96 Ford Bronco 5.8 Semi-High idle

Discussion in 'Domestics' started by majestek12, Jan 13, 2011.

  1. majestek12

    majestek12 Newbie

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    1996 Bronco, 5. 8, auto/E4OD, about 200k.

    Truck used to idle ~500-600 rpm. A few days ago the truck began idling at ~1000 rpm even after engine warmed up. The truck idles high while coasting, at 1000 rpm, and sometimes after higher revs only slows to ~1500 rpm without having a foot on the gas pedal. Other times will be coasting at about 1000 rpm and it will rise to 1500 and stay there on its own. Gets more prevalent after getting up to operating temperature. Truck would have erratic lag-response to gas from stop - no response to initial gas pedal travel, then immediate response past a certain point in travel. Torque converter would engage/disengage while driving at constant speed and in middle of respective gear range (not near shift point), correspondingly RPM would jump around. Truck does not want to seem to shift below 2200-2500 rpms - engine has to wind way out before transmission drops into next gear: most pronounced in the 1-2 shift, better between 2-3 and 3-OD.

    I tested the TPS and it showed erratic resistance on analog meter during throttle valve travel - was lower at idle, then would increase in resistance as throttle traveled to about 25% throttle, then would evenly drop in resistance up to WOT. Pattern was repeated in closing travel. I figured it was a bad spot in the "90% of your driving done here" range. Removed throttle body and replaced TPS. Cleaned throttle body while it was out with throttle body, not carb, cleaner. Reinstalled everything, I did not replace the throttle body to manifold gasket but did reinstall bolts to factory torque spec. Manually cleaned IAC while it was out and made sure gasket was in good shape. Also cleaned the MAF with MAF cleaner while the air intake was disassembled.

    Started truck - still idles at about 1000 rpm. I took it on a quick test drive, torque converter disengagement and associated engine revving was corrected as well as gas response. Truck still didn't want to shift below ~2200-2500 rpm and idled high.

    Truck idle drops to about 400 rpm when IAC is unplugged. When plugged back in rpm raises again to 1000 rpm. Iterated several times so doesn't seem to be a stuck IAC or bad motor.

    I've not done an in-depth check but there are no obvious/whistling vacuum leaks.

    Not sure what to look at next - PCV? Coolant Temperature sensor?

    Any help is appreciated!!
     
  2. nickb2

    nickb2 Wrench. I help when I can

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    Check engine light on? Might want to check into a revised IAC valve that ford had a TSB for back in 1996 for 5.8l bronco's and other ford light truck and mustangs. The part # is F6TZ-9F715-HA
     
  3. billr

    billr wrench Staff Member

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    Since you have changed the TPS and the IAC seems to be operating, my bet is the ECT. If it was a vacuum leak, PCV or other, then I wouldn't think the idle would drop when IAC was disabled.
     
  4. majestek12

    majestek12 Newbie

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    Thanks for all the input guys!

    Problem fixed - but to answer questions for general knowledge: CEL was not on, and unfortunately I didn't have a code reader that could read real time input to check sensor data. Perhaps with two older vehicles I should look into one or a breakout box. Thanks for info and the part number on the revised IAC, I will have to put that into mental memory.

    Kept messing with the truck last night and figured it out - turns out the incorreect driving pattern/info or an error code must have been set in memory. In my frustration with the issue I neglected to disconnect the neg battery cable to reset the ECM. After walking away and posting on here (and a cold beverage) I thought about it. Went out, fought the corroded battery cable off (will have to remember to put those anti-corrosion discs on it), let it sit for about 30 minutes then reconnected and started. Truck idled a little erratically while it learned but then settled right down to ~600 and stayed there. At first it ran so even I had to check the tach to make sure it didn't cut off. Road test proved problem fixed - proper shift patterns were restored, gas response was linear again, and torque converter engagment was correct. And, the truck no longer fights the brakes or feels like its trying to climb into the car in front of you when its stopped!

    Thanks for the input!
     
  5. keith

    keith Sr. Member

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    Maybe the battery cable was the problem.
     

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