After 4 yrs and now this car acts up.I have never changed the water pump and timing belt but there is a sticker on the engine bay for this change in 1997
I lost the reservoir cap 2 months ago but even then I have never seen steam come out of the reservoir until today in traffic(with ac on).The steam was emmtting from the reservoir (topless).I doubt the missing cover was the cause of the overheating because that reservoir water never boiled until today.I hope I have not damaged the head gasket because I drove in the 3/4 mark of temp gauge for about a mile and not within the red mark.The car is also chattering like how most 90/91LS's do with bad power steering pulley.I plan on doing this step "http://www.lexls.com/coolant.html" in case the radiator is clogged.The water in the reservoir keeps going up and down and this tells me the water pump is good.I maybe wrong, and please correct me if I am wrong.

 

It's a pressurized system so you need to use a cap to avoid exactly what you experienced. You might have caused your thermostat to go bad driving for two months without one. First put a cap on and see what happens.

 

Yes you were right and maybe driving the car without the reservoir cap caused the thermostat ot remain stuck.My friend who also has a 90LS and works at the junk yard followed me to his house.On the highway the over heating started and I even had to turn off the key and cruise with hard steering.We then parked on the side of the hwy and we took out the thermostat.I am usually a DIY guy but I never thought the thermostat was stuck since I had changed it 2 months ago during a tune up.Viola ,the car is not overheating anymore.I will even leave the thermostat out until winter time.
Thanks guys.
FYI I also noticed that on LS's when the car overheats, it will drive like there is no compression and you hear chattering noises.Some cars just shuts off.