2010 Prius, ~133k miles. Bought used at 126k mi, previous owners seem to have taken good care of the car. I usually put Shell 89 in it because I was going off the "88" listed in the owner's manual. I read somewhere on here that that was a misprint and it was suppose to say 87. So I put 87 in. A short while after that on the first (and only) tank of 87 I got horrible rumbling on startup and while idling. Car threw the P0401 EGR code. I decided to do the next fill with my usual 89, a bit after that the rumbling was only at start up and wasn't so horrifying. Filled it up again today with 89 and the rumbling is completely gone. Toyota won't replace the EGR valve in accordance with the enhanced warranty because it is a previously salvage title and I am not in a California emissions state. (That is a whole other story with the the CSR telling me it was covered and then the dealer refusing to honor it, they did confirm that the EGR valve should be replaced) Doing research on here I had decided to replace the PCV valve, clean the intake manifold, throttle body, EGR pipe, and have the EGR valve and cooler cleaned (or replaced if they won't clean it). I have already bought all the tools to do the PCV to EGR pipe, but I was going to pay for the EGR valve and cooler because we have an apartment and I don't want to risk spilling coolant. It also looked like that part was much more intensive. Now that the rumbling is completely gone, and I suspect if the engine light is reset it won't come back on, should I still pay to have the EGR valve and cooler fixed up? I will still do that DIY work I intended on doing.