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Discussion Starter · #1 · (Edited)
My 1991 302 has a poor, searching idle, which rises up and down and always feels like it's threatening to stall out. Sometimes it does. She will run like this but doesn't feel healthy. I discovered that if I remove any one of the vacuum tubes from the vacuum tree the idle becomes strong and the engine sounds appropriate. I did 120 miles this weekend with the HVAC vacuum tube disconnected and the truck ran well. MPG was just 7.5, though not sure if this is related or a mix of other issues (working on it.)

Does anyone have a theory or potential diagnosis? Thanks!
 

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1986 Bronco, 5.0 nothing special... yet!
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My 1991 302 has a poor, searching idle, which rises up and down and always feels like it's threatening to stall out. Sometimes it does. She will run like this but doesn't feel healthy. I discovered that if I remove any one of the vacuum tubes from the vacuum tree the idle becomes strong and the engine sounds appropriate. I did 120 miles this weekend with the HVAC vacuum tube disconnected and the truck ran well. MPG was just 7.5, though not sure if this is related or a mix of other issues (working on it.)

Does anyone have a theory or potential diagnosis? Thanks!
Have you looked at cleaning out your idle air control circuit? There should be a computer controlled plunger on the throttle body the controls your idle. If it is dirty, sticking or in general not working it will cause an erratic idle. you might also look at cleaning out the throttle body.
 

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Charlie don't surf..
'92 Ford Bronco XLT
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My 1991 302 has a poor, searching idle, which rises up and down and always feels like it's threatening to stall out. Sometimes it does. She will run like this but doesn't feel healthy. I discovered that if I remove any one of the vacuum tubes from the vacuum tree the idle becomes strong and the engine sounds appropriate. I did 120 miles this weekend with the HVAC vacuum tube disconnected and the truck ran well. MPG was just 7.5, though not sure if this is related or a mix of other issues (working on it.)

Does anyone have a theory or potential diagnosis? Thanks!
7.5 mpg is some crappy gas mileage..for reference, my '92 with a built 351, 37 inch tires and 4.56 gears get a little better than 8 around town..

Doesn't make sense why it would run better with the vacuum disconnected. Have you checked for codes?
 

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95 5.8L MAF XLT, Hedman Shorties/MF SS Y & Muff, E4OD, Man hubs, KYB Quads, 31x10.5x15, 304K miles
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Have you checked your Fuel pressure bleed down rate lately? You might have a leaky Injector or 2. Your 7.5 mpg sure "Doesn't" sound like things are running just right. Adding extra air to the mix and it running better/smoother sounds like, the Idle at least, is running "Rich". Start with the Basics and work your way up. Check your IAC valve, Fuel Pressure, etc...
(Edited)
Also if your engine is not getting up to "Normal" operating temperature (somewhere round 180 deg F) because of too cold a Thermostat or a faulty sensor sending a bad signal to the computer or, or, or, the Computer is designed to operate in "Open Loop" Mode with Preset settings which include Richer Fuel settings and not using the Oxygen sensor readings vs "Closed Loop" Mode with Normal Fuel settings (Leaner than Open Loop) and operating off of the Oxygen Sensor also.
 

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Yo 91Problems,
As advised:
Try a Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Code Reader.....

PCM stores the Self-Test program in permanent memory. When activated, Self-Test checks the EEC system by testing memory integrity and processing capability, and verifies that various sensors and actuators are connected and operating properly.
The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test. Run it around to heat the engine up and shift thru all gears including Reverse. Make sure A/C is off and transmission is in Park (automatic); or in Neutral for a Manual and; release clutch. Then turn off engine, all accessories/lights (close driver's door) , etc.

Do KOEO test First. Post Code(s) here according to KOEO and KOER.

A helper can assist you by counting the codes. Some use their smart phones to record them.

BEWARE OF FAN, BELTS, PULLEYS, HOT HOSES, IGNITION HIGH TENSION WIRES, AND ENGINE COMPONENTS⚠

Or ask local mom and dad parts stores if they will test it for you.

Or purchase a coder reader such as Equus 3145 Innova OBD I Code Reader for Ford EEC IV Engines at Walmart & most parts stores.

Check for vacuum leaks see my test, post #11

Idle Air Control (IAC)


➡"...bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection." by Ryan @ Fuel Injection Technical Library » Idle Air Bypass (IAB)
Read more.

The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced.


Gacknar wrote, "I can tell you one thing, if your idle did not tank when you unplugged the IAC then one of two things happened. It's not closed all the way, the idle set screw has been jacked with or you have a vacuum leak.
Testing..
"First let me say this little thing has many names. But they all talk about the same item under the hood. Here all the names I've had the torture of learning throughout the years:
•Idle Air Bypass •Idle Air Control •Idle Speed Control •Throttle Bypass Air •Idle Bypass •Inlet Air Controller
•Inlet Air Bypass •Intake Air Bypass •Intake Air Control.
This is really easy to test. First as with all problems you should gather the trouble-codes from the computer. Follow the codes for testing and repair. If you get a code that points to a problem with the IAB start the vehicle and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection.
The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced." By Ryan M.
□•□
177775
 

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Discussion Starter · #6 · (Edited)
Thank you all for the help, I took off the the throttle body and cleaned it thoroughly. Was as full of carbon as the worst ones I saw on Youtube. The idle is at least 50% more stable. First time taking apart part of the engine itself, and now I'm hooked. Hope these pics are as satisfing for you as they were for me.

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85 Bronco, 309ci I6 w/4bbl, np435, 4" lift, 37" Irok NDs, 4.56 w/ Detroit Locker and tru trac
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Holy moley!

Is the oxygen sensor clean and working correctly? I had a 96 f350 351w getting terrible milage, and it didnt have an oxygen sensor in it. Added one, and picked up almost 2mpg.
 
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