Unreasonably difficult idle problem, help needed
 

I have been fighting this crap so hard I am about to part out the car, so I must ask for help from the masses.

Ever since I finished building my intake manifold, I have been fighting with a strange high idle issue. My car starts fine and idles normally until the car is driven or something is done to activate any idle up such as power steering movement or ac activation. Once driven or the steering is turned, the idle shoots up to 2k rpm, you can hear the idle valve abruptly open. When this happens, it sounds like a vacuum leak hissing. The car will continue idling high, even if you switch it off and restart a few times. Here's the weird part, if you briefly unplug the idle valve, but plug it back in before the engine dies, t settles to a normal 800 rpm idle until you upset it again.

I have replaced the coolant tenp sensor, idle valve, ecu, iat, maf, both engine harnesses going all the way back to the ecu, crank and cam sensors, coils, plugs and wires and have replaced both intake manifold gaskets. This is going on with the factory vics manifold in place. Also of note, it got worse after trying to install and run a defective ms2 I bought from a deadbeat seller on here.

I'm pretty much at the end of my rope in solving this. Any insight will be helpful.

The car is a 2000 nb.

sounds like iacv is getting stuck. I woulda expected it to be resolve with replacement

So would I. The one I replaced it with was an oem mazda one as well. I get oem parts through my second job so all the replacement parts are oe.

TPS or TPS connector (likely TPS as you replaced the harness).

Spec Miata Community: 99 with sudden throttle cut out and looping RPM

Can a ODBII reader display TPS signal, I wonder.

I read the values and tested the TPS when the megasquirt was hooked up, and it returned good values with both ecus.

What exactly do you mean by "building my intake manifold"? That might have something to do with it

An unrelated project, I fabricated an intake manifold using my car to jig and test it on the dyno. These issues with idle, as stated, are using the factory intake manifold.

Are you sure you don't have a vaccum leak?

Positive. I installed new OE intake manifold gaskets, resealed the throttle body gasket and replaced all the specers, gaskets and crush rings on the injectors. All the intake bolts were torqued to at least factory spec. Vacuum nubs and hoses are also all accounted for. PCV is in good shape and the hose to it and the booster are new.

You can hear the idle valve open and stay open too far when this happens. Something is triggering it, but I am at a loss as to what. I also adjusted the idle to 800 rpm with the TEN pin grounded.

just shooting out ideas, what about ground to the tb?

I cleaned that and the ground to intake bracket and also moved them around to see if the valve cover bolt was a better ground location.

I changed the idle valve again today, and I'm cautiously optimistic. What rpm do/did your cars idle at when rolling (>10mph) but in neutral when on stock ecu?

not much higher than stat idle, if at all

Which coolant temp sensor did you replace btw?

If ECU sees low temp it brings it into warmup/fast idle.

NB only has one clt sensor.

Car is idling at 1200-1300 rpm not while rolling. After stopping id drops to 800-900 rpm. Idle bypass screw is all the way in.