91 5.0 Rough Idle, Dying Engine Codes 15, 33 - MustangForums.com

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91 5.0 Rough Idle, Dying Engine Codes 15, 33

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Old 10-13-2012, 06:54 PM   #1
Join Date: Oct 2012
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Default 91 5.0 Rough Idle, Dying Engine Codes 15, 33

I have a 91 5.0 that is unmodified from stock save for the headers and exhaust. It was running fine and then one morning it was idling very rough and dying. No changes were made to the vehicle to my knowledge.

First thing I did was run fuel injector cleaner through it just hoping it may be a dirty injector. No dice.

KOEO codes are 15 and 33. No KOER codes.

What should I check next to track this problem down?
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Old 10-13-2012, 07:12 PM   #2
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Code 15 indicates that the ROM inside the computer has failed.
Code 33 indicates that the EGR was commanded to move but failed to do so.

Start with 15, it's likely the cause of 33. New computer can be purchased through rockauto.com for around $100
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Old 10-15-2012, 01:33 PM   #3
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From what I've read the code 15 can either be an ECU failure or improper voltage supplied to the ECU. The voltage from pin 1 to ground should be 12 volts, mine reads 11.86. Does anyone know if this is within acceptable range?

I also took a look at the ECU and did not see any blown resistors or bulging capacitors. Is it typical for an ECU to be bad without showing any signs of physical failure?
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Old 10-15-2012, 01:41 PM   #4
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Disregard that last question, I just realized that the error refers to the actual ROM. It's entirely possible for a ROM chip to go bad without damaged components.
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Old 10-15-2012, 01:41 PM
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code 15, code 33, idle, rough

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