I would soak it for a few days with PBW. I've had a few containers (primaries, mash tuns) that started to develop an odor after several uses, not bad...just did not smell "clean". After a few days soak, they were good as new. PBW is really an indispensable tool in the homebrew toolbox.
I usually mix it with hot water, but it isn't critical. It will dissolve and break down any organic solids regardless of temp. Might help to tear down the keg as far as possible and soak all components.
I would think that an overnight soak with either would be all that was needed. Temp shouldn't have that big of an impact in removing the aroma if you start warm. You could also just replace with a warm solution every day (it would be a pain I know but if need be).
Just soaking them in PBW or OXY isn't enough. If this is the first time you are cleaning them then dismantle the kegs entirely, clean thoroughly and replace all the O-rings & lube them. Get a dip tube brush or make something to scrub the inside if the tubes, especially the Liquid/Out tube. Use a brush to scrub the inside of the keg. Then rinse & sanitize with starsan. At each step make sure you run the liquid (PBW, water,starsan) through each post & the relief valve. Cheers!!!
Yeah I have o-rings and such for all 3. I decided to tear them all down after I realized that 2 of them had slow leaks. I thought I cleaned them pretty well to start with, so I kind of freaked out when I was nearly knocked over by the vinegar smell. On a side note, I tore one down and now can't remember which post is which. The poppets and posts seem to be slightly different. Is there a diagram or description somewhere that I can use instead of the put-it-back-together-and-see-if-it's-right method?
The IN/GAS post has groves ground into the side of the nut part. You'll have to play with the poppets to see if there's a difference. Be sure to put a little keg lube on the rubber part of the poppet and the relieve valve, as well as the o-rings before you reassemble them. Cheers!!!
Your keg that smells like vinegar might not have been a soda keg. It could have contained some kind of pesticide or cleaner. Make sure you clean it really well. I would use Pbw and hot water and leave it for 2 days, flip it upside down after 1day to clean the top as well. Then sanitize, as stated earlier remove all old o rings and clean and disassemble everything.