92 Firebird, 84,000 miles, bone stock. I purchased this car with this issue. The previous owner had it in a shop numerous times to fix it, got fed up, and sold it. The car starts and immediately idles up to 1500 rpm, a few minutes later, the idle lowers to 500 and surges up and down. If you try to drive it at this point (not up to operating temp), it will barely accelerate and stumble (seems to be a lean condition). Once its up to temp the idle calms down, still high (1000 rpm) and it drives better. It will stumble slightly up to 2000 rpm and then run out fine. SES light comes on occasionally and then go out.

The fuel pump was changed and vacuum lines checked (by that shop...not a lot of faith in them) The last thing they told the previous owner was that the intake gasket needed replaced as it was causing a vacuum leak...they GUARANTEED this would fix the problem. I've shot carb cleaner all over the manifold, TB, and vacuum lines and found nothing.

 

Check all the small sensors, Tps, IAC, CTS to see if their bad. You also said it threw the check engine light out? Have you scanned it for codes? From my repair guide:

SELF DIAGNOSTICS
READING CODES
The following procedure applies to models with the 12 pin diagnostic connector
only.
1. Turn the ignition switch OFF. Locate the Assembly Line Diagnostic Link
(ALDL), usually under the instrument panel. It may be within a plastic
cover or housing labeled DIAGNOSTIC CONNECTOR. This link is used
to communicate with the ECM.
2. Use a small jumper wire to connect Terminal B of the ALDL to Terminal
A. As the ALDL connector is viewed from the front, Terminal A is on the
extreme right of the upper row; Terminal B is second from the right on
the upper row.
3. After the terminals are connected, turn the ignition switch to the ON
position but do not start the engine. The dash warning lamp should begin
to flash Code 12. The code will display as one flash, a pause and two
flashes. Code 12 is not a fault code. It is used as a system
acknowledgment or handshake code; its presence indicates that the
ECM can communicate as requested. Code 12 is used to begin every
diagnostic sequence.
4. After Code 12 has been transmitted 3 times, the fault codes, if any, will
each be flashed in the same manner three times. The codes are stored
and transmitted in numeric order from lowest to highest.
The order of codes in the memory does not indicate the order of occurrence
5. If one or more codes are stored, record them. At the end of the
procedure, perform a visual and physical check of each system.
6. Switch the ignition OFF when finished with code retrieval

 

The code was 44, which makes sense. I replaced all suspicious vacuum lines. I noticed the plug wires appeared to be original (AC Delco with the black plastic slip on covers still on them) so, tomorrow, I'll be getting new plugs, wires, cap and rotor. I also found the air pump lines had been removed and the manifolds plugged, no big deal, and the heater hoses are missing! Good thing I live in SW Fla.

 

I've changed the plugs, wires, cap, rotor and checked the vacuum off the line coming from the base of the tbi that goes up to the air cleaner and it was 18 to 19 at idle. It's still running the same, EXCEPT, the car has randomly died two days in a row on the highway with cruise control. The first time it started up within a few minutes, the second time it took 20 minutes. I'm starting to think it's the fuel pump.

 

OK, found a buddy with a fuel pressure gauge and checked between the fuel filter and the gas tank and only had 8 PSI. Bought an AC Delco fuel pump and replaced the pump and the short rubber hose that connects the pump to the metal line (It had a small hole and felt gummy, so I don't know if it was the pump, the hole, or a combination) and the car ran absolutely perfect. So, apparently the previous owner lied about the fuel pump, but I got a car with great paint, body, interior, low miles, runs awesome (NOW), all the options work, even the AC (very important down here) for only 1700 bucks plus the 175 I've put into it for a tune up and fuel pump. YAY!!! By the way, thanks for all your help Third Gen .Org. LOL. ONE F***ING REPLY!

 

Did you change the o2 sensor when they go bad it will cause fuel ratio issues and high idles, also check or replace the tps causes similer issues.