1986 Mustang Idle Tough Idle Problem

Discussion in 'Fox 5.0 Mustang Tech' started by erzenble, Jul 28, 2014.


  1. erzenble

    erzenble New Member

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    Hi, I have been trouble shooting my 86 mustang engine for months trying to figure out this idle problem and I have ran out of ideas. It is a 302 swapped into a 94 mazda b4000.

    The problem is when I first start driving it I will come to a stop in drive and the engine will surge slightly up and down 200 rpms, if I am at a stoplight and wait for a minute the rpms will drop from 900 rpms (where I have my idle set) down to 500 and start running very rough and the surge is gone. I will then start driving again and repeat this process. Also when the vehicle is down at 500 rpms if I put it in park right then it will idle up to 700 rpms and continue to run rough until I drive it again. The engine when first started cold will also sputter and when I give it gas will hesitate a lot then surge forward eventually. After that it runs smooth besides at idle.

    Here's what I have done:

    Replaced: ignition coil, wires, cap and rotor, and plugs. Fuel filter, oil and filter, idle air control valve, and bap sensor. Also the pcv valve and several vacuum lines.

    I have also cleaned the maf sensor, air/fuel sensor, air filter, fuel injectors and fuel system.
    I have checked fuel pressure, tps sensor readings, and checked intake vacuum (at 17 ) and vaccume leaks. Checked for codes and only got code 67 for neutral safety switch fault.
    I have checked timing but it was jumping all over the place at idle and perfect when reved up. I'm not sure if it was so erratic just from the erratic idle or not.

    Each thing I did seemed to help with power and help the engine run smoother and helped the idle slightly but never fixed the issue. I also beleive its running rich because I smell gas sometimes after I stop. If anyone has an idea I would be grateful, thanks
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  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

    It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 134,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
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  3. erzenble

    erzenble New Member

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    Thank you very much. That helped actually. Was an oxygen sensor ground was bad.
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