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1987 Ford Bronco 87 351 Bronco idling and runnining very ro

Tiny
CROSBY09
  • MEMBER
  • 1987 FORD BRONCO
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
My 87 351 Bronco just started idling very rough. It idles fairly rough when started but when put into gear it really starts shaking and idling rough. It seams to be taking in too much fuel because there is a fuel smell smoke when you apply the accelerator. I noticed a small fuel leak coming from in between the carb and the intake so I tightened down the carb. It still idles rough and just sound like it is getting too rich of a fuel mixture.
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Tuesday, March 24th, 2009 AT 2:28 PM

3 Replies

Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Do you have a check engine light and when was the last tune-up?

A large vacuum leak can lean the air/fuel mixture out to such an extent that an engine will not idle at all. An EGR valve that is stuck open at idle can have the same effect as a vacuum leak. So too can a loose positive crankcase ventilation (PCV) hose, a leaky PCV valve or the wrong PCV valve (one that flows too much air for the application). If somebody replaced the PCV valve recently, they may have installed the wrong PCV valve. The rough idle in all of these cases is caused by "lean misfire." The fuel mixture is too lean to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will show up as elevated hydrocarbon (HC) readings in the exhaust, which may be enough to cause a vehicle to fail an emissions test.
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Tuesday, March 24th, 2009 AT 3:53 PM
Tiny
CROSBY09
  • MEMBER
  • 2 POSTS
The pcv valve has not been replaced recently. The last tune up was about 3 months ago. I also noticed that there is a bit of a hissing sound.
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Tuesday, March 24th, 2009 AT 4:35 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Hissing points to a vacuum leak-
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Tuesday, March 24th, 2009 AT 5:06 PM

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