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What causes rough idle and internal backfire?

Tiny
BROKENFORD
  • MEMBER
  • 1991 FORD F-250
  • 130,406 MILES
I have a 1991ford f250 351. I changed the spark plugs, cap and rotor, fuel pressure regulator, fuel pump but still have low pressure on the rail. It barely keeps an idle when cold and once warm its very rough and theres an internal backfire when under load. Im really getting frustrated with it.
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Friday, November 4th, 2011 AT 1:06 PM

12 Replies

Tiny
WRENCHTECH
  • MECHANIC
  • 20,758 POSTS
What exactly is an "internal" backfire. Backfires are either through the intake or through the exhaust.
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Friday, November 4th, 2011 AT 1:22 PM
Tiny
BROKENFORD
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  • 7 POSTS
When I go to give it gas while in gear there is a "bang" sound that comes from inside the engine.
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Friday, November 4th, 2011 AT 1:33 PM
Tiny
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You have to mean intake backfire. That usually means an ignition issue. Check the plugs wires cap, rotor and timing.
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Friday, November 4th, 2011 AT 2:00 PM
Tiny
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  • 7 POSTS
I checked and replaced all of that except the wires. What could cause low fuel pressure? I canged the fuel pump and fuel pressure regulator.
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Friday, November 4th, 2011 AT 2:04 PM
Tiny
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What do you mean by "low fuel pressure"?
How low?
Is this a problem when running or rest pressure?
Does it hold rest pressure?
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Friday, November 4th, 2011 AT 2:23 PM
Tiny
BROKENFORD
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  • 7 POSTS
Ive been reading and regular pressure is supposed to be between 35-45 psi and mine reads 20 psi while running. About 10 psi in rest pressure.
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Friday, November 4th, 2011 AT 2:42 PM
Tiny
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  • MECHANIC
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If you don't have a restricted filter, try pinching off the return line and see if it spikes up. If not, you have a bad pump.
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Friday, November 4th, 2011 AT 3:37 PM
Tiny
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  • 7 POSTS
I bought and changed the filter and pump and it helped bring up the pressure a tiny bit.
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Friday, November 4th, 2011 AT 6:14 PM
Tiny
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Make sure you don't have a voltage drop to the pump.
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Friday, November 4th, 2011 AT 8:26 PM
Tiny
BROKENFORD
  • MEMBER
  • 7 POSTS
My coil looks quite old and rusty. Could it be that?
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Friday, November 4th, 2011 AT 8:32 PM
Tiny
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Causing low fuel pressure? No.
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Friday, November 4th, 2011 AT 9:07 PM
Tiny
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This confused me greatly. My dad unplugged the hose from the map sensor that goes to the plenum and plugged it and put a vacume gauge on the map sensor and put it to the right pressure and it ran perfect. As soon as the hose was back on the map sensor, it ran poorly. Whys that?
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Saturday, November 5th, 2011 AT 3:49 AM

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