I pulled my 91 Firebird with 305 TBI from it's winter home last week, put on the new 47mm bore TBI unit I got over the winter, and now once the car is warmed up the RPM's are moving around between 350-500, and it is stalling when I sit at a stop at traffic lights. Actually it's more like if I put the throttle down a little bit, and let it go (to move up at a stop sign or whatever). The RPM's sink like a stone and the engine cuts out. It's happened in drive, and reverse. I checked fuel pressure in park and drive, it stays steady at 12 PSI, even when the car drops in RPMs as decribed above. I checked all my plug wires with the timing light, they all show a signal, the injectors look like they're spraying fine. And vaccuum is at about 20 in/Hg with it in park. I unplugged the IAC, and it got much worse, just a little tap on the pedal and the thing almost killed. Also, it's been starting funny, it seems to fire up OK then cuts off a second after I release the key, but that's only happened three times, all today(it's pretty wet and rainy here today). Battery, Alternator, and starter are all new (bought them in fall). Could the bigger bore throttle body and standard bore intake manifold be throwing things off?
Please help, I don't want to get rear-ended.

 

I would recomeend adjusting the minimum idle speed screw on the driverside front of the TBI unit. It takes a Torx screwdriver.

To do this

1.)Jumper A and B on the ALDL connector under the dash
2.)Turn the key to the run position but don't start the engine
3.)Wait 30 seconds
4.)Unplug the IAC connector
5.)Remove the jumper on the ALDL connector
6.)Start the engine and let it warm up at idle with the transmission in Park/Neutral. A/C and electrical accessories off.
7.)Adjust the idle speed to approximately 600 rpm.
8.)Shut off the engine
9.)Re-connect the IAC connector
10.)The problem should go away.

I would also take some carb cleaner and spray around the TBI gasket and vacuum connections on a cool engine. If you have a vacuum leak the engines idle speed will pick up when the carb cleaner is sucked in. Just be careful and don't use too much as Carb Cleaner is flammable.

 

Thanks Fast, you always help, and your accomplishments are incredible, I tell all my car buddies, and they can't believe it. Do you think I would have a vaccuum leak, even when the mityvac gage hooked up to a port at the bottom of the TBI reads 20 in/Hg in park at idle?

 

You could have a vacuum leak, but I am leaning more toward a mis-adjusted throttle plate setting. The IAC valve is only able to flow a small amount of air and the air that bypasses via the IAC valve IS a vacuum leak. It is advisable to have most of the air going through the throttle plates themself, not the IAC. Also by having the IAC sitting at the minimum of counts, the idle logic cannot fail and cause a stall.

 

Ok, I tried what Fast suggested, now I'm even more confused. I disconnected the IAC, and it idled at 2000 rpm. I let it warm up to 185 degrees, and it was still idling at 2000 rpm. I tried to move the idle set screw backwards, it didn't do anything, the throttle stayed in the closed position, and the screw backed away from the throttle lever. I tried to push the throttle lever back against the screw, but it wouldn't move. I then tried to move the screw the other way, towards the throttle lever. This made the rpm's increase. So there is no way I can decrease the idle using the idle set screw, and even with the IAC disconnected, I think 2000 rpm's is way to high for being in park. Any suggestions?

 

make sure you did steps 1-5 correctly (as stated)-if you dont, then the iac wont extend completly. the idea is to get the iac to close the hole(vac leak) so that you can set the idle via throttle plates. this way the computer can adjust the idle more effectivly. you must give the iac valve enough time (30 secs or longer)to close before unhooking the A_B jumper

if this doesnt work then you have a vac leak.

 

I tried it twice, and I'm sure I waited more than 30 seconds both times. I also tried spraying carb cleaner around the base of the tbi and on all vac hoses. No change in RPM.

 

Ok I replaced all the vac hoses, and adjusted the idle screw trying to match the exact location of my two other throttle bodies. After letting the car warm up in the driveway, the idle seems to be up about 100 RPM, both in drive and in park, from where it was. I will drive it tomorrow and post how it goes. Thanks everyone for all the advice.