Engine Runs Badly When Warm

Discussion in 'Fox 5.0 Mustang Tech' started by Mantoga320, Sep 13, 2017.

  1. 1990 GT 5.0, manual, 240K. I've been chasing this problem for a long time. After a cold start up, it runs great and I can drive it around for a while. After a good warm up it starts to run badly - bouncing idle (600-800), stalls at idle, poor throttle response, trouble re-starting. If I let it cool down for about 15 minutes, I can restart and get a smooth idle again, until it warms back up. It seems to me like a bad sensor is coming into play as the computer switches modes from cold to warm. Any ideas before I throw more parts at it? So far I have: blocked off EGR, new throttle body, TPS, and Idle Air Controller, new plugs and wires, distributor cap and rotor, new MAF sensor, Coolant temp sensor, thermostat, ignition coil, PCV valve, O2 sensors. The computer was replaced some years ago.
  2. Run the codes
    I suspect a failing TFI
  3. what is your tps voltage at?
  4. Okay, thanks. I've had mechanics running the codes and nothing shows up. It's been intermittent, but recently getting worse. I thought about getting a code reader myself. Any recommendations? And any recommendations for a TFI module? I see MSD, Accel, Performance Distributors dynamodule, and Motorcraft. And it says there's a special tool for it. I'm living in Germany, so I have to ship stuff there. Usually from American Muscle, CJ Pony Parts, or LMR. Should I think about replacing the whole distributor?
  5. About 1.00. My son and I put on a BBK 70mm throttle body and EGR spacer as part of a trouble shooting and slow upgrade program. We were adjusting the throttle stop screw, and we'd set the voltage between .90 to 1.05.
  6. TPS Troubleshooting and testing

    Revised 23-Oct-2016 to add new link for 10 pin connector maintenance.

    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. There is no advantage to setting it to .99; that is a BOZO Internet myth, complete with red nose and big floppy shoes.
    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.

    No code 11 (two flashes when the computer dumps the codes)? A perfectly functioning system will always get a code 11, it is the everything is OK code. The 11 code is computer passed its internal self test.
    If you don't get an 11, you have computer or wiring problems.
    Please check and repost.

    Dump codes sticky

    Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
    I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
  7. Okay, thanks again. I've seen those threads, and went through some of the recommended steps. I'll check the codes myself, and for the TPS I'll check the reference voltage and use the ground wire instead. The throttle instructions had me grounding to the intake manifold. I'll order a TFI module just in case. And maybe a distributor (so many years and miles on the original). My work takes me out of town a lot, and in about 5 weeks I'll be able to get back to it. Good to have some parts standing by, even just as spares, with the delay in international shipping. I'm ordering an ACT sensor and a PCV valve/grommet/screen combo as well. If I can get past this annoying problem, we can start getting into "upgrading for fun and excitement" as a father and son project. These cars are great for bolt-on's, but I can see from the threads that the early-generation EFI's develop problems that leave people scratching their heads. "Dump the codes" - got it!
  8. The thing that sucks about any brand of new TFI's these days are they seem to be very undependable. I don't even trust a new made in China Motorcraft TFI as I had one that lasted about 10 minutes in my car before it failed. If you can find an older Motorcraft TFI with Motorcraft actually stamped on the TFI would the way I would go. But if you find one they are pricey, north of $100. But then again how much does one pay for a dependable TFI?
  9. Thanks for the info. I was assuming Motorcraft would be the way to go. I saw some product reviews saying the same thing, and about the distributors as well.
  10. rock auto
    Mantoga320 likes this.
  11. Rock Auto is a great tip, thanks.
  12. So have you run the codes?

    The TPS on these cars is non adjustable. You can twist it all you want but the ECU simply takes whatever voltage it's at during startup and marks take as idle reference. 0.5 to 1.19volts. You can chase 1.0 all you want but it Makes no difference. If the computer doesn't like the setting it will spit out a code 63/23.

    So, of it is sensor related have you dumped codes?
  13. I won't be back home to work on it until late October, unfortunately. I will dump codes and post what I get. For the TPS, the new throttle body we put on came with a TPS, so we followed the instructions to unplug the IAC, set the throttle stop screw for 800 rpm, and then plug in the IAC and check the TPS voltage between .7-1.1. We had a safety pin in the green wire and grounded to the intake, and rotated the TPS slightly to get the reading in the range. But every time the engine was really warm, it started acting up (bouncing idle, stalling, poor throttle response). So we need to fix whatever that is. I'm ordering a distributor with TFI to be waiting when I get back. On test drives during warm up, the car would have an occasional "jerk" under acceleration, and the CEL goes on and off randomly.
  14. you have taken a wrong turn...

    IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.


    Automobile computers use current sink technology. They do not source power to any relay, solenoid or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from positive to negative is what provides the energy to make the IAC, fuel pump relay, or fuel injectors work. No ground provided by the computer, then the actuators and relays don't operate.

    We are going to supply an artificial ground path to the IAC instead of letting the computer supply the ground.

    Start the engine and let it warm up.

    Take one of the cheap inline fuse holders with a 5 amp fuse in it. Use it to bypass the blue/white wire to ground. You'll have to get creative probing the back side of the IAC wiring with safety pins or paper clips. Since the computer doesn't supply any voltage , but supplies a ground, that can't hurt the computer. The 5 amp fuse protects you and the wiring if there is an internal short in the IAC coil.

    The engine should speed up when the fuse holder wire is grounded and slow down or stall when the fuse holder wire is disconnected from ground.

    Recommended procedure for cleaning the IAC/IAB:
    Conventional cleaning methods like throttle body cleaner aren’t very effective. The best method is a soak type cleaner used for carburetors. If you are into fixing motorcycles, jet skis, snowmobiles or anything else with a small carburetor, you probably have used the one gallon soak cleaners like Gunk or Berryman. One of the two should be available at your local auto parts store for $22-$29. Take the solenoid off the body and set it aside: the carb cleaner will damage some types of plastic parts. Soak the metal body in the carb cleaner overnight. There is a basket to set the parts in while they are soaking. When you finish soaking overnight, twist the stem of the IAB/IAC that sticks out while the blocker valve is seated. This removes any leftover deposits from the blocker valve seat. Rinse the part off with water and blow it dry with compressed air. The IAC/IAB should seal up nicely now. Once it has dried, try blowing through the bottom hole and it should block the air flow. If it doesn't block the airflow, there is still something that is gumming up the works. Reassemble and reinstall to check it out. Reassemble and reinstall to check it out.

    Gunk Dip type carb & parts soaker:

    Setting the base idle speed:
    First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

    Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

    This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

    Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

    Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
    Changing the mechanical adjustment changes the TPS, so you will need to set it. Anything between.6 and 1.0 volt is good. There is no advantage to setting it to .99; that is a BOZO Internet myth, complete with red nose and big floppy shoes.

    When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

    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. It includes how to dump the computer codes quickly and simply as one of the first steps. 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 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
  15. Okay, we'll get the codes, do those checks and follow those steps when I get back, and I'll post the results. And I'll have a few parts standing by. Thanks.