1G low idle, tried everything

[squeakanator]

My 90 GST has had problem idling ever since it's been rebuilt about 1000 miles ago. It idles around 700-750 rpms usually, but that's with the BISS (properly) adjusted all the way out. When I turn the A/C on it drops down to 600 and sometimes kills the car. Also, when putting it in neutral while cruising the RPM's plummet down to 400 rpms usually and barely pulls back up to 750 if it doesn't stall out completely.

I've tried everything, so far I've tested my ISC(good), tried blocking off the FIAV, replaced MAF, replaced ECU, replaced TPS(adjusted too), replaced all CTS's, boost leak tests(air-tight, along with the intake pipe), replaced O2 sensor, replaced plugs+wires, replaced coil and PTU, checked timing(5 degress BTDC), and checked ignition timing. I cant think of anything else to possibly replace that could cause this issue and I've searched till my eyeballs fell out. SOMEONE HELP

 

[d0sitmatr]

my 95 does something like this, but its never cut out on me.
the shop ran it on a computer and it gave me a bad IAC, they also recommended I change the fuel filter.
not Im not sure if the 1G has an IAC or not, but if your fuel filter is slightly clogged you might be having problems getting enough fuel through the lines.

 

[squeakanator]

I have a brand new fuel filter.... I've been thinking it could be an injector stuck open because when it idles low it sounds like it's missing one one cylinder and the fact that the IAC can't compensate for the AC at all even though it's working properly must mean I've got a serious mixture issue at idle that corrects itself as the RPM's increase, plus I have gone through half a tank of gas in 40 miles..... . I'm gonna check my plugs for fouling and see what I find

 

[steve]

Does your Idle Position Switch work?
I see cars all the time with the throttle body ground strap missing causing the IPS not to work.

Does your ISC step in and out when you turn the ECU on like shown in this video?

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[squeakanator]

I'll check to see if the ISC steps like that, mine being a 90 I believe it goes the opposite direction when the key turned to on then the 91+ cars. I checked the spark plugs and theyre fine, nothing unusual at all, no signs of running rich or lean... I'm pretty sure I did do the ignition test on the ISC before though and the motor just twitched, didn't really move at all. BUt like I said I'll double check and get back to you guys.

Oh and I don't have a ground strap going to my TB, however I do have one going to my intake manifold. How do you check to see if the IPS is working?

 

[steve]

If you haven't found the answer by searching my threads yet, the answer is simple.

Measure the voltage at the IPS when the ignition is on and the wire is connected to the sensor. When the throttle is closed the IPS should be shorting the signal to ground and so you should see close to 0v and when you open the throttle a little the IPS should open and the voltage will go high (5-12v depending on the year car)