Idles Sometimes But Won't Carry It's Own Weight

Discussion in 'Fox 5.0 Mustang Tech' started by 1badfoxmorgan, Apr 5, 2014.

  1. a 92 lx foxbody that has a 93 5.0 motor swap. I was driving around and stopped at a friends and tuned the car off, about 5 seconds later I turned the car back on and it ran like dog sh**. It would run but hardly run and it had a popping noise but not like a backfire
    My list of mods are as follows
  2. E303 cam
    BBk cold air
    Msd cap
    Msd coil
    Walbro 255 fuel pump
    New fuel pump relay
    New fuel regulator
    Checked red part of inertia switch and I couldn't push it down
    Explorer lower and upper intake
    BBk 75mm throttle body
    BBk shorty headers with exhaust
    Electric fan
    373's on a 95 gt rear end
    I just need to figure out why it idols but hardly responds to throttle imput half of the time and won't carry it's own weight
  3. I forgot to add that I have fuel rail noise coming from the passenger side fuel rail and almost 48 pounds of fuel pressure is what my sharader valve gauge is reading
  4. start with the basics;

    1: make sure the ignition timing is spot on

    2: check for vacuum leaks

    3: check the condition of the timing chain. easy enough to do, just pop the distributor cap, and turn the engine one way to take up the slack in the timing chain, then turn the engine the other way and see how far you have to turn the crank before the rotor starts to turn.
  5. Timing chain and timing is good, I had the timing set to 14 degrees and I turned it down to 10 to see if it would fix it, my inertia switch is reading 10v all around and my injectors are reading 11 and change, I pulled all of the plugs and I have great spark all around but they have small amounts of oil on the plugs, the motor is rebuilt but not by me. The car turns over fine and will run for a second and shut off. Anything else I can try? I'm in desperate need
  6. still havent checked for vacuum leaks though.
  7. Check fuel pressure:
    The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
    Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
    Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
    Turn the ignition switch on & start the engine. Observe the pressure: you should see 37-41 PSI at idle.
    Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.
    If you have a non-adjustable fuel pressure regulator,, you have either a restriction in the return line or a filing fuel pressure regulator.
    If you have an adjustable fuel pressure regulator, adjust the pressure using the above procedure to 37-41 PSI.

    Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

    Dumping the computer diagnostic codes on 86-95 Mustangs

    Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

    Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

    Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

    Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

    Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
    Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.



    If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

    What to expect:
    You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

    Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

    Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

    Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

    Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
    See AutoZone Part Number: 25886 , $10

    Alternate methods:
    For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

    Or for a nicer scanner see Equus - Digital Ford Code Reader (3145It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.
  8. That's my next task, I completely over looked it and hopefully that's the problem, what points could they be leaking from? Egr and fuel pressure ragulator? Is there any more
  9. Thanks you for the posts, I will do the test tomorrow and let you know
  10. vacuum leaks are myriad in location on a 5.0 engine. the upper and lower intake gaskets, the throttle body gasket, the egr valve, a multitude of vacuum hoses.
  11. Dumped the codes and they are as follow
  12. All those relate to emissions equipment, and the quantity of them tells me that some uninformed person removed the emissions equipment. None of the will cause the problem you are having unless the vacuum lines used to operate them are broken or flapping open in the breeze. That could be a source of vacuum leaks.

    Diagram courtesy of Tmoss & Stang&2birds
  13. Should I use the carb cleaner method
  14. I used carb cleaner and didn't notice any idol changes while spraying around the injectors, egr, intake gaskets, or the soft vacume lines, could maybe a clogged fuel filter do this? I changed the pump but not the filter. Or maybe a fuel pressure ragator? I have a video of the car doing what it's doing as well
  15. I missed code 87 the first time.

    Code 87 – fuel pump primary circuit failure. The fuel pump lost power while the engine was running. Check fuel pump relay, check inertia switch, wiring to/from inertia switch, red wire going to inertia switch for +12volts. Check the other side of inertia switch for +12 volts.

    Diagram of the fuel pump wiring for 91-93 cars.

    Fuel Pump Troubleshooting for 91-93 Mustangs

    Revised 08-Dec-2012 to add check for 12 volts at ignition coil to prove the ignition switch is good.

    Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
    It should run for 2-5 seconds and shut off. To trick the fuel pump into running, find the ECC test connector and jump the connector in the upper RH corner to ground.




    If the fuse links are OK, you will have power to the pump. Check fuel pressure – remove the cap from the Schrader valve behind the alternator and depress the core. Fuel should squirt out, catch it in a rag. A tire pressure gauge can also be used if you have one - look for 37-40 PSI. Beware of fire hazard when you do this.

    No fuel pressure, possible failed items in order of their probability:
    A.) Tripped inertia switch – press reset button on the inertia switch. The hatch cars hide it under the plastic trim covering the driver's side taillight. Use the voltmeter or test light to make sure you have power to both sides of the switch

    B.) Fuel pump Relay:
    On 91 cars, it is located under the driver's seat.
    On 92 and 93 cars it is located under the MAF. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors.
    Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
    C.) Clogged fuel filter
    D.) Failed fuel pump
    E.) Blown fuse link in wiring harness.
    F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
    for fuel escaping while pump is running.

    Theory of operation:
    Read this section through several times. If you understand the theory of operation, this will be much easier to troubleshoot. Refer to the diagram below frequently.

    Diagram of the fuel pump wiring for 91-93 cars.

    The electrical circuit for the fuel pump has two paths, a control path and a power

    Remember that the computer does not source any power to actuators, relays or injectors, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

    The control path consists of the computer, and the fuel pump relay coil. It turns the fuel pump relay on or off under computer control. The switched power (red wire) from the ECC relay goes to the relay coil and then from the relay coil to the computer (light blue\orange wire). The computer provides the ground path to
    complete the circuit. This ground causes the relay coil to energize and close the contacts for the power path. Keep in mind that you can have voltage to all the right places, but the computer must provide a ground. If there is no ground, the relay will not close the power contacts.

    The power path picks up from a fuse link near the starter relay. Fuse links are like fuses, except they are pieces of wire and are made right into the wiring harness. The feed wire from the fuse link (pink/black wire) goes to the fuel pump relay contacts. When the contacts close because the relay energizes, the power flows
    through the pink/black wire to the contacts and through the dark green\yellow wire to the inertia switch. The other side of the inertia switch with the brown\pink wire joins the pink/black wire that connects to the fuel pump. The fuel pump has a black wire that supplies the ground to complete the circuit.

    Power path:
    Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay).
    No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92 or later models the fuel pump relay is located under the Mass Air meter. Watch out for the WOT A/C control relay on these cars, as it is located in the same place and can easily be mistaken for the fuel pump relay.

    Relay: Turn on the key and jumper the ECC test connector as previously described. Look for 12 volts at the dark green\yellow wire (relay controlled power for the fuel pump). No voltage there means that the relay has failed, or there is a broken wire in the relay control circuit.

    Inertia switch:
    The location for the inertia switch is under the plastic for the driver's side taillight.
    There should be a round plastic pop out cover over it, remove it to access the switch button.
    With the test connection jumpered and ignition switch in The Run position as described above, check the brown/pink wire. It should have 12 volts. No 12 volts there, either the inertia switch is open or has no power to it. Check both sides of the inertia switch: there should be power on the dark green\yellow (inertia switch input) and brown/pink wire (inertia switch output). Power on the dark green\yellow wire and not on the brown/pink wire means the inertia switch is open.
    Press on the red plunger to reset it to the closed position. Sometimes the inertia switch will be intermittent or will not pass full power. Be sure that there is 12 volts on both sides of the switch with the pump running and that the voltage drop measured across the switch is less than .75 volts.

    Pump wiring: Anytime the ignition switch is in the Run position and the test point is jumpered to ground, there should be at least 12 volts present on the black/pink wire. With power off, check the pump ground: you should see less than 1 ohm between the black wire and chassis ground.

    Make sure that the power is off the circuit before making any resistance checks.
    If the circuit is powered up, your resistance measurements will be inaccurate.


    Control path:
    Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay.
    No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections coming from it. The ECC relay is located on top of the computer, which is under the passenger’s side kick panel. It is not easy to get to, you must have small hands or pull the passenger side dash speaker out to access it.

    Another possibility is that the ignition switch is faulty: look for 12 volts on the red/green wire on the ignition coil. No 12 volts with the ignition switch in the Run position and the ignition switch is faulty or the fuse link in the ignition wiring is blown.

    Relay: The light blue/orange wire provides a ground path for the relay power. With the test connector jumpered according to the previous instructions, there should be less than .75 volts.
    Use a test lamp with one side connected to battery power and the other side to the light blue/orange wire on the fuel pump relay. The test light should glow brightly. No glow and you have a broken wire or bad connection between the test connector and the relay. To test the wiring from the computer, remove the passenger side kick panel and disconnect the computer connector. It has a 10 MM bolt that holds it in place. Remove the test jumper from the ECC test connector.
    With the test lamp connected to power, jumper pin 22 to ground and the test lamp should glow.
    No glow and the wiring between the computer and the fuel pump relay is bad.

    Computer: If you got this far and everything else checked out good, the computer is suspect.
    Remove the test jumper from the ECC test connector located under the hood. Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer and everything else is connected. Turn the ignition switch to the Run position and observe the fuel pressure. The pump should run at full pressure.
    If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
    If it does run at full pressure, the computer may have failed.

    Keep in mind that the computer only runs the fuel pump for about 2-3 seconds when you turn the key to the Run position. This can sometimes fool you into thinking the computer has died.
    Connect one lead of the test light to power and the other lead to computer pin 22 with a safety pin.
    With the ignition switch Off, jumper the computer into self test mode like you are going to dump the codes. Turn the ignition switch to the Run position. The light will flicker when the computer does the self test routine. A flickering light is a good computer. No flickering light is a bad computer. Remove the test jumper from the ECC test connector located under the hood.

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
    for help on 88-95 wiring Mustang FAQ - Engine Information

    Fuel pump runs continuously:
    The fuel pump relay contacts are stuck together or the light blue/orange wire has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use an ohmmeter to check out the resistance between the light blue/orange wire and ground. You should see more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t jumpered to ground.
    If the wiring checks out good, then the computer is the likely culprit.

    Prior to replacing the computer, check the computer power ground. The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up next to the starter solenoid where the wire goes into the wiring harness.
  16. I changed the fuel pump relay and it did nothing, I have yet to check the voltage which I will do next and I checked the inertia switch and it wasn't tripped and it read 10 and change on both sides
  17. At idol I have around 46 pounds of fuel pressure and as I rev it it goes up even more
  18. No point in chasing the fuel pump as a source of the problem. You fuel pressure is high, which suggests the fuel pressure regulator is misadjusted, malfunctioning or there is a restriction or kink in the return line. Plan to troubleshoot and fix it AFTER you have the current poor running problem fixed.

    Does the poor running seem to get worse as the engine warms up?
  19. It stays the same after it warms up, and it does have some fuel rail noise on the passenger side fuel rail. I've been looking non stop and haven't found any vacume leaks
  20. I unplugged the maf and it was running near perfect so I suppose the maf is the problem