rough idle
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New User
Joined: Nov 2005
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From: harrisonburg,va
rough idle
hey fellas,got a question about an idle condition. i've got a 90 bronco, 351w, 128,000 mi. rough idle at start. have to feather throttle to keep it runnin. will idle on its own when warmed up but it's rough.did a tune up last month. pulled codes and got 12(no idle control) and 41(o2 sensor lean). changed iac when tuned up last month, but don't hav much confidence in the after market brand i put on it. when i unplug iac, it sputters and dies. checked for vacuum leaks, but found none. i may go to ford dealer and get another iac(orig equip this time). any thoughts or suggestions would be appreciated. i drive a truck so i won't be back home until later this week to respond to any thoughts. thanx.
Lead Driver
Joined: Sep 2002
Posts: 9,182
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From: Gateway to the West
First off, Welcome to FTE and the Big Bronco Forum. A greater group of people you will not find anywhere.
When you replaced the IAC, did you disconnect the battery for 5-10 minutes after you installed it? If not, the old stored IAC fault Code may still be in there which will "confuse" the ECA even if the new IAC is functioning perfectly.
Check your coolant level as well. The ECT (Engine Coolant Temperature) sensor can get false temp readings if the coolant level is low especially when the truck is cold, i.e. before the thermostat opens.
When you replaced the IAC, did you disconnect the battery for 5-10 minutes after you installed it? If not, the old stored IAC fault Code may still be in there which will "confuse" the ECA even if the new IAC is functioning perfectly.
Check your coolant level as well. The ECT (Engine Coolant Temperature) sensor can get false temp readings if the coolant level is low especially when the truck is cold, i.e. before the thermostat opens.
Last edited by greystreak92; Jan 30, 2006 at 08:44 PM.
Elder User
Joined: Jan 2005
Posts: 656
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From: Houston
Rough idle always makes me think of dirty Throttle body. These things can get very dirty and cause a poor idle. Have you cleanned yours lately? If not its easy and cheap. do a search on here for Throttle body and you will read about all the satisfied people who cleanned their TB.
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New User
Joined: Nov 2005
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From: harrisonburg,va
thanx for the suggestion poffutt,i'll chek it out. been a while since i had it off and cleaned it. i checked the tps and it showed good using my voltmeter. responded as it should through out entire range. checked antifreeze level and is ok. reset codes by removing neg batt cable for 10 min. and stil no change. pulled codes and showed 12 again and this time 72(map sensor). gonna check that w/ volt meter and vacuum pump. there may be a vavuum leak that i hav missed. thanx for the ideas.
Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
I had the exact same issue - same year & motor, too. 
I took the throttle body off and cleaned it out with a stiff brush and a can of carb cleaner (any solvent will work, even brake parts cleaner). I cleaned out the IAC and put a new TPS on. It idles great now.
I took the throttle body off and cleaned it out with a stiff brush and a can of carb cleaner (any solvent will work, even brake parts cleaner). I cleaned out the IAC and put a new TPS on. It idles great now.
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From: harrisonburg,va
thanx andym. actually i fixed it today with a new map sensor. tested the old one and it wasn't up to par. put the new one on and smooth as silk now. but i'm still gonna clean that tb just for good measure. thanx to all for the ideas.
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From: on da beach
My 96 Bronco 5.8 is idling pretty bad.
Pulled codes and I have 02 sensors codes.
Paul(conanski) mentioned that the 02 sensors are okay, and to check EGR. Reading this thread makes me wonder about the TPS, IAC, MAP sensor.
Andy, how did you go about cleaning your IAC.
I should start replacing some items, but guessing which one to do first is the question.
Pulled codes and I have 02 sensors codes.
Paul(conanski) mentioned that the 02 sensors are okay, and to check EGR. Reading this thread makes me wonder about the TPS, IAC, MAP sensor.
Andy, how did you go about cleaning your IAC.
I should start replacing some items, but guessing which one to do first is the question.
Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
Well, I would always recommend cleaning the TB if you have idling issues, regardless of codes. The IAC may have something to do with it as well. I just soaked mine in carb cleaner and blew it out real good. I have had IAC valves so gummed up with carbon that cleaning them didn't work. It can't possibly be your MAP sensor, you don't have one. But the MAF sensor could be dirty causing the computer to not know how much air is going into the intake.
But the MAF sensor could be dirty causing the computer to not know how much air is going into the intake.
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