5.0 Is Driving Me Insane

Discussion in 'Fox 5.0 Mustang Tech' started by J-mo, Jul 29, 2013.

  1. I know this is the most common problem for fox mustang but this has everyone stumped.
    Car was originally a 4 cyl. and I have converted it to 5.0 efi. harness and a9l computer. The car will start then die. I can feather the throttle for about 1 min and get it to idle but soon as you blip the throttle hard or restart it will immediatly surge to point of dieing. dies at stop light/signs. I can not get this car to idle right what so ever. here are the mods
    Stock 95 short block
    flo tek heads (gt40 aluminum X equivalent) 1.90in 1.56ex 180 runner 58cc
    B-cam ( I know its a dinosaur cam) (cam is 4 degress advanced)
    Explorer lower and upper (port matched to heads)
    Scorpion 1.72 stud mounted rockers
    Professional products 70mm throttle body
    Under drive pulleys
    Granatelli CAI
    Stock injectors
    Stock mass air meter
    Ceramic shorty headers
    High flow cats/ x pipe/ magnaflow mufflers (2.5in all the way)
    Aluminum flywheel/driveshaft
    New IAC with bypass (fully open)

    I have done the serging idle check list and everything checked good.
    TPS voltage at .98
    Cleaned 1o pin
    O2 are switching (both new)
    Mass air at .9 volts at idle
    Grounds are good.
    ACT and ECT voltage/ohms measure good (both new)
    IAC good (new)
    No codes
    Fuel pressure 38 plugged 42 unplugged (new pump)
    Timing set 10 degress spout out, 21 spout in (new distributor)
    New plugs/ wires/ coil
    Reset the KAM on every adjustment

    I have swapped PCM and mass air with known good parts off another modded non tuned fox
    so i'm starting to rule out electrical. The injectors and mass air come off a perfectly good car.

    The car will idle with the mass air unplugged or with a small vaccum line unplugged so the car i know is runing rich. the plugs are black and only have 20 miles.

    Can anyone point me in the right direction please.
  2. The car shouldn't be running rich. That set up with stock injectors and MAF should be running lean.
    Check to make sure your fuel pressure regulator hasn't gone bad.
    Oh your MAP sensor, you have it venting to the atmosphere or do you have vacuum to it? It's supposed to vent to the atmosphere. I heard that can do some funky stuff
  3. MAP is atmospheric. With the fuel pressure dropping when vaccum applied to it means it good??
  4. 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 125,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
  5. I already did the surging idle check list and posted the test result are good. Its dumping fuel and i know the PCM is working properly due to swapping with a known good one. 19lbs injectors are flooding out this setup. injectors maybe??
  6. Check your maf again. Try to start car with it disconnected.
  7. It stumbles at first then idles. Reinstall plug then it idles up then surge and dies. Maf reads .9 volts at idle. Replaced with known good one and same results
  8. That's strange. 1990Coupe had that same problem. It'd run with the MAF unplugged, then die if plugged in. I sent him another used MAF and it solved his problem. Sucks that's not your problem man.

    What is the FORD part number on your computer? I'll run it in our system to make sure it's the year your were told it was.
  9. I know. Its driving me absolutely crazy.
  10. Get me that 12A650 number...the full thing I'll run it for you.
  11. What year is your car and what year harnesses are you using? 86-88 harnesses will swap with each other, 89-90 are the same, 91 is unique and will not work with another model year. 92-93 are identical. The problem is with the headlight harness iirc.
    The 94-95 electronics arent even remotely compatible.
    SVT32VDOHC likes this.

  12. That's another big question, what year is your car itself? And what year engine harness was used? Did you keep the original body harness from selenoid to taillights? What year MAF and computer you have?
  13. Car is a 1993. Harness and a9l are 1989. Didn't use dash harness due to custom gauge cluster and fuel relay is external from factory. I will get you the mass number. First 2 I know are E9 so I know its an 89 year meter
  14. Yeah, post em and I will run em.

    These numbers:

    ID E8ZF-AA ...
    ID F2PF-AA ...
    ID F2PF-AARM .

    88-89 Mustang 5.0L only
    These numbers:

    ID E9ZF-AA .....
    ID F1ZF-AA .....


    Cougar '91-93 6-232 (3.8L)
    Cougar '91-93 8-302 (5.0L)
    Lincoln Continental '91-94
    Lincoln Mark Series '89-92 (8-302, 5.0L)
    Mustang '88-89 (8-302, 5.0L), ID E9ZF-AA
    Mustang '88-89 (8-302, 5.0L), ID F1ZF-AA
    Mustang '90-92 8-302 (5.0L)
    Mustang '93 8-302 (5.0L), exc. Cobra
    Sable '91-95 6-232 (3.8L)
    Taurus '89-90 (6-183, 3.0L, SHO)
    Taurus '91-95 6-183 (3.0L), SHO
    Taurus '91-95 6-232 (3.8L)
    Taurus '93-95 6-195 (3.2, SHO, AT)
    Thunderbird '91-93 6-232 (3.8L), w/o supercharger opt
    Thunderbird '91-93 8-302 (5.0L)
  15. Ok I will check when I get home and let you know asap
  16. E9ZF-12A650-A2A
  17. Found vacuum leak at egr spacer. Fixed and is better but no perfect. Still surges and the o2 aren't switching. Stuck reading 0.02 at idle and switch erratically on throttle. Measured voltage unplugged and the voltage slowly rises to .6 (both sides) but when replugged they immediately drop back to 0.02. What the **** is going on? How should I go about testing this?
  18. Im not sure if this would have anything to do with your problem, but it might be worth talking about. What year is the O2 harness? Is it the right one for your tranny? Is the loop in the right spot?
  19. I'm not sure the year o2 harness. The pins and colors match up. What do you mean loop?
  20. Sorry for the delay bud. What i mean by the loop is at the connector for the o2 harness. The connection is near the throttle body, close to the fender apron area. The o2 harness plug has 8 wires and is round. Peel back the tape about 6 inches or so, and look for a jumper wire. This is on the O2 sensor side of the harness, NOT on the car computer side of the harness. Its just a loop of wire that connects 2 pins together. The connector im talking about is at the bottom of this picture. You need to see where your jumper is, to make sure you have the right one for your setup. Yours might be fine, but why not check on it to make 100% sure, will take you less than 5mins. If you have the jumper in the wrong location, it can cause some issues. If you look at the picture, you will see an , A, M, J. This means, Auto, manual, and jumper. If you have an A9P harness and computer, then you want the jumper to go to the A pin, and the M pin is left open. And if you have a A9L harness and computer, you want the jumper to go to the M pin and A is left open. Hopefully you understand what i mean. Forgot to say, the jumper is usually purple/yellow, and is about 5" long. Goodluck