hello i have 92 firebird 305 tbi auto. i was having surging rpms around 2k it was bouncing up a couple hundred rpms. after looking into the problem i changed the IAC and TPS, didnt fix the problem, but created new ones. it turned out the surging rpm was coming from a very worn out distributor, so i replaced it and fixed that problem, but the car now has a very low eratic rpm, my tps is the non adjustable type, and ive had 2 more IACs professionally installed and it still wont hold an idle, after the last IAC installation it seemed like it was gonna run fine but as soon as i drove it around the corner it went back no idle. ive also changed the O2 sensor, manifold temp sensor, checked vacuum lines, checked grounds, timing...nothing. the idle in gear is around 500 but is constantly moving up and down and if i dont rev it up alittle at lights itll drop too low and die. im extremely frustrated and almost sick of dealing it, any help would be much appreciated.

 

If you drive it a while does it throw a code?

 

it threw a o2 sensor code, a manifold temp sensor code, and a code for the sensor on the stock air cleaner. i replaced the o2 sensor and manifold temp sensor and ive been running a open air cleaner but i now have the sensor that was in the air cleaner plugged into the harness. no codes now, but it didnt fix the problem.

 

I assume surge not at idle with engine at 2000. I would check fuel delivery. I would have a passenger monitor Fuel Pressure with gauge duct taped to windshield while you accellerate under load.

 

no the surging was happening while i was cruising around 2k rpm it would randomly spike up.
oddly enough i just put my stock air cleaner assembly back together and the car seems to be acting fine lol. my fan blows pretty hard im wondering if the fan blowing into the open air cleaner was causing the erratic behavior???

 

your either sucking air from the tbi mounting gasket or running lean. none of the tps sensors i have seen are adjustable. i have drilled the holes in them to be able to change the settings. try this.

https://www.thirdgen.org/tpimod2

 

Verify that the timing is set correctly, make sure you unplug the timing connector.

 

well putting the original air cleaner setup back on it didn't help, it looked like it did on the test drive but when I drove it home it went right back to the same thing, maybe worse, the idle wont stay still at all its constantly going up and down almost in rythm and eventually dies, it also has to be started a couple times to keep it running. I've checked the timing several times its right on, is it possible its a different style of IAC? the original one was the kind you twist to shorten it the one I got from Napa you wobble back and forth.

 

Subscribing to this thread because I have a very similar issue on my 92 z28.