1991 Ford Mustang Idling problems

Engine problem
1991 Ford Mustang V8 Two Wheel Drive Manual 100K+ miles

About three months ago I had my engine overhauled, however, after it was rebuilt it had idle control issues. At first, we (my mechanic and I) thought it was a timing issue and the timing gun confirmed that the problem was not related to a timing miscalibration.

The idle control issues occur only during engine start-ups and later works itself out after a couple minutes. If the engine is started cold, the engine always takes 2 minutes and 40 seconds to reach normal operation. If the engine is started when warm, it takes exactly 2 minutes to work through the issues (these times never vary). While the engine works though its timing issues you can smell the raw fuel emanating from the exhaust pipes. If the car is not allowed to work through the idling issue and is taken out on the road, the car will hesitate and knock during acceleration. When the timing issue settles down, the engine works fine with no further problems. However, when the vehicle is out on the road, the “check engine” light comes on and turns off after 5-10 minutes of operation.

On October of this year, I had an electrician connect his engine computer scanner to my car and his unit indicated no errors. The electrician believed that the ECM module on my car was damaged and was the cause of the engine timing issues as well as the false “check engine” alerts. I recently bought my own error scanner and after following the operating instructions, my scanner detected various faults that were saved on the ECM memory. I performed two tests with my scanner, key on engine off (KOEO) and key on engine running (KOER).

On the KOEO mode, this fault was detected: “34 EGR valve position sensor or pressure feedback EGR voltage high or sensor fault memory codes.”

On the KOER mode, multiple faults were detected: “92 exhaust gas oxygen shows rich bank 2 or fuel control circuit failure, ” “34 pressure feedback EGR sensor voltage is high, ” and “42 no exhaust gas oxygen switching detected – always rich.”

The memory codes indicated: “34 EGR valve position sensor or pressure feedback EGR voltage high or sensor fault memory codes, ” “91 no exhaust gas switching detected bank 2, ” and “96 fuel pump circuit failure or high speed fuel pump relay circuit failure.”

The results of these two tests pointed out that the electrician may have NOT performed his tests correctly. I am aware that I may have to replace the EGR sensor and the EGR valve. As for the other errors and the idling issue, can they be related to the malfunctioning EGR sensor and valve or are the other errors separate issues that need to be looked at and repaired individually? Sorry for the long post but I wanted to make sure that I posted my problem in detail. Thanks for the help…

These are the stats on my Mustang:

1991 Ford Mustang GT 5.0L (302 cu in engine) V8 HOEFI RWD

My engine scanner: EZ Scan 6000 by Autoxray
Do you
have the same problem?
Tuesday, December 25th, 2007 AT 1:29 AM

1 Reply

Well it seems that the ECT may be faulty, causing the ECM to work in the wrong loop, test the ECT, values should change as engine warms, but were you go to the wrong loop with a warm engine it is a poosiblity!
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Tuesday, December 25th, 2007 AT 7:12 AM

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