I have a 94 Bronco with the 5.8 liter and the check engine light has been coming on and off so I pulled the codes tonight and this is what it gave me. Codes 327, 332, 334, 337
I replaced the EVP about 3 months of so ago but now it is acting up again.
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So I guess my question is where is the PDFE located on my truck I cant find it and the Chilton’s book is no help. Also what else could be causing all these codes, could the EGR be bad? If so any way to check it, auto store want $150 for a new one. And help appreciated

 

There is no PFE on your truck. This is an "either/or" situation with some engines using the PFE sensor and others using the EVP sensor. The Pressure Feedback EGR sensor measures back-pressure in the exhaust system and sends this info to the PCM. In the case of the EVP sensor, the information is sent in the way of sensing the actual position of the EGR valve.

327 indicates the voltage in the EVP circuit was lower than it should EVER be. However, 337 indicates that the voltage coming back to the PCM is too high. These conflicting codes need to be verified. You may have wiring damage with this situation. You can check the VREF voltage coming from the PCM at pin 26 of the PCM. It should be pretty close to 5VDC. To check the wiring harness, you can jump between pins one and three of the EVP sensor and check the voltage coming back to the PCM at pin 27. With the jumper in place in the wiring harness connector instead of the EVP sensor you should get the same voltage at pin 27 as you got at pin 26. Once you have verified that the wiring to and from the sensor is not damaged or shorting you can move on to checking the EVP sensor operation.

The EVP sensor is a potentiometer that when it ages will show signs of wear in the way of "dead spots" or "spike points" in the voltage going back to the PCM as the potentiometer moves. To move the sensor manually, apply vacuum to the EGR valve and take a continuous resistance reading. Apply about 12-14" of vacuum and release it slowly. The resistance in the EVP sensor should change smoothly with no jumps or bumps in the meter. If not, the sensor is bad. If applying vacuum to the EGR valve does not affect the EVP sensor resistance either the EVP sensor is dead or the EGR valve is not moving (failed).

One note, the EVP sensor also has a ground that should be connected as well. without this ground intact, the voltage will be affected in much the same way a defective EP sensor would behave. I will look up the wire color information for this shortly but I must head for work.

 

Ok, some additional information about checking the wiring harness that I wasn't able to post earlier.
Belay that information! It is incomplete at best. Read on and you will see what I mean.

EVP sensor should have three wires running to it. BROWN/WHITE (pin1), GREY/RED (pin2), BROWN/LT. GREEN (pin3). The GREY/RED is always the signal return. For two-pin sensors the PCM uses the voltage coming back down this wire for information. For three-pin sensors the PCM uses both this and the third pin (in the case of the EVP sensor, the BROWN/LT. GREEN wire) as a comparison voltage to the signal return voltage in the GREY/RED wire.

Pin 46 of the PCM is the signal return terminal. The PCM assumes the necessary voltage drop through the wiring harness and takes the voltage from the third (BROWN/LT. GREEN) pin as a comparison. The difference is calculated and the PCM makes adjustments accordingly. The voltage at pin 46 of the PCM should be very close to the same voltage as found at pin 26 or slightly lower.

To check that the VREF (reference voltage) generated by the PCM power supply is good, meter between pin 26 of the PCM and a good frame ground. Double check the wiring harness TO the EVP sensor by checking for the same voltage at the BROWN/WHITE wire in the EVP connector in the wiring harness. If you have a good VREF voltage (5VDC approx.) then jump the BROWN/WHITE wire to the BROWN/LT. GREEN wire and check the voltage at pin 27 of the PCM. If you still have good VREF voltage, the problem does NOT lie in the wiring harness. You can do the "wiggle" test on the harness to make sure there are no weak or loose connections between the sensor connector and the PCM.

To test the EVP sensor itself, probe the pins that connect to the aforementioned wires and run the test by applying vacuum as described in my previous post. As mentioned, the resistance should change smoothly as the vacuum is slowly released. If it does not, the EVP is bad. Note: Only run this test with the EVP sensor disconnected from the wiring harness or your readings will be tainted by the electronics in the PCM.

One thing to note here is that the VREF feed for the EVP sensor splits off to both the TP (throttle position) sensor and the MAP (manifold absolute pressure) sensor somewhere in the main harness between the firewall and the individual sensors. You might do well the check for damaged wires at all of these sensors just in case the harness is damaged and shorting at one of the other sensors being fed VREF.