Aaron is undoubtedly right when he says it serves a purpose and reduces pollution and combustion temperatures, so you should fix it. But I must mention that if you don't, the vehicle probably won't run any differently, and the knock sensor will control the detonation. So at least until you get it fixed, you can just crimp off its vacuum hose and go ahead. You shouldn't disarm the whole solenoid etc. because that also controls fuel tank purge venting.
Try cleaning the valve, but don't be surprised if it doesn't help. I think that the later (starting, I think, in mid-88) ones had a tendency to wear funny and hang up intermittently, not related to carbon buildup. Although I'm pretty sure it's irrelevant for the '90 in question, you should note also that the Haynes procedure for testing these does NOT apply to all versions. The one on my 87 had a back-pressure valve in the diaphragm, which allowed it to open only when there was sufficient exhaust back-pressure. This means that a good one will flunk the vacuum test.