I have a pretty weird case on my hands right now - 1992 LS400.
Recently I have been struggling with getting my LS400 running right after accidentally knocking ur the open circuit relay and it not running for awhile. I found this problem fixed it, and now the car is running very poorly. On start up you have to keep your foot slightly on gas to prevent it from dying instantly. Also if the car has not warmed up enough yet, it may also die once put in reverse. At this point i’m able to get out and driving. The car will barley accelerate unless you tap the gas pedal, in that case you can get it to accelerate fast ish, but still very sluggish. I can then get to a stoplight, where I will stop and the car will stall/die. Then have to put it in park start it up again and go. These issues all seem to be related and all go away after about 15-20 mins of driving, apart from the misfire feeling at 2.2k ish rpm’s. This consists no matter what. I recently replaced the battery, spark plugs and spark plug wires; prior to this it would run relatively smoothly, sometimes a sluggish start but never the dying or misfire. Fuel pump was also replaced recently. The car will have a check engine light after 3 mins of running and then after driving for a little bit it will disappear; have not been able to catch it and check it. Where do i start?!

 

Can you expand on what you did exactly?

Have you tried to read any engine codes? Any vacuum leaks? Your symptoms could be caused by a bad airflow meter (google 22250-50010) but I'm guessing, hopefully there is a code stored.

 

You can check the codes even when the light is not on. they store in the computer. It does sound a bit like the AFM might be bad, they are know to not last forever. The Intake Air Temp (IAT) sensor inside the AFM goes bad alot and can't be changed on it's own, the whole AFM needs to be changed.
http://www.lexls.com/tutorials/intro/codereading.html

 

Found saved codes and I have code 12 and 31
What would the code 12 - RPM signal one. 31 - is looking like AFM.

 

12 is an ignition issue I think. Ya know, like a misfire. I don't remember off hand which bank the 12 is for. I know there's some other threads here about code 12 if ya search for em. I'll look later when I got more time. Plugs, wires, cap and rotor, coil. Could be anything, the OBD-1 doesn't break it down for ya. The 31 is the AFM which is known to go bad. I would clear the codes, then drive it, and see if those, or other, codes come back.

 

AFM consists and so does the 12. Ive replaced plugs, wires, and 1 ignition coil (It went bad), have not done the ignition coil on the top of the motor. I would doubt that it is the plugs, wires or ignition coils. So im kinda left with cap and rotor so I will look into those. Could a timing issue also cause code 12? With some of my reading im seeing a lot of talking about the issue with code 12 is the crank shaft position sensor, does that sound right to you?

 

Code 12 has to do with your camshaft or crankshaft sensors. The timing could be off, distributor caps/rotors could be shot, then the actual cam/crank sensors or the wiring going to them. I would check in that order and go from there for code 12.