TechTips

Put The Pedal To The Metal And This Baby Rocks
Check Engine Light Comes On Only During Hard Acceleration

by Paul Mensink

The call sounded pretty routine. It was a '91 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 that 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.

Like A Rubber Ball
Two days later, the car 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 the technician's findings, 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 informed 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 once again the PFE sensor sampling tube had blown off of the sensor.

This definitely indicated that we had a back pressure related problem with this vehicle. 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 convertor was restricting exhaust flow. This restriction could be the reason for the sampling tube being blown off of the sensor.

Drop That Exhaust
I suggested that the technician pull the exhaust down and inspect for any possible obstructions or related problems. The inspection should start 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 found that the rear catalyst was in good order and not restricted at all. What he found instead however, was that the flexible union between the y-pipe and the hanger at the cross-member had collapsed. That flexible pipe is made of a double-wall construction and it was the inner wall that 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.

Questions Remained
The remaining piece to this puzzle was why the condition occurred only under hard acceleration. Shouldn't a collapsed inner wall restrict all the time? Even under normal acceleration? We also wanted to know why the PFE sensor voltage hadn't been higher than normal at idle or during moderate acceleration.

The remaining piece 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.

So it didn't happen again, replacement of the motor mounts put the car back on the road with normal operation and a satisfied customer.

Did You Know ...

Paul Mensink has 15 years of automotive experience, 10 of which he has focused directly on driveability. Mensink is the team leader for the Ford Department. He coaches 600 to 800 callers every month, from all over the United States. Mensink knows what hands-on technicians are experiencing!

Each month, technicians from AutoLine Telediagnosis share valuable TechTips with our readers. As an ASA member, you're already signed up to use the AutoLine technical diagnostic service at a discounted rate.

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AutoInc. Magazine ®, Vol. XLIV No. 12, December 1996