High idle...
 

I've been searching and searching... starting to go a little nutty.

I'm hoping this is a common simple fix that I just haven't stumbled across yet using the search.

The car, 87 TurboII, starts fine. After I rev it a little bit the idle will drop down to 800ish (somewhat shakey) for a brief moment and then something is kicking it right up to the 1200ish mark...??

I don't want to play around w too much on my own. I already messed with the BAC and varaible resistor. Neither one does anything.

Any help would be much appreciated...!!!

What does it do when fully warmed up?

Possible bad thermosensor.

It just stays at 1200.

I'm thinking about doing this:
Rotary Resurrection home of the budget rebuild.

But don't want to get in over my head...???

Perhaps you already are.

There's no reason to execute the nuclear option before exploring less drastic (and consequence laden) avenues.

1200 rpm is exactly where the idle is supposed to be until the thermosensor tells the ECU the engine has warmed up...a bad sensor (which is different from the one that feeds the gauge) will never allow the ECU to transition to normal running no matter the coolant temp.
IF the sensor was bad (the fsm has test procedures), deleting the rest of the emissions would not solve your problem anyway.

Perhaps the thermowax needs adjusting.
Maybe the TPS.

If you've recently worked in the area, check the throttle cable hasn't done something weird...in fact, you could disconnect it altogether, see if the idle drops.

Point being, there are several rather simple fixes to explore before an emissions delete (which is going to bring its own set of idle problems to the party).

I loosened the throttle cable 100% and idle didn't change.

The BAC that was on there had no screw adjustment. It's just blank inside, don't know if it was done by someone or original from Mazda since it's has to be over a decade or two old... So I changed it for another BAC intact w the screw. Adjustment raised the idle to 2000 but only dropped it back down to 1200 lowest. Variable resistor adjustment thing did nothing to help.

I'm in uncharted territory here so I'll guess my next move it to locate the thermosensor and my manual. Thanks clokker. :)

And the idle screw itself? Next to the dash pot on the throttle body

I'm going to mess w it now but I don't think it's that only bc something is kicking it back up to 1200 from 800.

Set the TPS. W/the engine as hot as it can get (20 minute drive) set the Green/Red wire of the TPS to 1 volt w/key to on.

FC3S Pro v2.0:&nbsp TPS adjustment for 1986-1988 RX-7 Turbo II models

I tested the TPS dead cold. 1.687 ohms rested. OL & beep @ WOT.

I don't get why I have to warm it up if the TPS is unplugged...?

Satch, I do have a single green/red wire on the harness side. Test it for volts...?? Unplugged...???

The engine needs warming because the throttle plate which rests against the TPS is more open when cold than when fully warmed up which changes the output signal of the sensor. The sensor outputs a voltage signal w/key to on so when setting the G/R wire to volts the sensor needs to be plugged in or the sensor is not powered (no power equals not voltage output). The reading can take place on either side of the connector but it is usually best to take it from the emission side of this harness because it takes into account the plug conection quality or lack thereof.

And w/the ohm method the throttle plate is still in a different position which affeects the ohm reading of the sensor even when unplugged.

Again, 20-25 minutes of driving is sufficient to completely warm the engine and cause the throttle plate to find its proper positioning. Idling the engine for a few will not achieve this.

Ok. I'm going to take her for a spin to get lunch. Thank you.

Edit: Didn't get out of the driveway. New used BAC is shooting a tiny leak & won't go below 1500rpms. Going to put original one w no adjustment screw back. Is there a way to bench test these things...? I do have one more used, one of them has to be good.

So after a good drive TPS measures @ 1.53 ohms lowest. Adjustment screw will raise ohm value but will not drop it lower then 1.53.

Could the throttle stop screw be stopping it from going lower then 1.53...???

Edit: 1.00 @ TPS using throttle stop screw to get lower gives me no idle / stall. Raised throttle stop a little & she's @ 800 smooth. Should I continue to chase getting TPS to 1.00 exact or leave it...?