So I'm talking to anyone who finished this dlc case.  
 
I went after the gearhead after connecting the missing wrench at the car lot.  Even my partner suggested doing that, and I was thinking I would collect all the evidence I could before interrogating Valdez.   But it turns out if you go to the race first, it ends with the Leutenant congratulating Phelps on getting Valdez to squeal about liking boys  (which I hadn't gotten to yet).   
 
Overall, pretty disappointed with that case, it wasn't really interesting.  Short and no big set pieces.... Well, it was that bug that ruined it for me, and of course I was docked a star because I hadn't gotten all the information...    
oh well... time to catch an arson! 

I immediately interrogated Valdez because I figured he's one of those self important types who doesn't like to be kept waiting.

I thought the idea of busting illegal street racers in 1940's LA was pretty cool though.

Yeah, same thing happened to me. It's a bit of a let down, especially if you compare it to the other DLC. "Naked City" and "Slip of the Tongue" feel way more fleshed out.