My car has multiple personalities.

-Sometimes it fires right up and runs & idles like a champ
-Sometimes it fires right up and runs like a champ, but wont idle
-Sometimes it fires right up, then dies after 30 seconds. Subsequent attempts to restart it last about 2 seconds before it dies again.
-Sometimes it wont fire up at all

-Sometimes all of the above, within a 5 minute timespan, in no particular order


So far I've checked for error codes (none), and vacuum leaks (none), and tested the AFM (fine). Anybody ever had weird intermittent symptoms like this?

 

Yes. Sometimes these symptoms are caused by a failing O2 sensor. The ECU doesn't always figure out that the sensor is bad. When it does, it goes into limp mode and drives great. When it does not, it tries to adjust mixture using the bad or not-connected sensor and then it runs terribly.

Try checking the 'green lamp'--bottom center pin in the 6-pin test connector you used to check for codes. If you rigged up an LED test set to check the codes, you can even use one of the lamps on that set to check the 'green lamp' pin.

Connect between top center (+12v) and bottom center (ground = on) pins on the test connector. With the car at operating temp, you should see:

At idle: lamp on
At cruise: flashing on and off
Accelerating: lamp on
Decelerating: lamp off

If you can't get those indications, your ECU is not controlling the mixture properly. At steady speed, you should get about 8 flashes every 10 seconds.

If not, check wiring from O2 sensor to ECU; O2 sensor; ECU.

 

Hey thanks Calpatriot for the excellent reply. God knows how long its been since anyone changed the O2 sensor on this thing.

Anyways, I put a new one in and... same thing. Starts up immediately then stalls after a couple seconds. Its gotta be something electrical, because when it actually does decide to run, it runs great. Is there a way to force it into limp mode? That could probably tell me something...

 

Sweet. Have the same problem, but already know my ECU is toast, gonna change the O2 sensor for the hell of it.
Fixing ECU when I get back home in 2 weeks.

 

These cars will run just fine without an 02 sensor, that is no the cause of you problems. However it very well may be sensor related. Check the more pertinant sensors, like the TPS, MAP, and Water thermosensor.