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Rich in one bank, lean in the other, equal on load

Discussion in '1994-2004 V6 Mustang Tech' started by kitsune1324, Aug 7, 2019.

  1. kitsune1324

    kitsune1324 Member

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    Hey guys, I'm having issues with tuning my mustang after I recently had the timing assembly replaced (the tensioner arm literally broke off...). I was wondering if anyone had any insight as to why both banks would be opposite of each other then sync back up under load (basically just revving). The car has a supercharger cam but it's only a .54/.54.
    Here's a log from a warm start, no revving, just idle: https://docs.google.com/spreadsheets...it?usp=sharing
    Here's a log from a warm start with revving (in this one you can see the short term fuel trims equalize when on throttle): https://docs.google.com/spreadsheets...it?usp=sharing


    The tuner I'm working with had this to say:
    "Looking over this data log I am seeing the fuel trims showing opposite from each other when the car is at idle (one bank rich and one bank lean). You are right though that the fuel trims clean up when the engine is under load. This could be the result of the cams being out of time and showing this type of issue when at idle as that is when the duration would be most prevalent and clearing up with the RPM's since the cams are turning faster and this would clear up then.

    Without having the vehicle present this is hard to determine just from a data log though. For something like the cams being out of time I would have a performance shop inspect the car before pulling things apart."

    So it's a possibility that the camshaft is not synchronized with the crank, but could this possibly be something as simple as the camshaft synchronizer?

    I'm stuck at the moment so any help is greatly appreciated!
     
  2. Pete fender

    Pete fender Pete Fender

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    The lean side could be retarded by one tooth one one of the cam PHASERS.
    Cam synchronizers could be another problem, especially aftermarket.
    One could be not quite aligned.
    Check both...
     
  3. Pete fender

    Pete fender Pete Fender

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  4. Pete fender

    Pete fender Pete Fender

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  5. kitsune1324

    kitsune1324 Member

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    I appreciate the replies, Pete. Correct me if I'm wrong, but I'm pretty sure this car doesn't have any cam phasers as it's just a single cam 3.8. It's a heavily modified 3.8 (4.3 now) but I'm not sure how to be 100% sure if the cam is correct or not as I don't understand why, if the cam is not adjustable by electronics/etc, the issue would clear up on throttle. I would assume that the issue would persist, even under load.
    The car still has the OEM motorcraft cam synchronizer, unsure if that could be the problem or not.
     
  6. Pete fender

    Pete fender Pete Fender

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    ''The tuner I'm working with had this to say:
    "Looking over this data log I am seeing the fuel trims showing opposite from each other when the car is at idle (one bank rich and one bank lean). You are right though that the fuel trims clean up when the engine is under load. This could be the result of the cams being out of time and showing this type of issue when at idle as that is when the duration would be most prevalent and clearing up with the RPM's since the cams are turning faster and this would clear up then."

    I misunderstood, since the above mentions two cams, which would not be a 3.8.
    There is only on cam position sensor., that only has a signal at top dead center.
    It controls both banks.
    The cam timing control both banks.
    I believe even the V8 has only one Cam sensor.


    Possibly the lean bank has a O2 sensor problem?
    P0171 or P0174 may be the code. If the right side wasn't so hard to unplug (Short wires) I would swap the to upstream O2 sensors to see if the lean and normal side switches, then you know for sure.
    Some people install new O2 sensors every so often.
    Also, check for a vacuum leak.
    Most of us do not have a smoke machine that will find a vacuum leak, so I use a cheap cigar.
    Take loose a handy, larger vacuum hose, and with the engine off, blow smoke into the manifold.
    If you have a vacuum leak, it will be where the smoke comes out.
    No smoke, no leak.
     
  7. kitsune1324

    kitsune1324 Member

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    No worries, the tuner wasn't very clear when he wrote that.
    I have replaced both front O2 sensors with the good Bosch sensors, it's weird though that the issue clears up when you give it any sort of acceleration. I can try swapping the sensors, I thankfully have an extra set of O2 extenders..
    I have smoked the intake and patched all leaks that could be found.
     
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  8. Pete fender

    Pete fender Pete Fender

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  9. Pete fender

    Pete fender Pete Fender

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    One last suggestion.
    The 3.8 has a balance shaft, that is driven by the camshaft.
    If the timing chain were to wear or stretch, not only would the valves be running a few degrees off, but your engine balance would also be off, at idle.
     
  10. kitsune1324

    kitsune1324 Member

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    The idle speed is fine, so no issues there, I even changed the desired idle. The only problem is the car has a cam so it's always a little bouncy. The timing chain was literally replaced no more than 40 miles ago..I'm wondering if they screwed something up?

    I threw in a new set of NTK O2 sensors in bank 1 and 2 and saw no change, I changed the spark plugs and did a compression test while doing so and still saw no change. You can see the results of the compression test in the attached pic (spark plugs are lined up as if they were in the engine, the bottom of the pic is toward the front of the car). It's hard to tell in the picture, but plug 4 was definitely rich, all the other plugs were somewhat optimal. I have pics of the individual plugs if anyone wants to see. I ordered some O2 extenders to do a voltage test (cut the sensor wire of one of the banks and read the voltage/etc), so before I do that I plugged the bank 2 sensor into bank 1 and bank 1 sensor into bank 2 to make sure things didn't get swapped for whatever reason. Good news, the car ran worse so I'm pretty sure the sensors were plugged into the right place. Interesting note, the datalog showed what I've been seeing with the fuel trims do the same thing, but on the opposite banks. So it could still be a sensor wiring issue. The last thing I noticed is when trying to hold the RPM's at a steady level around 1.5-2k rpm's it sounds like there may be a small misfire? Here's a short video I took of it. It's hard to hear in the video but there's a low pitched thumping noise while trying to hold the rpm's steady, whenever it thumps, you can feel it in the car. Could this be a misfire or something?

     

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  11. Pete fender

    Pete fender Pete Fender

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    Check your plug wires at the coil.
    Only other thing I can think of, now, would be a leaking injector on #4.
    Engine is to the left, coil pack is to the right in this .jpg...
     

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  12. kitsune1324

    kitsune1324 Member

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    I literally forgot to add that part haha...I checked all the woring today and even threw on a NEW msd coil pack, unfortunately nothing changed.
     
  13. Pete fender

    Pete fender Pete Fender

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    Did you put the wires as in the diagram?

    I think you may still have a bad injector or loose injector plug, jiggle the #4 injector wires, if you can reach them, while the motor is running.
    It sounded like a dead miss.
    You could clear your codes, and keep running the motor.
    It eventually will throw a code you can work with, narrowing it down considerably.
    Check both current and pending codes.
     
    Last edited: Aug 16, 2019 at 8:15 PM