Reading lots of rough idle and no start threads. This is a must on these 20 year old LS400. Remove the PCV valve hose and inspect it for a crack on the back where it connect to the throttle body. Even a "small" crack or brittle looking hose should be replaced immediately or risk be stranded. Found mine a couple days ago with months of perplexing. Photo thanks to Yamae

 

Thanks for the tip!

 

I just replaced this fancy hose with 2 feet of 1/2" rubber fuel line with a loop to make the "J" turn. Cost $3. Hope this saves someone some cash and a long wait for an order to ship.