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Thread: What causes Declutch dip and stall?

  1. #1
    Fuel Injected!
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    What causes Declutch dip and stall?

    I've got an issue (one of many) where when the car is Declutched at most any speed over 5mph, the rpm drop to like 4-600 rev up to 1200, then back down and up and down a few swings then settles out at idle. During this event, ipw doubles on the downswing and goes lean while rpm swinging up.
    Also, if I'm backing out of the driveway (coasting downhill) Declutched, it will do the same thing once i hit 3-4 mph, unprovoked (no throttle blips, just while idle), occasionally leading to a stall.
    I'm using $88

  2. #2
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    It sounds like stall saver mode is engaging. In order to prevent a stall the ecm will advance timing, open IAC, and sometimes add fuel. Decel Enleanment and possibly DFCO are the tuning areas to work on.

  3. #3
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    $88(and all other MPFI 60V6 masks) have "declutch" detection code since the only mask that actually used a clutch switch for the ECM was $DF(91-93 3.4 DOHC) from what I can remember. that may be what causes the transition from idling normally to suddenly going nuts just from moving at a slightly different speed.

    check the thresholds in your calibration, around 4MPH sounds right for the detection code. there's also a "vehicle moving, add x amount of steps to the throttle follower" value as well, which might also cause this, but I would expect it to occur when going from 0 to 1 MPH.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  4. #4
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    I've been playing around with the tune to chase down this behavior, but I think I've been barking up the wrong tree. I've found an exhaust leak upstream of the o2 sensor and i think that's the actual problem.

  5. #5
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    Exhaust leak fixed, but problem still remains. I'm running into this issue whenever i cut fueling at idle to chase blms down, which I have never been able to pull down. I'm thinking this motor like to idle rich and I'm about to give up, just accept it and move on. When i add fueling to the idle areas, this Declutch stall and hunting doesn't happen. The rest of my tune is pretty decent and i should just be happy with that. If anyone has some more insight, I'd really appreciate it!
    Ron

  6. #6
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    So if you stop on a hill, press the clutch, and allow the car to start rolling, at 3-4 mph this surging will begin? This sounds like easily repeatable behavior. If so you can watch logs to see what's changing. Timing usually affects idle speed right away but IAC action is a little slower. If you're getting a large increase or decrease in timing when surging starts then you can try to hunt down a table or constant that's responsible. Same with IAC changes.

  7. #7
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    Thanks. I'll try that. I haven't messed with timing a whole lot and that would make sense. I could probably add some timing in the 600 rpm line to build a ridge.
    Ron

  8. #8
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    I bumped up timing, Then fueling until i hit the limits on both and backed off. The result is steady hot idle, but cold is still stalling when declutched. Still can't get idle blms above 111 without stalling behaviour.

  9. #9
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    While researching another issue, i stumbled over a possible answer in the volts vs. Added pw in usec. The values i was using where for different injectors, so I've revised the table to reflect the injectors I'm using now. I have a feeling this caused much of the problems I've been experiencing.

  10. #10
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    Good find. It is very important to get correct values in these cells. Your fueling will vary for no apparent reason and it will be extremely difficult to track it down.

  11. #11
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    I've had a chance to burn a bin with the updated volts vs. Low bpw and that solved the drivability problem. I suspect the bpw was being held open longer and another calc was fighting it.
    I still can't get the blms up though. At least it's getting closer.

  12. #12
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    Update:
    Given the insight learned on this site (thanks to all) I've found the problem!
    In $88, the table f132 Alpha-N airflow multiplier vs iat intake runner temp. Reducing the multiplier made a HUGE difference. Now I've got to totally redo my idle ve table :( :)

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