Idle Dropping/Almost Stalls in Traffic

[armos]

Originally Posted By: BrandonVA
could a zero tach signal be caused by a cheap TPS sensor? I replaced mine recently but did NOT use OEM. I think it was Bosch or standard.


The TPS shouldn't have any effect on sensing engine RPM.

I'm suspicious of the crank position sensor. I don't know if there's any way to test that though, and I hate to guess. Also, even if it is losing the crank signal, that doesn't necessarily mean it's the sensor itself causing the problem.
From looking on RockAuto it looks like a cheap part. I assume it should be simple to replace, so I'd be inclined to try replacing it.

It seems that the ECM would set an error code if the signal from the crank sensor was suddenly disappearing, but maybe it isn't intelligent enough to realize what's happening. Regardless of error codes, it looks apparent that it's losing the RPM signal, and the ECM can't run the engine properly without that.


Caveat - the dash gauges might be using buffered, massaged outputs from the ECM. As such, the tachometer might not be a literal copy of what RPM the ECM is actually sensing. If that's the case, then my theory would be invalid. I'm assuming that when the tach says zero, it's because the ECM is truly reading a zero signal from the sensor.

 

[Vikas]

Originally Posted By: armos
Quote:
will drop to zero on the rpm guage and my car will almost stall.

So your tach signal is disconnected from reality.
Can you explain why do you say that? Losing ignition for a split second would indeed give the exact symptoms. Because of the inertia of the flywheel, the engine continues to run and thus able to catch up again but it was "stalled" for that fraction of second. The electronic RPM gauge seems to be working fine. Or you meant the same thing?

 

[armos]

Originally Posted By: Vikas
Originally Posted By: armos
Quote:
will drop to zero on the rpm guage and my car will almost stall.

So your tach signal is disconnected from reality.
Can you explain why do you say that? Losing ignition for a split second would indeed give the exact symptoms. Because of the inertia of the flywheel, the engine continues to run and thus able to catch up again but it was "stalled" for that fraction of second. The electronic RPM gauge seems to be working fine. Or you meant the same thing?


I say "disconnected from reality" because it's not showing actual engine RPM at the time of the failure. This is likely a clue to what's wrong.
I think we're saying the same thing about the ignition. At least on older cars, the tach reading is based on pulses from the ignition system. So if it drops to zero, then it means the ignition has stopped. If the engine were misfiring for a different reason, but ignition pulses were still happening, then the gauge would still be showing actual RPM bouncing around 400rpm or whatever.
This can also help diagnose a car that won't start - if you crank the starter and the RPM needle doesn't rise, then it's not getting ignition.

However - I'm not sure any of the above applies on newer cars. On those cars, I'm guessing that the tachometer might be controlled by the ECM, not the ignition system. If so, then the RPM reading is probably a reflection of the pulses that the ECM is receiving from the crank position sensor.

I guess the critical question is whether this vehicle's tachometer is controlled by the ECM, or the ignition. That could point the finger in different directions.

 

[SHOZ]

Factory tachs are notoriously inaccurate in the extreme ranges. Drops to almost zero could really be 200-300.

I'd still go with adjusting the throttle plate. It's a Ford trait to stall when slowing down to a stop sometimes.