Where is the intake air sensor on a 1990 toyota corolla

SOURCE: Car/1990 Toyota Corolla

If Biggy1 has described it right, I think when you remove the WIRE to the spark plug ( not the plug itself ) and oil jumps out, there is a cylindrical seal that needs replacing and one does not need to even remove the cylinder head.
I have the same problem with my corolla. On the other hand if Biggy meant when removing the SPARK PLUG oil was on the contacts....

SOURCE: Removing Toyota Corolla 1990 Car Stereo

the plastic frame around it should snap out with a flat head screwdriver to get access.

SOURCE: Sensor Wire/Fan 1990 Toyota Corolla

These wires could go to a temperature sensor for the radiator fan. The best thing to do is to bypass the sensor and straight wire the fan to come on when the engine is running. I had one to over heat because the sensor went bad and wouldn't let the fan come on.

SOURCE: 2000 Toyota Corolla P0171

When the fuel trim is high teens or in the 20s then its the MAF that the problem. If fuel trim is normal, 2-5 then its the O2 sensor.

MAF-
Buy "Electrickleen" at the parts store.
Remove the MAF sensor, WHILE COLD, and blast the wire inside with cleaner.
The "hotwire" gets gummed up and changes the resistance of the wire so you get a bad input to the ecu and it trys to lean out for a rich condition that doesnt exsist.When it cant adjust anymore it trips the po171 code.

SOURCE: Hi, I've sent my toyota

To have so many different codes at the same time indicates that the first place to start looking is the wiring harness itself. Locate the MAP (manifold air pressure sensor - code 31) and IAT (inlet air temperature sensor - code 24) connections. Disconnect and reconnect each sensor a few times and check that the connecting socket and pins are clean and free of corrosion - the codes indicate intermittent breaks. Check all fuses are clean and well mounted. Examine wiring harness for chafing, abrasion and signs of wear. Any intervening connections between the wiring harness and electronic control unit (ECU) need to be checked over and should be free of oily grime.