The call sounded pretty routine. It was a 1991 Taurus 3.8L AXOD-E with a "check engine" light on. The technician had already performed a PCM self-test and retrieved a Code 332 in continuous memory and a Code 332 during the KOER test. That would indicate low EGR flow. No big deal. Inspection of the EGR components found that the PFE sensor sample tube, which hooks into the EGR supply pipe, was disconnected from the PFE sensor and had melted the PFE sensor due to heat from the exhaust. Figuring someone else had left the sample tube disconnected from prior inspection, the technician replaced the PFE sensor and reinstalled the tube. He then retested the PCM for trouble codes. None were present, so the car was returned to the owner.

Back again
Two days later the car was returned to the shop with the same symptom, codes and PFE sensor damage. Seeing the duplicating symptom and damage, the tech decided it was time to call AutoLine. I listened to the technician's story and agreed that he had performed all the correct tests. After reviewing his information, I found that the light initially came on during acceleration. I suggested he install a different PFE sensor to replace the damaged one and then monitor the PFE sensor voltage on the signal circuit while driving to verify that the car did not have excessive exhaust back pressure.

The technician told me that the car seems to run just fine with the exception of the CE light coming on while driving. That sounded odd, but I still wanted to see the readings while the car was being driven under the related condition.

After the test drive, the technician called back. He told me that the PFE sensor voltage was normal until he went into a hard acceleration mode. Under hard acceleration the PFE sensor voltage read approximately 4.5 volts, then dropped down to a normal KOEO reading of approximately 3.25 volts. He opened the hood only to find that again the PFE sensor sampling tube had blown off of the sensor. This definitely indicated that this vehicle had a back pressure-related problem.

I quickly reviewed the exhaust system layout. The 3.8L uses two catalysts, one for the front and one for the rear bank of cylinders. However, the EGR system draws exhaust flow only from the rear bank of cylinders. Knowing that the PFE sensor voltage was normal at idle, I suspected the possibility that the honeycomb in the rear converter was restricting exhaust flow. This restriction could be the reason for the sampling tube being blown off of the sensor.

Exhausting
I suggested that the technician pull the exhaust down and inspect it for any possible obstructions or related problems, starting with the rear catalyst, then on to the muffler and related tubing. The following day the technician called me back with his findings. As he was inspecting the exhaust he initially found that the rear catalyst was in good order and not restricted at all. But what he soon discovered was that the flexible union between the Y-pipe and the hanger at the cross-member was made of a double-wall construction and it was the inner wall that had collapsed. The technician replaced the flexible union with an improved design that is also of double wall construction, but is made of a woven wire material with greater flexibility.

Puzzle solved
The remaining piece to this puzzle was learning why the condition occurred only under hard acceleration. Shouldn't a collapsed inner wall restrict all the time? Even under normal acceleration? Why hadn't the PFE sensor voltage been higher than normal at idle or during moderate acceleration? It all fell into place when the technician performed a brake-torque acceleration test and found that the rear of the engine was rocking back far enough to collapse and maintain force on the flexible union. To prevent a recurrence, the motor mounts were replaced, putting the car back on the road with normal operation and driven away by a satisfied customer!

Paul Mensink is an IDENTIFIX Ford specialist and team leader. He is ASE master and L1 certified with 18 years of diagnostic experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 1998 IDENTIFIX. All Rights Reserved.

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