I just had this problem with one that my wife bought at a yard sale. The filler cap was stuck with lime deposits in the threads. It would turn back and forth a little, but no further.
Frank's answer is a good start. What you need:
Small knife with a sharp point.
Large flat screwdriver.
Calcium-Lime-Rust remover ("CLR" or other brand)
Something to protect your hands while you torque on a hot filler cap. I used a rubber pad made for removing stubborn lids.
Use a knife point just inside the circle on top of the cap to pry off a hard rubber button. This exposes a screw; note how far down it is in the hole, since you need to return it to the same setting when you put things back together. Unscrew the screw (left to loosen). This wasn't easy, but I worked it back and forth and eventually could crack through the lime in the threads. Remove the pressure-relief spring underneath the screw.
Now you've got a little hole you can trickle some water into. You won't get much in, but keep at it until you can hear a little water sloshing inside.
Now put the pressure-relief spring back in and screw it down to the same place as before. Press the rubber button back in. Plug the steam cleaner in and wait for it to heat up and make steam. Once you can hit the trigger and get steam, unplug it and wait a couple of minutes.
Hold the trigger down until steam stops coming out, then try to twist the cap off. It didn't work right away, but I kept twisting back and forth and eventually it broke free. Then it seemed like it was spinning without going anywhere, but actually it just takes a lot of turns to get it off.
Once it's off, clean the crud off the threads so it won't get stuck next time. The male threads (on the plug) came clean with just a spritz of CLR and a rinse under the tap. More stubborn deposits might need a brush. Then I sprayed CLR down into the female threads, filled the tank up with water, and poured it out. Quite a lot of loose crud came out, so I repeated several times. Make sure all the CLR is rinsed out in the end, that stuff eats metal if left on for more than a few minutes.
What if the spray valve is plugged too, so you can't release the steam pressure? DON'T unscrew the pressure relief again while it's warm, that's likely to scald the skin off your hand! Perhaps you can wait until it's completely cool, then trickle in CLR + water, heat it up, and work the trigger in hopes the CLR will dissolve the plug.
mark
November 2010
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