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1986 Ford F-150 Rough idling 302 V8

Tiny
CRMIV67
  • MEMBER
  • 1986 FORD F-150
  • 130,000 MILES
I have been trying to solve this problem for a while now. My truck is idling rough. I thought I had it fixed but it is back to idling rough. It started out idling rough and cutting out on me at stop lights and stop signs. I have replaced the IAC, idol position sensor, PCV valve and hose, fuel pressure regulator, EGR valve, new plugs, rotor, cap, cleaned the intake, checked vacuum ( tested 19 ) tested fuel pressure ( tested about 35 ). When I replaced the regulator I found a broken injector and replaced that. The truck ran great for about 2 days and now it is back to idling rough but not cutting out. Any ideas, thoughts, or solutions you guys have would be much appreciated.
P.S. I did not replace all these things at one time. It has been a test and replace situation. I think I am getting close to the fix but the low pressure fuel pump has been in the back of my mind since the beginning. I can hear the high pressure pump run when I turn the key but it is hard to turn the key and have my ear at the tank.
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Monday, July 29th, 2013 AT 4:55 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,269 POSTS
If it is just a rough idle, have you checked for vacuum leaks, especially around the intake? Check it when the engine is cold by using this guide.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please go over this guide and report back.
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Monday, July 29th, 2013 AT 7:55 PM
Tiny
CRMIV67
  • MEMBER
  • 7 POSTS
I have checked for leaks and did not find any. I also checked the gasket around the intake.
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Tuesday, July 30th, 2013 AT 3:46 AM
Tiny
CRMIV67
  • MEMBER
  • 7 POSTS
I should mention that this problem only occurs when the motor is warm.
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Tuesday, July 30th, 2013 AT 4:27 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,269 POSTS
When the engine is warm, have you checked fuel pressure? Have you removed the EGR to check if it is carboned up?
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Thursday, August 8th, 2013 AT 6:27 PM
Tiny
CRMIV67
  • MEMBER
  • 7 POSTS
I have not checked the fuel pressure when it is warm but I replaced the EGR valve and the position sensor.
I will check the fuel pressure and see what it is. Thank you for the suggestions.
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Friday, August 9th, 2013 AT 3:55 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,269 POSTS
Let me know what you find. Also, make sure the intake isn't carboned up where the egr mounts.
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+2
Friday, August 9th, 2013 AT 2:06 PM
Tiny
CRMIV67
  • MEMBER
  • 7 POSTS
I cleaned the intake where the EGR mounts. I checked my fuel pressure when the motor was hot. I have 30 psi but when I rev the motor it drops slightly. When I let off the gas it drops to 28 and then returns to 30. I left the gauge on to see if it held pressure and the pressure actually increased to 39 psi. I am thinking that is the reason when I start the motor, even if I shut it off when it was running rough, it runs smooth for a while. Am I correct in thinking that? What is my next step replace the fuel pumps? I can hear the external pump run when I turn the key. Should I replace that one too assuming it is wearing out?
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Friday, August 9th, 2013 AT 2:13 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,269 POSTS
Did you disconnect the fuel pressure regulator and check it?
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Saturday, August 10th, 2013 AT 7:30 PM
Tiny
CRMIV67
  • MEMBER
  • 7 POSTS
I just replaced the fuel regulator and filter about a week ago.
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Saturday, August 10th, 2013 AT 7:38 PM

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