Yes you absoultely need a gasket for the cap, it should be found way up in the cap, not down on the flange. That car has what is called a surge tank, its radiator cap is not located on the radiator and that tank (which some will mistake for an overflow tank) sees full EOC (engine oil coolant as they (GM engineers) called it) pressure.
I was a design engineer on those exact vehicles from 92-94 but during that time I worked on 95-99 model year vehicles. I did engine packaging for GM on L, N, and J cars. What that means is the engine was in the car and I (and everybody I worked with) designed everything that attaches to the motor and tranny. Radiator hoses (EOC lines they had just started to use the red stuff, thats why we called it EOC) Transmission oil cooler lines, (TOC lines) Fuel lines, shift linkages and shifters, vacuum lines, battery cables, Throttle and criuse control cables, engine mounts and exhaust, and EGR and vapor return lines.
Theres a funny story about how that surge tank came to be shaped like it is. Turns out the 1st prototype ones were smooth on the outside and had support ribbs in the inside to make it withstand all the pressure. During some of the very first tests the prototype ones developed very serious bulges from the enormous pressure built up during Over heat cycle testing. we went back to the manufacturer for several months to try to get them to increase the strength of the plastic or try to get them to guarntee they could make a tank that wouldnt bulge and develope stress marks in the plastic. Also tried to redesign the tank ribs to strengthen the outside surface so it wouldnt bulge like that. Meeting after meeting on the subject and the still no solution. The issue was elevated and now they were haveing a meeting inviting all sorts of experts even from other platforms to come and review the problem to see if they could help solve it.
We surely didnt expect to solve the problem in that meeting, but were hoping for a resolution soon (hopefully in the next 4-10 weeks). The meeting had dragged on for more than 3 hours, everybody had persented their material and more than a dozen people had spoken and presented, each one more educated and impressive than the previous. At the end the speaker had asked for comments or conjecture. The room lay silent for a solid 30 seconds, then a very old man at the back of the room made some noise while he stood up. The project manager motioned in his diretion and everybody turned to look at the old man. He said, in a broken but commanding voice, "Why the hell dont you leave it that way!!!" the room broke out with laughter, it took at least 2 minutes for everybody to stop laughing. When everybody had stopped laughing and the room quieted down, he was still standing, and most still faced him. He said "Im serious as hell!!!" Again the room roared but not for near as long. As he made his way to the front of the audtorium/hall and went into a ramble about a catenary, and this failure (the bulge) developing the perfect shape to define our starting point for the new shape of our surge tanks. Nobody could beleive it, half of tha attendees left saying it would never work.
I was very close to this issue from the very start I was the liaison engineer on the V6 powertrains at the time and watched closely. The next few weeks would tell we had the manufacturer all spooled up to make tools as fast as possible and get the prototype parts back as soon as possible.
Many gathered for the test but not the man that suggested the change. He was conspicuously abscent. He was an old stress engineer one of my friends had dealt with him and told me later that he wsa so confident that his suggestions would work he didnt even want to witness the test.
Sure as hell he was right. The brand new design all bulged out even before the tests could handle more than 3x the pressure before showing any signs of deformation and even then it was the sonic welds that held the top and bottom togeather that failed first.
I really learned alot working at GM and wouldnt trade it for the world.
