I'm at a loss as to what's up with my T/A...
To begin with, I have a 91 Trans Am and I have had 2 (yes... 2) new 350 engines and a tranny installed in my car within the last 6 months. The first 350 I had installed decided it was a piece of crap and self-destructed, luckily it was under warranty. My car originally had the 305. I replaced the ECM, the PROM (the correct one for the 350), distributor, spark plugs and wires, all 8 fuel injectors (22 lb), knock sensor, TPS solenoid, IAC valve and the throttle box when I had the 350 installed. (All new parts.)
The problem is, I can't seem to figure out why my car idles rough. On a cold start up, the RPMs will waver up and down. It has a very shaky uneven idle. But the thing is, it doesn't do it all the time. Some times it will idle almost normal. Another thing is, when I slow down to a stop the car jerks, or lurches (this isn't the brakes, already replaced the rotors, and it's not the transmission). When I had the 305, the car did the same thing and it did it on the first 350 as well so I had the injectors replaced and then the problem stopped and it idled smoothly. Now that I have this second 350 in, with the new injectors, I can't figure out why it is doing this again. I took it to a repair shop and they ran computer diagnostics but said they couldn't find anything wrong with it, but I think they were on crack... lol They checked the timing and said it was where it should be. Does anyone have clue as to what might be wrong with it? Could one or more of my injectors have gone bad? Sometimes I smell gas fumes and when I accelerate under any load it will make a gurgling detonation sound. Don't know if that means anything or not... Any help will be appreciated....

 

Could be more than one thing going on
The gurgling/detonation I can't say

The jerking/lurching thing leads me to think you should check your intake plumbing
If the corrigated hoses are cracked, they will let in an extra amount of air when they flex open at the broke part and screw up all of your input readings. Check all of your snorkels for cracks/breaks.
We had a little ****-pot in the shop years ago that baffled us intil we found the break in the snorkel going to the TB

If that's not it, check all your vaccuum hoses

 

on my carb'd 350, I also lunge at idle and the rpms surge but it is not noticable when your moving. I have been told it is either too high of an idle (which would also cause the car to start dieseling when you turn it off) or their is a vacuum leak.

With my carb, I have tried spraying starting fluid around this and that and listening for the idle to jump but had no luck. I also plugged my vacuum lines one at a time to see if that solved it but no luck. I eventually decicided my carb gasket must be bad but I havent replaced it yet.
Hope that helps.

 

Thanks for the responses, I will have those things checked out. Much appreciated!

 

Not sure if your car has it, but try checkin your MAF. That could have something to do with your rough idle.

 

I would hook up a fuel pressure gauge just to see if everything checks out there. You might at well ohm your injectors to just to see if anything obvious pops out at you. I would also go over all the vacuum lines and spray around the intake, runners, and plenum to make sure you don't have a vac leak.

 

The 1991 Trans Am will not have a MAF sensor on it from the factory. I would start simple, has the check engine light ever come on during this problem, did the shop find any codes at all? As was already mentioned here, eliminate the possibility of vacuum leaks, then search the board for the IAC reset procedure.

If the IAC reset procedure doesn't help, it may be a good idea to remove the IAC and inspect/clean it and the passages. If those check out, you may want to look at your purge canister to see if it is filling with fuel.