'97 Z-24 Rough Idle When Started

I have a '97 Z-24 5 speed. The car will start and idle fine for about 15 to 20 seconds and then starts to surge and bog down and you have to play with the throttle a little bit to keep it running. This goes on for about a minute then everything is good. The car will then run great with no problems at all. The car will do this every time it is started no matter how long it was off whether it was a few minutes or a few hours. I put a new plugs, a new IAC, and a new fuel pressure regulator in it. I found a vacuum leak and fixed that also. I have also run a few bottles of injector cleaner through it. Any help would be appreciated. Thanks.

Surge and bog down? You mean like loping?

If so, you might have more intake leaks [vacuum].
Loping idle is vacuum leak. 30 year old lawnmower, or a new car on the lot. If it sucks air where it ain't supposed to, it lopes.

Brick on pedal, block TB arm, anything to get a STEADY idle. 1200rpms or so... Feed starting fluid spray, A LITTLE AT A TIME, EVERYWHERE. Rev increase is the leak.

In a tough spot to pinpoint? Light a book of matches, blow it out while the heads are burnin', and feed the smoke to the leak.

I have a 98 Z-24 5-speed and mine does the same thing. I mean the exact same thing. If I rev mine to over 3k for a few secs it will clear up. Mine only does this when cold or after sitting a while. Does yours do this too? I see you did plugs, IAC, fuel pressure reg and injector cleaner already. I haven't done the IAC yet, but I don't think that is the problem. I did take off my Throttle body and clean every inch of it. I was leaning towards the EGR valve stuck open for those seconds or a bad fuel pump, maybe a the MAP sensor.

I don't have the money to buy a fuel pressure gauge or even buy the new parts right now. I think I will block off the EGR valve on the intake side to check for that vacuum leak, but I am not sure if this will cause engine codes.

I'll update when I tried more things.

I have cleaned the throttle body and also had my mechanic try a new MAP sensor so if it didn't work I wouldn't have to buy it. The MAP sensor did nothing. The car still acts up a little bit when started but doesn't seem to be as bad if I use high octane fuel (92,93, or 94) ??? I'm still averaging about 30 MPG.

If you have access to an OBD-II scan tool and/or an oscilloscope, check the upstream O2 sensor (scan tool is to verify if the problem starts when the engine enter closed loop, as I expect). If you're over 100k, just replace it, it needs it anyway. Checking it, the voltage should go between .1 and .9 volts with a fairly smooth waveform. It should go to a higher voltage when you snap the throttle and a lower voltage when it's coasting aster the snap. The PCM ignores the O2 sensors until they begin producing a signal, and the hotter they get the more accurate they get.

Try a full induction flush. BG makes a kit that I use in my shop (professional kit that requires a flush aparatus, not a DIY job), works great. I don't think it will fix your entire problem, but it should make it run a bit better than it does on premium. I've got luck before and had it fix things that I really didn't think it would fix, but the induction system was so dirty I had to start there to eliminate it as a possibility. It will also usually make the O2 sensor waveform better... a reason to try this first. I think the reason it does better on premium is that you have so much carbon buildup in the cylinders (and intake manifold, and on the valves, and well, everywhere after the throttle plate). The carbon in the cylinders effectively raises the compression ratio which makes it require premium. If I recall correctly, the 2.4 is pretty close to needing mid-grade as it is.