Code 96 Wot Miss/backfire

Discussion in 'Fox 5.0 Mustang Tech' started by Buddy, Aug 26, 2012.


  1. Buddy

    Buddy New Member

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    '93 5.0, basically stock, except for headers and PS and AC deletes. It starts fine, idles great, accelerates and cruises good; but on WOT misses and backfires like crazy. I pulled the codes and got a 96, fuel pump relay secondary circuit open. I replaced the relay, but didn't solve the problem. Any suggestions?

    Thanks for the help!

    Buddy
     
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  2. jrichker

    jrichker StangNet's favorite TOOL
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    Code 96 causes & tests 91-93 models. – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay or battery power feed was open - Power / Fuel Pump Circuits. The fuel pump circuit lost power at one time or another.

    Clear the codes by disconnecting the battery and turning on the headlights for about 5 minutes before reconnecting the battery. This will clear any remaining codes. Drive the car for several days and dump the codes again. In many cases, this clears the 96 code.

    Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump relay is located under the Mass Air Meter on Fox bodied stangs built after 91. On earlier model cars is under the passenger seat. On Mass Air Conversions, the signal lead that tells the computer that the fuel pump has power may not have been wired correctly. See Mustang Mass Air Conversion | StangNet

    Diagram of the fuel pump wiring for 91-93 cars.
    [​IMG]

    Look for power at the fuel pump - the fuel pump has a connector at the rear of the car with a pink/black wire and a black wire that goes to the fuel pump. The pink/black wire should be hot when the test connector is jumpered to the test position. To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower RH corner to ground. No voltage when jumpered, check the fuel pump relay and fuse links.

    [​IMG]


    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.

    [​IMG]

    91-93 Models:
    Using the diagram, check the dark green/yellow wire from the fuel pump relay: you should see 12 volts or so. If not the relay has failed or is intermittent. Check the inertia switch: on a hatch it is on the drivers side by the taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up the plastic trim. Check for voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black wire on the fuel pump relay: it is the power feed to the fuel pump. Good voltage there, then the fuel pump is the likely culprit since it is getting power. No voltage there, check the Pink/black wire, it is the power feed to the fuel pump relay & has a fuse link in it. Good voltage there & at the dark green/yellow wire, swap the relay.

    All testing is done with the ignition switch in the Run position. Do not forget this crucial step.

    The pink/black wire s should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the fuel pump has opened up.

    With the test jumper in place the green/yellow wire should be the same voltage as the pink/black wire +/- 0.25 volt.

    If not, look at the red wire: should have the same voltage as the battery positive terminal +/- 0.25 volt.
    If not, then check the yellow wire on the EEC relay located on top of the computer. This one is hard to get to. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the computer has opened up.

    If the red wire does not have the same voltage as the battery positive terminal +/- 0.25 volt and the yellow wire on the EEC relay does, then check the red/green wire on the EEC relay. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the ignition switch is defective or the fuse link in the ignition wiring harness has opened up, or the EEC relay is defective.

    All testing is done with the ignition switch in the Run position. Do not forget this crucial step.

    The pink/black wire s should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the fuel pump has opened up.

    With the test jumper in place the green/yellow wire should be the same voltage as the pink/black wire +/- 0.25 volt.

    If not, look at the red wire: should have the same voltage as the battery positive terminal +/- 0.25 volt.
    If not, then check the yellow wire on the EEC relay located on top of the computer. This one is hard to get to. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the fuse link for the computer has opened up.

    If the red wire does not have the same voltage as the battery positive terminal +/- 0.25 volt and the yellow wire on the EEC relay does, then check the red/green wire on the EEC relay. It should have the same voltage as the battery positive terminal +/- 0.25 volt. If not, then the ignition switch is defective or the fuse link in the ignition wiring harness has opened up, or the EEC relay is defective.

    Diagram courtesy of Tmoss & Stang&2birds
    [​IMG]
     
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  3. Buddy

    Buddy New Member

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    Thanks for the help. Ok, I cleared the codes, went for a test drive, and rechecked the codes, both engine off and engine running. No codes, but the problem is still there. Start, idle, cruise, partial acceleration are all good. However, at WOT it stumbles, backfires and misses like crazy. Any suggestions?

    Buddy
     
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  4. jrichker

    jrichker StangNet's favorite TOOL
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    The code 96 is a phantom problem, sometimes it really is a trouble source and other times, just a random occurrence due to testing or maintenance.

    Is the problem more evident on a hot engine than when the engine is cold?
     
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  5. Buddy

    Buddy New Member

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    Seems to be about the same, hot or cold. This morning, it would pull strong for 1-2 seconds, then start popping, hesitating, etc. I checked the fuel pressure:

    KOEO: 28
    idle: 32
    vac. line disconnected: 41 I don't see any fuel in the vac line.

    If it's the fuel pump, what brand and size do you recommend? Pretty much stock now, but would like to upgrade to about 300 rwhp at some point.

    Thanks for all the advice!

    Buddy
     
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  6. jrichker

    jrichker StangNet's favorite TOOL
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    The fuel pressure looks OK.

    Was the KOEO 28 referring to a computer trouble code?
    The code 28 is rather odd, I haven't seen it before. It points to a problem with the TFI module or the PIP sensor. I would try the TFI module first since replacing the PIP means replacing the distributor. The distributor must be removed and the gear pressed off to get access to replace the PIP sensor.
     
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  7. Buddy

    Buddy New Member

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    My mistake, 28 is not a code, the fuel pressure was 28 with the key on, but engine off. There are no codes, either engine off or engine running. Fuel pressure at idle was 32 and fuel pressure with the vac. line disconnected was 41.

    Buddy
     
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  8. jrichker

    jrichker StangNet's favorite TOOL
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    The popping and backfiring indicate a lean fuel mixture. The fact that you don't have it any other time than at WOT suggests a fuel delivery problem.

    1.) Check the fuel filter, replace it. Look for water and dirt in the tank side of the filter when you take the old filter off.
    2.) Inspect the fuel lines while you are under the car for dents, sharp bends or damage.
    3.) Check the fuel pump relay socket for evidence of corrosion or damage. Also check the wiring connector in the rear of the car that connects the fuel pump power wiring to the body. It is the connector with pink/black and black wires shown it the code 96 test path I posted.
    4.) Find a gas station that sells gas without any ethanol in it and try a tankful. Be sure to run the current tank contents down as far as you dare.
    5. ) If you replace the pump, a 155 LPH capacity pump is your best choice for your future plans. A lot of people oversize the fuel pump by buying a 255LPH pump thinking that the fuel pump regulator will just pass the excess gas back to the tank. It does, but… Did you ever consider that circulating the fuel around as a 255 LPH pump does will cause the gas to pickup engine heat? What happens to hot gasoline? It boils off! With most of the 5.0 Mustangs having the carbon canister removed or disabled, the car stinks like gas, and the gas mileage drops since the hot fuel evaporates away into the air.
     
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  9. Buddy

    Buddy New Member

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    Ok. Thanks for all the help, Jrichker. I'll try the steps you've outlined.

    Buddy
     
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  10. Buddy

    Buddy New Member

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    Ok, now I'm stumped. Here's a recap: basically a stock efi 5.0. It runs fine under every condition, except when it hits about 3600 rpm under WOT. In other words, its fine at idle, partial acceleration all the way to redline, WOT up to 3600, cruise; all ok. Under WOT when it hits 3600, I get popping, backfire, loss of power. I went to an autocross in June, everything was good. Parked it during July while I was out of town, when I came back in August, I noticed the problem and I've been chasing it every since.

    I've replaced the fuel pump relay, checked the fuel pressure (under all conditions, except actual driving), replaced the fuel filter, replaced the fuel pump, replaced gas in the tank with nonethanol gas; checked the gas lines, checked the electrical connections.

    Any other suggestions would be appreciated!

    Buddy
     
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  11. gearheadboy

    Mod Dude

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    Did ya pull the cap and clean it? Cap, and rotor area are my guess.
     
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  12. stangman11

    stangman11 Member

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    I have been chasing the exact problem with my 87 for months now also, I went a step further and pratically replaced the entire ignition system with no luck so if you find a solution please share it.
     
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  13. Buddy

    Buddy New Member

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    Problem finally solved! Turns out it was a bad ignition coil. I found a mechanic with a portable engine analyzer. We hooked it up and I went for a test drive. All 8 cylinders were breaking up. He suggested replacing the coil, which I did - with a Mallory. Runs great now.

    Thanks for all the suggestions and comments.

    Buddy
     
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