idle/fuel problem
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Freshman User
Joined: Mar 2008
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idle/fuel problem
94, f150, 4.9L, unknown milage
at idle the rpm jumps up and down. just replaced plugs, wires, cap, rotor. helped a little. then we tried to set the timing, first problem is the timing mark is on the bottom of the pully, has anyone ever heard of this?
the mark was of no help so we timed it by ear, when idle is fine it lacks power and throttle response, and throws black smoke out of the tail pipe, also misses when revving (but this fixed the idle) when idle is rough it runs fine unless you hit the throttle quickly then it misses and shoots out black smoke.
can't find a vaccuum leak anywhere, think it's some sort of fuel problem due to the black smoke but can't figure out what. i'm going to replace the fuel pressure regulator tommorow and see what happens.
anyone have any other ideas?
at idle the rpm jumps up and down. just replaced plugs, wires, cap, rotor. helped a little. then we tried to set the timing, first problem is the timing mark is on the bottom of the pully, has anyone ever heard of this?
the mark was of no help so we timed it by ear, when idle is fine it lacks power and throttle response, and throws black smoke out of the tail pipe, also misses when revving (but this fixed the idle) when idle is rough it runs fine unless you hit the throttle quickly then it misses and shoots out black smoke.
can't find a vaccuum leak anywhere, think it's some sort of fuel problem due to the black smoke but can't figure out what. i'm going to replace the fuel pressure regulator tommorow and see what happens.
anyone have any other ideas?
Super Moderator
Joined: Jan 2008
Posts: 40,035
Likes: 1,523
From: Maine, Virginia
I've had all those symptoms on my '88 F-150 w/ 5.8L and I had to replace the idle air control motor, EGR valve and sensor and then ultimately the onboard processor. The truck wasn't giving any codes and no one could figure it out. Finally a Ford Tech suggested the processor may be bad. I bought a rebuilt one from a parts store and it still didn't solve all of my problems. Ultimately, I had found a chaiffed injector wire that was shorting everything out. After I re-insulated the wire, the truck ran like new one.
Tim
Tim
Elder User
Joined: Dec 2006
Posts: 550
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From: Hillsboro, OR
Black smoke is too much fuel or oil burning. You realy need to set the timing as fmr9 says above. Doing it by ear these days just won't cut it. These engines are just too touchy. Do a search for "Timing 4.9" and follow the procedure. Even better, if you plan on keeping the truck, invest $15.00 and get the Chilton's for it. It will be worthless for troubleshooting but it will be a lifesaver when you need the steps to do something or the torque settings. Good luck.
Senior User
Joined: Dec 2007
Posts: 128
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From: Florida
Possibly an internal vacuum leak by/from a weak PCV
Pull the PCV hose and plug it with a bolt.
Crank the engine and see if it smooths out.
Even if this is not the problem it should run smoother.
All to many times the PCV (from the crank case *oil/rich burn*) will be an internal vaccum leak.
Just because a PCV "rattles" dose not mean its good.
There is a spring on top of the weighted plug inside. if the spring is weak it will allow a weak vacuum to draw air from the crank case prematurly and can cause idling problems .
It's also possible to have a good idle with the plug wires in the wrong placement.....Only to have the simptoms you say missing ,blowing out the exhaust once the throttle is pressed .
Check your TPI setting and make sure it is set correctly with a meter.
This is the reason I tell people to purchase the $10 PCV instead of the $2 ones.
Crank the engine and see if it smooths out.
Even if this is not the problem it should run smoother.
All to many times the PCV (from the crank case *oil/rich burn*) will be an internal vaccum leak.
Just because a PCV "rattles" dose not mean its good.
There is a spring on top of the weighted plug inside. if the spring is weak it will allow a weak vacuum to draw air from the crank case prematurly and can cause idling problems .
It's also possible to have a good idle with the plug wires in the wrong placement.....Only to have the simptoms you say missing ,blowing out the exhaust once the throttle is pressed .
Check your TPI setting and make sure it is set correctly with a meter.
This is the reason I tell people to purchase the $10 PCV instead of the $2 ones.
