1987 Ford Bronco 87 351 Bronco idling and runnining very ro

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.

3 Replies

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.