Engine Help With Never Ending Hessitation/bad Idle Problem I Cant Figure Out...

Discussion in 'Fox 5.0 Mustang Tech' started by more4u2c, May 2, 2014.


  1. more4u2c

    more4u2c New Member

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    Hello Im new here and I hope someone can help me out with my hesitation/idle issue. OK so I bought a completely stock 88 mustang Lx 5.0 AT ASCMclaren edition about a month ago and it had this hessitation/idle issue that I thought would be a easy fix... man was I wrong! The car was like a barn find as It was owned by a old lady whom passed away and I got it for a great deal at a estate sale. Im estimating that the car sat for a few years cobwebs everywhere. the owner kept evey recipt and service record and the newest one was in 2007 so im assuming it was parked and stored after that.

    So the issue started out with hessitation when acceleration gradualy from idle to about 2k RPM. after 2k RPM it smooths out and drives normally. at idle it basiclly bounces around at first itll idle fine then the idle dips down to about 500rpm then the car goes all panic mode trying to get the idle back up to normal bounces around then levels out for a second or two then dips down then freaks out etc.

    now at first this issue only happened after the car was ran for 30 minutes or longer as long as the trips were under a half hour the car runs great, normal idle no hessitation. but now it seems to be getting more worse. It no longer takes a half hour now its like within 10 or 15 min of starting the car. and its getting more violatant with the hessitation and idle going hella crazy. so it is getting progressivly worse.

    Here is what Ive done to fix the issue that have failed each time...

    Replaced intake with GT40 (explorer) intake and fuel injectors
    Both coolant sensors
    ACT sensor
    fuel filter
    fuel pressure regulator
    fuel pump
    plugs, cap, rotor, wires, coil, and distributor
    BBK H-Pipe (no cats)
    all new vaccum lines
    Set timing to 10 degrees
    removed all grounds cleaned suface with grinder. (amazing how ford attached them to painted parts)
    G3 Alt upgrade w/ upgraded ground wires

    Ran all computer test and get 11 codes and a all good 9 for the cyl bal test.

    Removed and cleaned the IAC, EGR, MAF.

    My question is this with no codes in the computer and all self tests come back good. can a MAF, TPS, etc be bad but the computer still thinks its good? Or as long as the computer has no codes it means all the sensors are good?

    Any info or ideas whats wrong with my new fox would be great.
    #1
  2. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    Perform base idle reset
    #2
  3. more4u2c

    more4u2c New Member

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    I did that yesterday except for the TPS reset. due to the fact I don’t have a volt meter to set it. I can say that when I started the car with the IAC unplugged the base idle was barely above 500rpm. moved it to 800rpm turned her off then plugged the IAC in then started and idle for 2 min then off then back on w/ all accessories for two min then off again. It seemed to help at first but then went right back to her old hesitation/bad idle ways. Do you have to "set" the TPS? I have read conflicting posts some say its a real thing you need to do and others say its not needed due to the computer zeroes out that sensor every time the car is started...
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  4. more4u2c

    more4u2c New Member

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    oh and new o2 sensors
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  5. 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.
    #5
  6. jAEded

    jAEded Active Member

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    jrichker, we just like to see it posted over and over again. :)

    more4u2c - multimeter might be a good investment. You can get one pretty cheap.
    #6
  7. more4u2c

    more4u2c New Member

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    thanks guys ill check it out. one more thing I have the Idle adjustment plate installed on my car were these a factory intalled thing? Or did the privious own put it in you think?
    #7
  8. jAEded

    jAEded Active Member

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    The previous owner did it. With the mods you have you shouldn't need one.

    My top end is wide open and my car idles good without the adjuster plate.

    It would help with your issue if you could remove some variables.
    #8
  9. more4u2c

    more4u2c New Member

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    So I sprung for a multimeter lol and tested my tps it was at 1.4 and wouldn't go down from there so I figured I'd buy a new one if it fixes it cool if not return it and well it fixed it about 85% I can definitely drive the car now and it definitely has a lot more power or throttle response. But I still have a very slight idle wiggle I'm assuming it's because of that idle adjuster plate so I'll remove that tomorrow and do another base idle reset. Can anyone tell me after such a reset will it take a few trips/miles for the computer to learn how to idle just right or will it do it within a few minutes?
    #9
  10. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    computer will adapt right away
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  11. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    you can drill out the holes to make them oblong to give you more adjustment
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  12. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Setting the TPS at .98 or .99 volt is a Bozo Internet Myth. When the computer powers up and initializes the TPS sensor, whatever it reads is the zero starting point for it. In other words your .98 volt becomes 0 volts and the computer watches for increases in voltage from that point upward.

    Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy. Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.


    Wire colors & functions:
    Orange/white = 5 volt VREF from the computer
    Dark Green/lt green = TPS output to computer
    Black/white = Signal ground from computer

    TPS troubleshooting steps:
    1.) Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.
    2.) Use the Dark Green/lt green & Black/white wires to set the TPS base voltage. Try this... All you need is less than 1.0 volt at idle and more than 4.25 at Wide Open Throttle (WOT). You'll need a voltmeter with a 1 or 3 volt low scale to do the job.

    The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running. Set the voltmeter on the 2 volt range if it doesn’t auto range.

    Here’s a TPS tip I got from NoGo50

    When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

    (copied from MustangMax, Glendale AZ)

    A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

    B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

    C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

    D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

    The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

    TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.

    TPS will not go below 1 volt

    Note: Make all resistance checks with the ignition switch in the OFF position. Failure to do so will result in incorrect results and may possibly damage the meter.

    Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 1.5 ohms. Higher resistance than 1.5 ohms indicates a problem with the 10 pin connector, computer or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.

    [​IMG]

    See the graphic for the location of the 10 pin connectors:
    Diagram courtesy of Tmoss & Stang&2birds

    [​IMG]

    See the graphic for the 10 pin connector circuit layout.
    [​IMG]

    Unplug the white 10 pin connector to do some resistance testing. It is good time to clean the connector pins and examine the connector for corrosion, broken wire or other damage. See http://oldfuelinjection.com/?p=85 for help in this department.

    If the resistance on the TPS Black/White wire and pin 1 of the white engine fuel injector harness 10 pin connector, is more than 1.0 ohm, you have bad connection or broken wiring. Repeat the test using the pin 1 of the white body side 10 pin connector and battery ground. You should have less that 1.5 ohm. More than that is a damaged signal ground inside the computer or bad connections or wiring.
    Last edited: May 3, 2014
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  13. v8only

    v8only Active Member

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    So you've been throwing parts at it, and haven't pulled codes yet? You can't even begin to troubleshoot a computer controlled vehicle without pulling codes first. Even if the light isn't on, a code can still be stored. Try running koeo and koer codes
    #13
  14. more4u2c

    more4u2c New Member

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    Um I've checked for codes re read my post smh... There are no codes it checks out both 11s and the balance test is is a pass. And I'm not just throwing parts all willy nilly I'm checking each part before replacement to see if it needs replacing. After the tps replacement it fixed it a lot but I still get slight hessistation upon slight gradual acceleration. But I'd say the tps took 95% of my issue away. Every part I replace gets me a lil closer to it running right lol but the tps by far helped the most but now I'm at a stand still idk where to look next...
    #14
  15. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    What did you get for resistance readings between the black/white wire on the engine side of the TPS wiring connector and battery ground?
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  16. more4u2c

    more4u2c New Member

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    I got .54 to .57
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  17. Killa

    Killa Active Member

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    My 95 GT 5.0 did the same thing, I replace IAC with advance auto part brand, and it burn that IAC up every 3-4 month, I suck it up and bought one from the dealer, last me a year.
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  18. more4u2c

    more4u2c New Member

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    Iac will cause hesitation? I thought it was only used to control idle? How do I test the iac? I'm thinking it might be a bad computer cuz its not everytime that it had this issue some trips is just fine but others it's fussy and wants to hessistate... is there a way to test the computer at home with my multimeter?
    #18
  19. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    There is no simple way to test the computer with a Multimeter other than to make sure it is getting good voltage at pin 1 & pins 37 & 57 with the ignition switch in the Run position,

    If the computer was at fault, you probably would see some codes, and it wouldn't give an 11 when you dumped the codes.

    How much progress have you made with the Surging Idle Checklist ?
    #19
  20. more4u2c

    more4u2c New Member

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    well that link for some reason seems to be missing IDK it might be my works firewall blocking me but, I have google searched that term "surging Idle Checklist" and have done things they say to do with no luck. Ive cleaned and widened the salt and peper shakers, checked my vaccum, set my TPS (LOL) and the ref is at .5 its crazy how this continues to be an issue I totally thought the TPS fixed it but only for a day then she started to act up again. maybe I should just swap out the TB for an exploer and see if that helps but I hear thats a PITA cus the linkage isnt right and you have to swap out the stock one. I kinda wanna get her running right before any more mods :(
    #20

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