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1991 F150 5.8L engine Rough Idling on startup for few minutes
Problem I have a 1991 F150 5.8L engine with automatic transmission. When starting the truck, the truck idles rough for few minutes and I have to press on gas pedal sometimes to keep engine going but the whole rough idling lasts only for few minutes and after that the truck idles normally and I can drive it without any problems. What has been checked or replaced so far:
1- ECT, IAT, EGR sensors(checked)
2- Idle Air Control Valve or IAC(replaced)
3- Checked reference 5V on PCM.
4- Ignition Control Module(replaced) with NAPA expensive part. Other symptoms:
1- Disconnecting the SPOUT wire from Ignition control module during rough idling period makes rough idling problem go away. However, connecting the SPOUT wire back to ICM after that does not bring back the rough idling problem again.
2- After driving truck, stopping and then immediately starting the engine does not result in rough idling.
3- when truck idling normally, the SPOUT signal is a square waveform with peak amplitude of about 13V and duty cycle of 70 to 80%.
4- Rough idling seems to be random and doesn't always happen on startup
Tomorrow my plan is to look at PIP signal during rough idling as well SPOUT signal. My thoughts were the PCM is acting up since problem goes away after disconnecting the SPOUT wire from ICM but that does not explain why the rough idling doesn't come back after connecting the SPOUT wire back to ICM.
Any thought or ideas on what to check would be greatly appreciated.
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