I'm not familiar with any LS430 that doesn't have a cap. Can you post a photo of yours? There seems to be two basic designs, one for the US and Japanese market, and the one we have in the UK. The former has a different expansion reservoir joined to the radiator, whereas the UK one has a separate expansion reservoir on the left of the engine compartment. Unless you have an import you need the radiator with the nozzle on the far left which connects to the expansion reservoir. Nearly all of the aftermarket ones, including the US don't have that, so I'm guessing that's the most important thing to look for. All of my three LS430's have had caps.