Take it apart, its a good learning experience. I just took apart my Pentax A50mm 1.4 and got it cleaned relubed and back together again, biggest thing was getting the 2 halves of the gears for the focus back together, near impossible to the get the angle just right to start them, and there are like 6 or 8 possible clock positions, it doesn't work after reassembly unless its clocked right so mark what is aligned with what before its fully apart. Also I don't know about that lenses construction but mine had a ring under the front cap held by 3 screws, if you loosed those 3 screws (I removed the ring entirely) you have to reset the fine adjustment on the focus afterwards (not that hard).
You will need a set of these
Amazon.com: Screwdriver, JIS Type S Set, 5Pc Plastic Handle: Home Improvement as those phillips screws aren't phillips, and possibly an adjustable lens spanner wrench, may also need a super tiny flat blade screw driver.
Also at the suggestion of others I used regular automotive white lithium grease (good in heat and cold and doesn't seperate) to relube my lens (grease VERY sparingly) seems to work perfect though its a tiny bit stiffer on the focus than some of my other lenses, the extra dampening is kinda nice.
If the blades are greasy (mine werent) its rather hard to clean with a physical tool without damaging them, seperate that part from the lenses and try blasting it with automotive brake cleaner with the aperture closed (dont use the stick on the can or get too close) then work it a bunch of times with a pick or something operating where the lever would go and spray it off again. I've never tried this myself so I don't know what will actually happen (sorry
) but brake cleaner eats grease without eating plastic or rubber and evaporates fast without a major residue (dont get it on the other lens parts though). The grease likely seeped out of the other areas when the lens heated up in the sun (or just cheap grease) you'll likely need to clean everything up and relube it.
EDIT: if its only jamming wide open I have a hunch that the problem is mechanical and something is corroded or bent, but youll likely see the issue with it apart, on mine i just needed to unscrew the back cap (watch for small parts) and I could unscrew the rear lens element housing by hand allowing access to the blades and associated parts.