4000 rpm idle
 

Help Please

1993 Miata with early version of PNP2. 2005 VVT normally aspirated engine and VVTuner. Also has a Innovate MTX-L air fuel gauge. Track only trailered car.

Car has been together like this for 2 years. PNP2 in car for 4 years with another previous engine.

Recently had a dyno tune to address idle and driveability issues. Shop changed from previous open loop to closed loop. I drove one session on track, loaded onto trailer. Got home and with cold engine started right up and idled good and put the car on jack stands for suspension work.

Did not mess with any wiring of MS or any wiring anywhere, just suspension and brakes.

As part of suspension work had to pull radiator, so when refilled I started it up and it idled fine so got to temp and shut off. Refilled radiator next day and started it up. Idle zoomed to 4000 rpm.

Been emailing with Matt Cramer who has been extremely patient and helpful for a novice like me but so far no solution

Took off IAC and bench tested and all was good. Solenoid worked and 10 ohms across terminals. Got another loaner IAC and tested and got same results. Installed loaner and 4000 rpm.

Test across IAC wiring connecter and always 12 or more volts

A friend who knows a lot more than me offered to help diagnose. We installed a 2 year old open loop tune and got same 4000 rpm

My friend took a video of him changing settings while I measured across IAC connector. Nothing he did resulted in any change of voltage – 13.9 to 14v with car running and IAC disconnected.

Video here
More info: with IAC disconnected and engine on cold decent idle, the air-fuel gauge will randomly peg to 22.4

Tune attached

Anyone got any ideas? I’m wondering if the megasquirt chose to malfunction internally.

With the IAC unplugged will it idle normally or go to 4000?

Every time I've had a crazy high idle like that it's a big leak on the intake manifold. For 4000rpms it's going to be big, like the brake booster line or something. Obviously that's not the issue if it idles low with the IAC unplugged, but just wanted to clarify since it wasn't clear in the post.

thanks for your response

it idles normally with IAC unplugged

and I should have said I disconnected the brake booster vacuum line and plugged it at the manifold and no joy there either

with IAC plugged in, I manually covered the inlet port with rubber gasket material held in place with fingers, had someone else start the engine, idled normally

relaxed the tension on my finger and big sucking sound and 4000 revs

Ill bet you need to invert your IAC duty cycles. (100% is closed)

If you mean change valve mode from this to "Normal, 0% = off" then already tried that and no change to 4000

if you mean something else, please explain in more detail so a noob like me can find it

https://cimg4.ibsrv.net/gimg/www.mia...b3e8abf121.png

have you done the iacv test and determined the actual valve range?
many older Miata's the valve flips out if you're far out of it's range and starts pegging open or closed
also make sure you're using the right frequency because different units and valves use different ones

the thing I don't understand is that without any changes to tune or wiring, the car was fine on one start and 4000 revs on the next

and the same with two different IACs

the IAC is stock 2005

sorry I'm a real noob, so not sure I understand the question

do you mean this, or bench testing IAC with 12v and ohm meter?
if this, then all I get is 12v at the IAC connector with both on and off settings

https://cimg5.ibsrv.net/gimg/www.mia...e8cbcec781.png


https://cimg9.ibsrv.net/gimg/www.mia...6a84c1cbcc.png

thanks for the link

I'm going to show my ignorance here

I read thru that and not sure it is applicable for my application

it discusses stepper valves 4-wire and 6-wire that can move in two directions

the Miata is a PWM 2-wire and is not a stepper per DIY --- "To determine if it’s a stepper or PWM valve, find the valve on your engine, it will be attached to the intake manifold, either directly or via a hose. How many wires does it have? Generally a PWM valve will have 2 wires, sometimes 3, and a stepper motor IAC will have 4 wires, sometimes 6. The MS2/MS3 stepper circuit was designed with GM/Jeep valves in mind, however it works great on many different stepper IAC systems, including most all USDM, Euro, and Japanese vehicles that use steppers, among others."

Under Idle Control, in my early release PNP2, I have the Algorithim options:
On/Off valve
IAC Stepper Moving Only
IAC Stepper Always on
PWM Warmup
15 minute IAC
PWM Closed Loop
IAC Closed loop moving only
IAC Closed loop always on

And to test the IAC circuit,the link says to turn the "Idle valve type" to "none" but I don't have that option. Only to set Algorithm to "none" Is that the same thing?

And the link shows a Programmable On/Off Outputs as having Stepper Out 1 and Stepper Out 2. I don't have those options but do have "PTC6 - IAC1" and "PTC7 - IAC2" Are those equivalent?

For your purposes, ignore everything but the instructions on finding the range, the low and high numbers, the screenshot of the menu you already posted. Once you figure that out, input it into the closed loop screen. Make sure none of your other settings (like initial valve duty table) are outside this range.
That will be a good start.

thanks, I'll look into that

Well, after a lot of back and forth email to Matt Cramer, and two other guys looking at it, the consensus was an internal ECU problem

Mailed it to DIY and finally got it back fixed and with new firmware

Of course, that needed several adjustments to the older tune, and back to the dyno shop.

So another bunch of money spent but it now seems good driving around the subdivision

Got a track day this weekend so hopefully it will be good there too.