the less common '01 DPFE Sensor), pulling the old one off the rubber hoses, disconnecting the wire harness from it, putting the new one back onto the rubber hoses (the same way it came off), re-attaching the wire harness and bolting it back to the firewall.
If you hook the rubber hoses up wrong, you'll get a CEL. Nothing harmful, but you'll need to swap them. :)
DPFE Sensor Failure WorkaroundsKeep in mind that these work-arounds might not be 100% legal depending on emissions laws in your state. You should probably go "back to stock" for emissions testing if your state requires certification. No one but you will be held liable for legal trouble you get in by using these troubleshooting methods to determine EGR and DPFE malfunction.
Work around #1 (Temporary) : http://www.focushacks.com/index.php?modid=66 All this does is stops your EGR Valve from being able to open. If your car is running like crap, try this first. If this fixes the problem, then you know it's likely your DPFE sensor, or less likely, some other EGR Malfunction. All EGR malfuctions I've ever seen on the Focus (a LOT of them) were DPFE related. This method will eventually cause a check engine light (CEL) due to "insufficient EGR flow"
Photo of EGR Block-off. Shown: a vacuum line splitter with caps on it. A plastic golf tee, screwdriver bit or anything that fits snugly inside the tube to block off air will work as well:
Work around #2: Take the hose off the EGR Valve like shown above, except then pull both hoses off the DPFE Sensor, and re-attach the hose that used to go to the EGR Valve, to the "Ref" hose port on the DPFE Sensor. This only works if your DPFE sensor is starting to fail but hasn't gone completely kaput yet. What this does is instead of opening the EGR valve when the PCM says to, it will now apply vacuum to the "Reference" sensor for the DPFE, and thus, the "Hi" sensor will see more pressure than the reference sensor sees, so the PCM is tricked into thinking that the EGR valve is working properly. If the DPFE sensor is completely shot, you'll get an "Insufficient EGR flow" CEL either way, hooked up this way or the original way, so it doesn't really hurt anything. The bonus here is that hot exhaust gases aren't destroying the DPFE sensor when it's hooked up this way, so the DPFE sensor will likely last longer. Remember to plug the 2 metal tubes that come off the larger metal EGR pipe. Failure to do so will result in hot exhaust fumes being released under the hood. This is not only noisy, but could possibly result in carbon monoxide getting inside your car while driving.
Photo of DPFE workaround, using the older style DPFE sensor. Notice that the vacuum line that used to go into the top of the silver EGR Valve is now hooked up to the "REF" port, the port on the passenger side, of the DPFE sensor, and the other DPFE hose has been removed.
Work-around #3: FocusSport/SCT chip with EGR system disabled. Ovbiously an expensive route, but if you were looking at a chip anyways, you can leave everything hooked up like stock, even if your DPFE sensor is totally done for, and just order your FS/SCT program(s) with the EGR disabled. Basically, you can save the hassle of buying a DPFE sensor AND a performance chip, by just getting the chip and having the EGR system disabled.