What causes the "engine" warning light on a 1994 Nissan truck to come on and go off ?

Various things. It could be your Throttle Position Sensor, or something to do with the distributor, or maybe O2 sensor. My suggestion is you get it to someone who knows how to pull codes off it and have them look at it.

Yes. Loose gas caps are know to cause a EVAP code to pop up. Sometimes the rubber seal on the cap itself dries and cracks and causes it to leak. Most part places check codes for free(Autozone, etc) Should have it checked. May be something else(crank sensor, misfire code,etc). Loose EVAP system hoses could also set a code(purge solenoid, etc) - can be detected with a smoke machine, but will cost you to check at a local repair shop to do. Also, if you have a 2.7 liter engine, make sure you change your oil every 3,000 miles. Seen alot of those motors with oil pressure problems. I've heard oil sludges up in the oil pickup and starves the engine of oil. 3.2's and 3.5 engines aren't bad though. These engines I've seen at our shop from time to time have valve cover gasket leaks. Sometimes they leak oil into the tube where the spark plug is located and causes misfires. A easy way to check for this is pull a spark plug wire out of the valve covers one at a time and look for oil on the boots. Hope this helps.

I´d check you lambda sensor. Before they go out for good, they start flashing that warning light, then you get a couple of missfires and finally you´re looking for a toll truck.

You need to get one of those home computer units that you can get to hook up to your engine, and it will tell you what the 'check engine' light means.

gas cap?

O2 sensor, take it to auto zone for a fre diagnostics test

check your engine oil or air filter.

electricity and a little light bulb.

NO DOUBT.......without question its the blinker fluid.

Maybe there's something wrong you ever think of that!!!

oil change, break fluid...