88 5.0 Ho Convertible Rough Idle Stall Backfire Issue

Discussion in 'Fox 5.0 Mustang Tech' started by tekdc911, Jul 31, 2013.

  1. bought a 88 5.o lx convertible had issues from the start knew it had problems when i got it $900 5.0 HO with 90 k original miles rough very rough miles from the way it acts car is tight motor acts like it has plenty of power when it acts right for the 1/2 a second that it acts right in intervals
    after 45 mins stalls out and 30 mins or so later it fires back up but is bogged down
    stomp the pedal and it just very slowly revs if you let off of it fast it grabs and idles up to where it should of been when you had it floored

    ok have replaced and checked

    checked for vaccum leaks

    have not replaced the pip sensor
    i figured by its age it would probably have a leaking capacitor on the ECM board havent looked

    the pip and tfi act the same when bad dont they ?

    Here's the WHOPPER went to buy a ECM and cannot for the life of me find one for the convertible 5 speed 5.0 HO
    only for the 5 speed sedan would this one work?
  2. replaced the fuel filter aswell
  3. Dump the codes
    tekdc911 likes this.
  4. is thats what its called if i look it up ?
    im a chevy man my old lady wanted this car and i know they scoot so figured id get it right :)
  5. Check fuel pressure. Is it mass air?
    tekdc911 likes this.
  6. no its the SD
  7. thats the main problem im having
    from what i understand no one has the ECM for this motor so either im going to have to add a mass air or rebuild this one myself
    3 parts stores same answer not available
    one actually told me good luck finding it cause he didnt see it happening
  8. will the sedan 5 speed ECM work on my convertible ?
  9. What's making you think its the processor?
    tekdc911 likes this.
  10. its doing random things at random times
    from half throttle to no throttle it acts like its backwards accels as you let off the pedal
    some of the gauges arent working
    but there is a receipt and boxes in the trunk for oil pressure (doesnt work)and water temp(half ass works)
    fuel ( works when it wants to)
    unless there is something else that can go out and makes it randomly fall on its face and bog down after 10-20 mins of operation then eventually stall out
    until it cools for 30 mins to an hour
    was going to change the PIP
    but the distributor looks fairly new
  11. So its runs fine at first then bogs after 20-30 min of driving then has to sit for 30min to run ok again?
    tekdc911 likes this.
  12. yes sometimes others its rough from the first thing in the morning
    koeo was 22 11
    map sensor out of range ?
  13. Don't waste your money by throwing parts and money at a problem. Test, diagnose and determine the root cause of the problem. Computers seldom fail, it is usually a sensor or wiring.

    The first step is to dump the codes. Post your results with the numerical codes and I will try to give you 5.0 Mustang specific code definitions and fixes.

    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 (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30-$36.
    stykthyn likes this.
  14. I had a '89 T-bird that was doing random bad things, poor idle, erratic running , stalling, etc.--everyone said the ECM rarely fails, but no one could figure out what the problem really was. I spent $600 with "experts" who all failed to correct the problem but charged me lots of cash for their time anyway.
    Finally a friend said to try a used ECM. I found one for $23 shipped on Ebay. We plugged it in and that car ran like new instantly, no further problems! I wish I had been able to see what the problem was before spending all the money!
  15. You can use a computer from a sedan/hatchback. It doesn't need to be from a convertible.

    You just need an 86-88 5-spd speed density computer. Should be cheap and readily available on Ebay.

    However...before relplacing the computer, run the codes. 1988's do not have a functional check engine light. Did you run the engine running codes as well?
    tekdc911 likes this.
  16. like i said i just bought it and im not a ford man i happened to the answer to this i believe
    while replacing vaccum hoses i failed to properly connect the one going to the brake booster
    it came loose going down the road car came unglued so much power
    FAIL it had no brakes >.< had to stand on them so im hooking it back up and noticed that there where 2 bolt holes on the fire wall
    something was missing i later found some under the hood pics and found that it was a vaccum manifold that went there
    cruise control and ac vent now work
    at the same time i realized that it is running rich couldnt smell it with all the lucas injector cleaner i had in it
    fuel pressure regulator ?
    its something to do with the vaccum in relation to the fuel delivery
  17. clutch depressed ?
    ill do this tomorrow
    the first koeo was 22 11 but i didnt have the clutch pressed
  18. Code 11 - Computer passed its internal self test.

    MAP/BARO sensor operation and code 22

    Revised 19-Jul-2011 to add functional descriptions for MAP and BARO operation.

    On a Speed Density car, the MAP/BARO sensor is connected to the intake manifold and acts to sense the manifold pressure. Lower vacuum inside the intake manifold when combined with more throttle opening measured by the TPS means more airflow through the engine. As airflow increases, fuel flow through the injectors needs to increase to keep the air/fuel ratio where it needs to be. When manifold vacuum increases, the engine is either decelerating or idling, and it needs to reduce the fuel flow through the injectors.

    On a Mass Air car, the MAP/BARO sensor vents to open air and actually senses the barometric pressure due to changes in weather and altitude. Its purpose is to set a baseline for the computer to know the barometric pressure. As barometric pressure decreases, it leans out the fuel flow to compensate for less oxygen in the air. When the barometric pressure rises, it increases to add fuel since there is more oxygen in the air. The fuel requirements decrease as altitude increases, since the atmospheric pressure decreases.

    Disconnecting the MAP or BARO sensor will set code 22.

    Misconnecting the BARO sensor to vacuum on a Mass Air car will cause the computer to lean out the fuel mixture.

    Code 22 or 126 MAP (vacuum) or BARO signal out of range. The MAP or BARO sensor is pretty much the same sensor for both Mass Air & Speed Density cars. The main difference is where it is connected. Mass Air cars vent it to the atmosphere, while Speed Density cars connect it to the intake manifold vacuum. Its purpose is to help set a baseline for the air/fuel mixture by sensing changes in barometric pressure. The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it. There a very few DVM’s with a price tag under $40 that will measure frequency, but there are some out there.

    The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.

    Baro or MAP test using a real frequency meter - run the test key on, engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout. And oscilloscope is very useful if you have access to one or know of someone who does. With an oscilloscope, you can see the waveform and amplitude.

    If it is defective, your air/fuel ratio will be off and the car’s performance & emissions will suffer

    Some basic checks you can make to be sure that the sensor is getting power & ground:
    Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
    Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

    The following power on check requires you to turn the ignition switch to the Run position.
    Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.
    tekdc911 likes this.
  19. The MAP/BARO sensor is mounted on the firewall behind the upper manifold on 86-93 Mustangs.

    had a hole in that line replaced same error
    made a poor man in a pinch vacum manifold with some brass nipples and galvanized 3/8" t's
    its no longer flooding

    so another question
    is it possible that the fan clutch is heating up and not moving enough air it seems to run warm now that its firing right
  20. forgot to add this question also

    is a thermostat required on these motors or will it run with out it ?

    also is there a supposed to be a gasket between the high rise and the intake or is this a machined fit ? i sprayed some intake cleaner where it meets but it didnt change idle so i guess its sealed but you would figure a gasket goes there
    #20 tekdc911, Aug 4, 2013
    Last edited: Aug 4, 2013