I was replacing vacuum hoses few days ago, had my UIM off. vacuum hose on fuel pressure regulator was poped off. what would this cause? constant low pressure or high pressure?

 

I don't think that would effect anything except for hot starts. The stock FPR is 1:1 and nothing changes that.

 

Actually, this is the vacuum & boost reference for the FPR. If it is popped off, or omitted, you will not get fuel pressure tracking with boost; which = lean! The system is set up to maintain about 43# of fuel pressure above manifold pressure. Under certain circumstances, it may be increased slightly more than that.