Throttle Actuators on engine shutdown: self-check?
 

Hey all! There's absolutely nothing wrong with my Z. I'm an electrical engineer and love to know all about this car I recently bought, and had a "curiosity question".

I noticed early on that the ECM definitely "does something" right after shutting the engine down. The electronic throttle system always produces a frequency while the engine is running (that digital mid-frequency hum/whir from the throttle/VVEL actuators, likely due to Pulse Width Modulation). Right when you shut the engine down, the frequency continues for a bit (probably because it doesn't just shut the throttles immediately), and then you can hear the system "moving around" a bunch, with some associated clicking/clunking for about 3 seconds, and then it goes silent.

I absolutely LOVE hearing that. It makes my nerdiness go insane. haha

Just curious, does anybody know exactly what it's doing here? Is it just doing a self check of the actuators, or maybe a calibration/linearization? Or perhaps it's doing something to make the engine happier while it sits unused (maybe like my Z tucking itself in? haha)

In the military they say: "That's above my pay grade:icon14:". LOL

You are probably hearing the "Spring Test." During this test the ECM operates the throttle to check if it can react to the command and return to the default (home) position. This self-test is used by the ECM to determine that the spring and motor inside the throttlebody are working correctly.

Ooo that sounds about right! Where did you lear about that ByThaBay? Rep!

I read about it in one of my books, I think it was called "diagnosis and repair of automotive electrical and computer systems" something like that.

:iagree: WTF?!?!?!
Can I borrow it?

I think its opening them when you turn the car car on KOEO, and closing them when you shut the car down.. i think lol

hey guys are you sure it's the throttle bodies? I've been told it's the electronic EGR valve doing its self check/self freeing of its shaft on shutdown. Concept is pretty much as ByThaBay described it.

Both my previous Mazda3's did it, gf's 2012 Mazda2 also. but all 4 cars have electronic throttle bodies too... it would be unusual if the butterfly flaps moved back & forth after shut down. You'd expect them to shut instantly & remain shut as a means of preventing an engine 'running away'

I'm not 100% sure about this, but I've been looking at data from the OBD2 port and it looks like when you press the off button, over the next 2.5s the throttle bodies close completely and then re-open a tiny bit.

ah! nice observation :)