When my car is stopped at a light in gear the idle drops way down to the point that the car vibrates and sometimes surges with the tach needle moving up and down between just below 400 to 500 rpm. I'm pretty sure there are no vacuum leaks and everything is in good order. Any suggestions out there?

------------------
Mongoose-NCTGA-1990 Pontiac Formula/305 TBI-700R4 with Edelbrock Performer TBI intake,TES headers,and STB,K&N,MSD-6AL,Holley laser shot coil,Accel Extreme 9000 wires,Jet Stage 2 chip,TCI shift kit with corvette servo,Richmond 3.73 gear with Auburn 3 series posi,high flow cat,SLP 3" cat back,Eibach sportline springs,Energy Suspension poly throughout,Lakewood LCA and adjustable panhard bar package,SSM subframe connectors,IROC wonder bar,Tokico 5 way adjustable struts and shocks,Autometer electric fuel pressure gauge,Alpine,JL Audio,MB Quart,92 GTA taillights,92 GTA aero wing and hatch,Steve Spohn LCA relocater kit and torque arm,MacEwen White face gauges,84 T/A cold air induction air cleaner,Grant Formula GT wheel.

 

What is the voltage gauge doing? If it's bouncing around a lot then the ecm will surge the idle blah blah blah.
Check your TPS and IAC settings. You might have the min idle screw turned too far down and that's why it's going down really low. You might want to put the paper clip in the aldl, turn car to the on position, pull out the IAC connector after after the IAC resets. Turn off car, take out paper clip, start car and adjust the min idle screw. I think the stock TB unit usually has a cap of some sort over the min idle screw but it's most likely it's been adjusted before (probably incorrectly). After you adjust the idle to the absolute min idle turn car off, plug in the IAC and see if you still have a problem, if so then it's probably your TPS sensor, check the voltage and if that's good then your alternator is giving you problems like me. I need a new alternator, anybody have any sugestions? I'm looking for at least 140amp but I really want idle amps down low, I contacted powermaster but they said the 200amp unit is over $300. I'm looking to spend less than that. BTW, it's my b-day, finally turned the big 20. So no longer am I a "teen" lol.

------------------
, Jon (350 TBI!)
91 Red My website

 

I've tried all that before and the problem still persists. The volts gauge shows no change when it occurs either.

 

Strange, how about a vacuum leak. Have you checked for that?

------------------
, Jon (350 TBI!)
91 Red My website

 

Vacuum leaks have been ruled out and since leaks tend to cause the idle to rise instead of drop I've mainly been concentrating on the electronics. I'll probably go through everything again this weekend just to double check in case I missed something. After some reading though, I'm leaning toward the TPS but I won't rule out the IAC.

 

My car does the exact same thing. I thought it was normal but i guess i was wrong. If you find out whats wrong pls post it so i can fix it too.