Close



Results 1 to 17 of 17
  1. #1

    Default 1986 GT wandering idle

    Short story first.This car belonged to a life long friend that died last year at 53. I have and still have my 83GT.He bought his 86 new,,now I'm the caretaker of the car until his wife can sell their house and move back to Alabama. The car is stock except for a set of shorty headers and an H pipe with no cats.The O2 sensors are connected and screwed into the exhaust system..The car starts just fine with an idle around 1200 and settles down to 900 or so in about 20 seconds,,as it warms up it begins to wander betweeen 800 to 1500,up and down,and then stays at 1500 and wont come down.Now if your driving the car this scenario continues,untill its shut off,,restart it after a short period it will run normally for a while but eventually will go back to 1500...any ideas??When I first brought it home on a trailer it ran real rich,barely running,i changed the oil and plugs and found a rotted out vacume line under the intake plenum,and now I got high idle.The car is a hoot to drive,its an unmolested example of the classic 4 eye and gets thumbs up stares everytime I drive it,,just like my 83,,but the 86 is stronger..One more thing,the car did sit a lot,Charlie had gotten too big (450+) pounds and couldn't work on the car anymore,and had a hard time driving it when it did get driven because of his size,and its got over 250,000 miles on her,but its in good shape otherwise,and always garaged.I think the headers and H pipe are from Borla or JBA

  2. #2

    Default

    One more thing,,yes I did replace the rotten vacume line

  3. #3
    FEP Power Member Ourobos's Avatar
    Join Date
    Apr 2008
    Location
    Waikoloa , Hawaii
    Posts
    1,879
    1986 CHP SSP Coupe

  4. #4

    Default

    My 86 had surging idle. I installed a new IAC,new MAP sensor, new TPS and set it correctly, and set the ignition timing. Idles perfect now.

    Ed

  5. #5

    Default

    My condolences on the loss of your friend.



    I got my '86 GT from, my BFF.
    We went through college, then grad school, then even moved to the same area out of State.
    My friend bought the '86 GT brand new, custom order the options he wanted. I drove him to pick it up.
    I know the history of my car very very well. I also know a lot about the engine/concerns/etc the '86 5.0.


    About the '86:
    Congratulations, it sound like it's an authentic '86 Stang GT.
    And, not some fake China clone. ;-)


    About the '86 5.0L Stang engines:
    In order of likely-hood for producing a hunting idle problem:

    1) The IAC gets filled with carbon from the EGR valve, and sticks.
    Get a new **FORD OEM** IAC.
    Note: A short-term solution is to clean it out with Acetone.


    2) The EGR valve may not fully seat. Get a new **FORD OEM** EGR.


    3) There were a few TPS units that were bad, and moisture would get in, and they would produce erratic signals
    If it's original, get a **FORD OEM** TPS.
    The '86+ TPS units are not adjustable. Sorry, only fools can adjust a non-adjustable part. :-)


    4) Sorry, the program storage in PROM(like a write-once flash-drive) was very very very very limited. The did a lot of hacks to get fit, what was fitted.
    What they did not have room for was the ability to limit the "integrator" wind-up in the "PID" control loop that controls the IAC and idle speed.

    So... even on a 100% perfect '86 5.0L engine, there is that chance that the idle will hunt. This is from a classic "Integrator wind up in the PID control algorithm". I think I learned that, and they teach that, in the 3rd year of engineering in a Control Systems class.
    The hunting idle will most likely happen when the engine is cold, or warming up.
    But, without that "Integrator" limit, or a more advanced control algorithm (that there isn't room for), even when the engine is warm and happy, the idle can go into hunting mode.

    The solution? Shut off the engine, and restart the engine.
    Even after my engine was rebuilt, and I had a new IAC, new EGR, etc, etc, the idle would hunt when the temperature was below ~70F->60F.


    Some people move to the MAF-based setup.
    That cures that hunting - assuming stock engine, and no other problems, of course.
    I did that for a number of years. But, when it was time for an engine rebuild, I decided I wanted to go back to SD. My coil is relocated to the passenger-side. And, that had some impact on my decision. Not much, but not zero.


    Btw:
    Any pictures?
    Any pictures?
    Any pictures?


    Yea, we leg-hump cars like mad in these forums. Just like it should be!

  6. #6
    FEP Power Member
    Join Date
    Nov 2007
    Location
    San Mateo, CA
    Posts
    2,271

    Default

    Cleaning the IAC valve is quick and easy. Had to do it on my 89 with 150k miles.
    Fox Body/3rd Gen MCA Gold Card Judge
    84 SVO 24K miles, 85 Mclaren Capri Vert. 84 GT Turbo Vert.
    88 Mclaren Mustang Vert 20K miles, 89 Mustang LX Sport Vert,
    03 Mach 1 7900 miles, 74 Mustang II, 69 Mustang, 67 Mustang, 07 GT500,
    14 Mustang CS/GT, 15 F150 FTX Tuscany, 16 F250 Crewcab, 67 Tbird 47K miles

  7. #7
    FEP Super Member erratic50's Avatar
    Join Date
    Dec 2016
    Location
    Nebraska
    Posts
    4,575

    Default

    Clean the throttle body. The back of the butterfly gets caked in carbon then the hunt and idle like crap. Also check your timing. They have a tendency to hunt a little more slightly advanced than they do at bone stock 10 degrees. Personally I always put up with it for the additional tire frying raw power right now of being turned up. 13.5-14 degrees is usually about all the SD E6 cars want. This is especially true with the VM1 ECU tune which is more aggressive on advance than the VR1 or DS1, etc. A bit of extra base advance on a VM1 makes an E6 head 86 run like a raped ape though. With a guy on the stick that can get the launch just right they will out-do a 93 Cobra on 0-60. (Or at least mine always did)

  8. #8

  9. #9

    Default

    A fool can adjust a TPS on an 86 if your aftermarket throttle body has a built in adjuster.

    Ed

    Quote Originally Posted by stangPlus2Birds View Post
    My condolences on the loss of your friend.



    I got my '86 GT from, my BFF.
    We went through college, then grad school, then even moved to the same area out of State.
    My friend bought the '86 GT brand new, custom order the options he wanted. I drove him to pick it up.
    I know the history of my car very very well. I also know a lot about the engine/concerns/etc the '86 5.0.


    About the '86:
    Congratulations, it sound like it's an authentic '86 Stang GT.
    And, not some fake China clone. ;-)


    About the '86 5.0L Stang engines:
    In order of likely-hood for producing a hunting idle problem:

    1) The IAC gets filled with carbon from the EGR valve, and sticks.
    Get a new **FORD OEM** IAC.
    Note: A short-term solution is to clean it out with Acetone.


    2) The EGR valve may not fully seat. Get a new **FORD OEM** EGR.


    3) There were a few TPS units that were bad, and moisture would get in, and they would produce erratic signals
    If it's original, get a **FORD OEM** TPS.
    The '86+ TPS units are not adjustable. Sorry, only fools can adjust a non-adjustable part. :-)


    4) Sorry, the program storage in PROM(like a write-once flash-drive) was very very very very limited. The did a lot of hacks to get fit, what was fitted.
    What they did not have room for was the ability to limit the "integrator" wind-up in the "PID" control loop that controls the IAC and idle speed.

    So... even on a 100% perfect '86 5.0L engine, there is that chance that the idle will hunt. This is from a classic "Integrator wind up in the PID control algorithm". I think I learned that, and they teach that, in the 3rd year of engineering in a Control Systems class.
    The hunting idle will most likely happen when the engine is cold, or warming up.
    But, without that "Integrator" limit, or a more advanced control algorithm (that there isn't room for), even when the engine is warm and happy, the idle can go into hunting mode.

    The solution? Shut off the engine, and restart the engine.
    Even after my engine was rebuilt, and I had a new IAC, new EGR, etc, etc, the idle would hunt when the temperature was below ~70F->60F.


    Some people move to the MAF-based setup.
    That cures that hunting - assuming stock engine, and no other problems, of course.
    I did that for a number of years. But, when it was time for an engine rebuild, I decided I wanted to go back to SD. My coil is relocated to the passenger-side. And, that had some impact on my decision. Not much, but not zero.


    Btw:
    Any pictures?
    Any pictures?
    Any pictures?


    Yea, we leg-hump cars like mad in these forums. Just like it should be!

  10. #10
    FEP Super Member erratic50's Avatar
    Join Date
    Dec 2016
    Location
    Nebraska
    Posts
    4,575

    Default

    I usually take a drill but to the screw holes on my TPS. Then I can dial the voltage in so it's right where I want it and screw it down tight. In the past I have buttered the bolts up with JB weld first so it can't move if the bolts come loose. Not needed and makes for a pain to replace the TPS next time.

  11. #11

    Default

    The voltage on a to a hardly matters. The range isn't .99-1.0 volts, its anything from .67-1.27 or something like that. Anywhere in that range, it is working correctly. Just make sure base voltage is at least 3.5 v higher with no dead spots. That's it.

    Want it to run good, not miss or hunt at idle? Run codes. Once you VERIFY no major problems exist, then you set idle. Unplug Ian. Turn screw till it barely runs, plug in iac. Done.

    I've had cars with over 300k miles with zero hunting at idle.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  12. #12

    Default

    TPS voltage (partially) dictates to the computer how much load the engine is experiencing, which affects fueling injected based on it, and the manifold absolute pressure (previously known as intake manifold vacuum), and ACT, and ECT, and etc., to look up table/s in the computer for air:fuel ratios. I remember all the magazine articles... All of the pioneering successful early EFI modifiers and racers (and a number of those who created and now still market adjustable/enhanced control systems), when EFI was still a baby, found that the best idle/off-idle air:fuel-ing came from ensuring EEC-IV idle TPS voltage was at about 0.98v, and having acceleration enrichment being anticipated for quickly instead of happening as more so of a reaction to acceleration. A slight higher voltage (again, partially) providing a slight richer air:fuel ratio, as an engine warmed to operating temperature and at idle should have. There is a difference, and it has a measurable affect on most efficient idle and off-idle quality and responsiveness... theoretically, the same way an efficient carburetor's idle and transition circuits' calibration when right and proper idle mixture screw adjustments have... albeit EFI doing so with many more compromises, and mediocrity, and lesser efficiency ... again I say, lol, shooting for "stoich" or 14.7:1 AFR (and the other fixed AFR ratios contained in factory EFI lookup tables) for ALL scenarios is far and away more about catalytic convertor efficiency than it is about FUEL efficiency and efficient and powerful engine and vehicle FUNCTION.
    Last edited by Walking-Tall; 01-26-2018 at 04:08 AM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  13. #13
    FEP Super Member erratic50's Avatar
    Join Date
    Dec 2016
    Location
    Nebraska
    Posts
    4,575

    Default

    TPS is a big factor in how fast an engine will rev up when you mat the gas. Base timing is also.

    You are supposed to put the idle at the correct RPM also.

    Personally I always loved how fast my car would rev up when it's idling at 900 with the TPS at 0.98 with a crapload of base timing. Also when the light turns green just ease out on the clutch, no need to give it any gas unless you're in a hurry.

  14. #14

    Default

    And if you get a new IAC, take the diode out of the line that plugs into it.

  15. #15

    Default

    Yeah, except if you know how the computer works, the theoretical .98v is reset every time you turn the key on and off. If it's .98v key on engine off, its probably 1.1v at idle. Its all based off of the 5v reference voltage, which is regulated (and not very exactly) off the computer input.

    Each and every time you start the car, the computer checks tps voltage and adds it into a table.

    I've had everything from 1.29v to .8v reading on the same car. If it had any appreciable effect on fuel trims, then it would have significantly changed gas mileage or power, which it did not.

    Anyone who has ever seen a megasquirt or quarter horse tuning log, it shows the to a voltage. Unless you specifically uncheck the check voltage at startup, it will reset each, and every time, the car is started and enters it into a chart as x.

    My guess is there is a ton of placebo effect. I remember when I was 16 and spent all day doing a tune up on my car. Took it around the block feeling pretty good about how much smoother it was running and it felt like it had more power. A week later, I pop the hiid and realize there is a spark plug hanging off the wire. When I tightened it back in, I realized I forgot to tighten down one while side of the motor, they weren't even hand tight.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  16. #16

    Default

    Quote Originally Posted by Haystack View Post
    Yeah, except if you know how the computer works, the theoretical .98v is reset every time you turn the key on and off. If it's .98v key on engine off, its probably 1.1v at idle. Its all based off of the 5v reference voltage, which is regulated (and not very exactly) off the computer input.

    Each and every time you start the car, the computer checks tps voltage and adds it into a table.
    Is there reference EEC-IV engineering design information for these statements somewhere? I keep seeing such statements being stated, and I love to learn. TPS is a potentiometer, a variable resistor (most resistance at idle, least at WOT), and 5v is easy to maintain with a 12v system... that's why VREF is 5v and not 12v.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  17. #17

    Default

    Guys way smarter then me, mainly joel5.0

    http://sbftech.com/index.php/topic,1....html#msg12363

    In a few places he explains that anything from .8-1.0 v is acceptable, but in others he says that as long as you have no dead spots and no tps code, you are golden.

    The exact voltage numbers actually do vary by car and actual computer tune, but the .49-1.19v is the accepted range.

    Each time you turn the key, the computer takes the lowest voltage from the tps signal wire and uses it and the base to measure from for wot.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •