1992 Ford F-150 Misfire at idle

Tiny
MSTRAWDER
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  • 1992 FORD F-150
  • 4.9L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 246,610 MILES
I've had intermittent problems with low idle and erratic RPM's since I got this truck. However, I had a rebuilt long block put in approximately 2 weeks ago, as the old one finally gave up the ghost. Since then, the overall driving has been a lot smoother. I pulled the codes after getting it back to try and clear the CEL. I received 1 for EGR valve opening not detected and O2 sensor reading lean in bank 1. Replaced the EGR valve, EGR valve position sensor and EGR vacuum solenoid. Cleaned the tube and passages best I could while I was at it. Changed the O2 sensor and cleared the codes. CEL disappeared and has not reappeared. I'm getting the bucking/surging which is a problem I've read with too much EGR gas. Going to put a restrictor plate in there to fix that.

The main issue I'm having is best described as a miss or hesitation at idle. Besides the above, I've replaced the distributor cap/rotor, MAP sensor, cleaned the throttle plates, replaced a few vacuum hoses (checked for leaks), checked the vacuum with a gauge (steady at 18-20), mechanic checked fuel pressure when the motor was put in (said at 40 PSI), new PCV valve and new plugs. Driving is fine, it only seems to miss at idle. I have not checked the resistance on the plug wires and I noticed last night that the terminal on the ignition coil looks rough.

Would either of these explain the idle miss? If not, what else should I look into? I plan on taking the entire intake off and cleaning it heavily in the next few days.
Sunday, June 1st, 2014 AT 7:37 AM

7 Replies

Tiny
HMAC300
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Hard to tell it could be corrosion on coil, something in one of the spark plugs a bad plug as well. If everything seems right recheck for a vacuum leak an dor clean the egr out as may be it's staying open partially. One other thing check the vacuum line going to purge canister, see fi the valve is staying open at idle which would cause a vacuum leak to fuel tank, just disconnect line from engine, block it and see if it straightens out. The purge valve shold open aroun d30 mph and close then as well.
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Sunday, June 1st, 2014 AT 7:46 AM
Tiny
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All the EGR components except for the tube are about a week old. Going to really clean out the ports when I take the intake off. Would a problem in the ignition coil allow it to start up, but still miss at idle?

Also, the vapor canister is actually cracked at the top.I replaced the vacuum lines going from the canister to the ports on either side of the IAC. Based on what you're saying, the crack in the canister could definitely cause the symptoms of a vacuum leak, or is it only that valve? Lastly, I were to test the plug wires, what resistance should they be at?
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Sunday, June 1st, 2014 AT 8:02 AM
Tiny
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No it's the canister as well and yes the coil could do that as you increase rpm it would throw more spark out so it would overcome some electrical problems
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Sunday, June 1st, 2014 AT 8:10 AM
Tiny
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Ok, if I disconnect that purge valve, then regardless of the state of the canister, if it's the culprit, it will fix the problem correct? Also, even though it starts up fine, the coil could still be bad to the point of not generating enough spark to idle properly?

The oddest part about it all is that it idles pretty well at first. The issues usually, but not always, seem to creep up after the engine has reached operating temperatures. Started it up at 5:00am and it idled well. By the time I drove about 17 miles to work and put it in park, it was hesitating/missing again.

Thrown enough parts at this thing. Really want to narrow this down. Thanks for the time in replying.
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Sunday, June 1st, 2014 AT 8:35 AM
Tiny
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S you increase speed the coil has to throw a steadier spark. So it's faster. This may be a bad pcm as well so scan for codes try wiggling different parts on truck sensor wise and see if it makes a difference. I have no further suggestions
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Sunday, June 1st, 2014 AT 8:43 AM
Tiny
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Just to follow up. I cleaned the intake and throttle body completely. Replaced all vacuum lines. Put the restrictor plate in the EGR valve. Completely bypassed the smog pump and hoses. Replaced the plug wires, coil, and re-gapped the plugs to.44.

Still have a bit of a rough idle. It was suggested that the distributor or pickup could be worn or bad. Would a worn distributor allow a smooth start, but miss continually at idle? I'm running out of ideas here.
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Saturday, June 7th, 2014 AT 5:19 PM
Tiny
HMAC300
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Ya it cold do that but normally i'dcheck other things like compression and for other vacuum leaks likeintake spray some choke cleaner around manifold to see if it gets sucked in anyplace. Also if pickup coil was bad it wouldn't run most of the time. Check dist cap for carbon tracking and cracks as well as around coil tower.
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Saturday, June 7th, 2014 AT 7:04 PM

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