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Stalls when releasing throttle from moderate (2K-3K) RPM

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This is on a 1996 626 2.0L, manual transmission;

When cold (open loop): Runs fine (starts right up, throttle response is perfect) Idles smoothly @1000rpm, throttles up fine, releasing throttle is fine, settles smoothly back to 1000rpm.

When warm (closed loop): Idles OK, not perfect (a bit erratic @750rpm) Throttles up fine, but when releasing throttle, will drop below 500rpm and usually stalls pretty quickly. After stalling, restarts and returns to same idle as before throttle test.

When test pin connected (TEN-GRND) such as for checking timing, it will not hold idle at all.

Things checked:

  • No engine codes are tripped
  • New plugs, gaps are all correct, color looks good
  • No vacuum leaks can be found (listened all around the intake and associated vacuum lines; Double checked by spraying starter fluid all around engine comp't, no variations in idle detected)
  • Fuel pressure is normal (30-35 at all times)
  • TPS is normal (0.75 closed, 4.5 WOT, snappy / quick response to throttle change) OBD2 shows 13% at idle, around 23 @3000rpm
  • IAC valve has normal resistance (7ohm), reads 7.5-8.5v fairly consistently during normal operation (I am not sure if that voltage is normal; couldn't find any reference on that) BUT does not seem to make a difference when unplugged; it stalls the same whether connected or not, and idle does not seem to change in erratic nature. (any further ideas on checking IAC for operation?)
  • Throttle body removed and cleaned, IAC removed and cleaned (after testing as mentioned above) but these were not that dirty to begin with
  • MAF readings seem normal (OBD reader shows 40 during idle, 120-130 @3000rpm)
  • Timing is set at 12BTDC (had to do some trickery to keep it running with TEN-GRD shorted, though)
  • Oxygen sensor readings are normal
  • No exhaust leaks
  • Battery and alternator charge are good
  • Disabling EGR by capping off the vacuum to the EGR valve makes no difference

Things I am considering next:

  • Installing a vacuum gauge to watch for fluctuations that might indicate something according to https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/
  • If a vacuum leak is shown, check the EGR for stuck-open (remove and clean it, pretty much)
  • If intake valve issues are shown, take to a shop to confirm valve issues with the head
  • If no symptoms / vacuum gauge reads healthy, run a can of intake cleaner thru the system (though I've never seen a dirty intake show this level of stalling)

Does anyone have any ideas or suggestions about anything I might have missed? I'm perfectly capable of missing the obvious, after all... The biggest mystery to me is how it can run so smoothly until entering closed-loop, and even then accelerate with plenty of power, but suddenly stall that easily. Seems like it must be some kind of sensor problem, but I can't think what else to check.



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So, today I ran my vacuum test and it showed healthy vacuum (19-25) with no significant fluctuations or drops, no symptom of vacuum leaks or exhaust restrictions. Pretty smooth.

Then, I ran SeaFoam thru the intake as per directions, it made a LOT of smoke (like in all those videos out there) but only a small improvement to where the car can just barely catch and hold idle 70% of the time when coming to a stop / releasing the clutch after moderate RPM drop.

I drove 40 hard miles (WOT, release, WOT, release, .... repeat 100 times) on the freeway, because it's still not broken in (but the break-in period shouldn't suffer from this issue anyway.)

Then I took another compression test, giving 122 120 115 123 which actually seems pretty good for being early in the break-in period.

Next steps, I will try resetting the ECU by removing the battery cable, depressing the brake for 30 seconds, then hooking it back up. If that doesn't help, I'll fiddle with the idle set screw (yes I know this is forbidden, but I moved it before knowing that, tried to set it back to where it was early in this process - it never had any effect, making it worse or better, so that's not the root cause anyway)

Last resort, if none of that helps, I'll probably humble myself and take it to a shop for diagnosis.


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Resetting the ECU did not help; Finally, I decided to fiddle with the FORBIDDEN IDLE SCREW.

Now, I have something very clear to say about this; I agree completely, 100%, wholeheartedly, this should NOT be adjusted to solve idle issues if the physics of the engine haven't changed. By that, really, in my case I am talking about displacement.

When I rebuilt the engine, at the advice of the machine shop, I had the cylinders bored 0.020" over to take care of some light scarring that had appeared from 240,000 miles of use. That essentially changed the displacement of the engine by a marginal amount, but my theory here is that it was just enough to cause it to idle roughly and stall like this. I have no guidance from any of my research, and no feedback from any forums on this topic, so I will go with my theory that this idle set screw exists solely for the purpose of when the engine mechanicals change in such a way (or, perhaps, shearing off some of the head due to decking, sleeving the cylinders, replacement of pistons with non-stock, aftermarket cams, etc.) Since this is my theory and nothing I've actually read anywhere, please don't take it for gospel, and for God's sake, PLEASE don't read this posting as open license to fiddle with that idle set screw. I only did this after a thorough and exhaustive examination of all the sensors, vacuum lines, compression figures, thorough cleaning of the intake manifold, fixing of all exhaust-related issues, O2 sensor monitoring, TPS reference voltage span checks, IAC cleaning and testing, and everything else that could possibly have affected my idle, and finally resolving on a theory related to how the set screw is configured according to factory engine specs and displacement.

I'm writing all this so anyone else that happens upon this thread might benefit from it. Even at that, I'm still not 100% convinced that I haven't missed something along the way, but for now the car at least idles more or less stable, and can hold idle with the TEN-GND connector shorted (for timing and such.) Speaking of which, I read from various other posts that the idle screw "doesn't seem to do anything" - IF you go this far, and IF you think it's necessary to change it, after reading all of my blazing warnings above, you might consider this is the base idle, just like base timing, and needs the TEN-GND connector shorted in order to properly set to spec. If you try doing either of those things (timing or idle) without TEN-GND shorted (diagnostic mode,) the ECU is going to do it's life's purpose and compensate to keep the engine running nominally, thus negating everything you do, thus making it appear like the set screw is "doing nothing."

Anyway, I hope this helps someone in the future.

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