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Thread: 1996 F150 2x4 4.9L HEAT PROBLEM

  1. #1
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    Question 1996 F150 2x4 4.9L HEAT PROBLEM

    I wonder if anyone could give me a clue where to look for an intermittent program that is driving me crazy.
    The problem is after the truck (I6, Manual Trans.) warms up I turn off the engine and restart it after about 10 minutes or so it will stumble for about a minute. I get the following DTC codes P0171 (System Too Lean- Bank 1) and P0385 (Crankshaft Position Sensor B Circuit Malfunction). After the minute it runs fine but I get the DTC error codes during the stumble period. If I let the engine fully cool down, reset the DTC Codes the engine runs fine without any errors. This problem has occurred several times.
    I have replaced the Distributor Cap and Rotor, cleaned the Fuel Injectors, replaced the Air /Fuel Filters, new Plug wires.
    Nothing seems to work and the problem goes away before I get, the "hood up."
    Any help would be appreciated.

    Xmike

  2. #2
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    leolkfrm's Avatar
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    sounds like you need to look at the sensor readings with a good scanner
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    I couldn't get the truck to fail again and display the error codes. Took truck up to a local Automotive Repair and they couldn't get it to fail again. I notice some of the Vacuum holes on top of the engine were in bad shape. I had them change them out for new ones. I guess I will just have to wait until it happens again. Thank's for your response.

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    The problem has occurred a couple of times since my last entry. The last time I was pulling my boat on a warm day and I stopped at a gas station. I ended up plugging the vacuum line on my EGR valve to get it to run properly. After that I didn't have any more problems or error codes. I took the Truck to a different Automotive shop and as before they couldn't get it to fail. I had them replace the EVR Solenoid thinking that it may of caused the problem. Heat, Warm days and Load seem to elevate the problem. I am now thinking that it has to be either the DPFE Sensor or the PCM. Different Forums have said that the DPFE Sensor goes bad more often than the PCM.
    The question is why wouldn't the PCM show an Error Code if either were bad? I don't want to keep replacing parts to find the problem. And the problem won't stay long enough to find it.

  5. #5
    Senior Moderator chevychase's Avatar
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    Code po171 02 sensor picked up a lean condition on bank 1 ... this is the side of engine with #1 spark plug. Usually can clean maf sensor resolve problem.
    Look for leaks down stream of the maf/ mass air flow sensor. A dirty air filter will cause symptom. Reason you got the cps code because of misfire /lean condition.
    But check wiring at cps for damaged spots. Pcm/ computer is not telling you something is bad ... just telling you you had a misfire.
    If maf or cps went bad then it would have a code for that. If cps is failing you will end up with a no crank.

  6. #6
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    Why would just plugging the vacuum tube to the EGR cause the Truck to run fine and without Error Codes unless it was in this circuit? Today I replaced the DPFE Sensor and will see if that takes care of the problem. But it may not occur for weeks now that the weather is changing. If the problem comes back I will try looking for Vacuum Leaks, check the Air Filter, check the Wiring and clean the Maf Sensor.
    Thanks.

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