does anyone know how hard it would be to change a main bearing seel or a timing cover gasket on a 1990 sedan deville? what would be involved?

You didn't say if front or rear seal. On front wheel drive engines you should be able to replace them WITHOUT pulling the engine.

The best suggestion I can make you is to buy the 'Haynes' GM repair manual for $15.00 [Haynes #38031]. I got mine at 'Discount Auto'. The book will take you through the process step by step. If it's the front seal it's not so bad---the rear is a bit tougher.

Carl

you might as well forget changing that timing cover gasket withou that motor out. you have almost take off the entire top of the engine and then the water pump and some big crank pulley. My old motor is out of my car and I still haven't gotten around to taking off the timing cover and changing the chain. Everything is sooooo tight over there...

You have to pull the motor or trans to replace the rear main seal, no other way.