Miata Rev Hang on Idle
 

On my VVT swapped NA, I'm having this strange issue where if I blip the throttle, the RPM jumps up to around 2000RPM, hangs out up there for 2-3 seconds, and then it comes back down. I've been told it may be due to a vacuum leak which I will investigate further soon, but I'd also like to see if there's something in the tune I am missing. I'm using pretty much the base tune for a 91 Miata, with some VVT control put in from a VVT base tune, and with the help of Mr. curly I have a bunch of fuel settings dialed in. I've attached the tune associated and the log showing this behavior. The ECU reads my TPS correctly so I don't think that's the issue, and my TPS lever thing hits the stopper when it's closed. I'm using all NB2 engine accessories like the cam and crank sensors, TPS, ISCV, etc and I'm using TunerStudio with an MS3. TIA!

You have virtually no control over your idle. Could be a vacuum leak, or could be throttle is held open, or could be the by-pass screw is opened too far.

Right now, when ECU tells the IAC to fully close (PWM Idle Duty = 0), you are still idling at 1260 RPM.

Fix that. Then you will eventually need to move to Closed Loop Idle / Use Initial Values, and populate the Initial Values Table.

DNM

He was running closed loop, hence it going to 0%, not sure why the attached tune has open loop.

it’s also at 1-2 degrees advance, idle ignition is trying to pull rpm’s down aggressively. So actual idle is probably around 1500.

I messed around with some closed loop settings since posting this, so are you suggesting I should fix mechanically induced idle issues in open loop before I go to closed loop?

Yes.

It is also helpful when Tune and Log correspond.

If Curly is already working with you, then I will graciously bow out.

DNM

If you run closed loop idle control, the default behavior is to revert back to the "last position" value when you stab the throttle unless you use the initial values table.