When my car gets up to opp. temp the low coolant light comes on "coolant is not low" but when I loosen the cap and release the pressure it goes off then after a little bit it comes back on again. My question is could it be a bad overflow tank cap not letting the pressure out possibly due to a sticky ball valve?
Thanks.

 

When hot it's supposed to be pressurised. Don't undo the cap!

The cap has an excess pressure relief valve but there's no reason to suspect it's faulty from your symptoms.

There's no ball valve. Not required.

Sounds like the sensor is faulty. It's in the tank and be VERY aware the tank and its nipples etc are horrifically fragile due to all the heat cycling.

 

Once again, what he said... lol

But yea, the pressure valve is their incase of a overheating situation where the pressure is too high. It'll relive pressure before it blows out something. Heater core, radiator, Compressor seals, something more expensive.

The reason the light turns off when you vent it is because once you relive the pressure it allows the fluid to expand and so no longer below the low level sensor.

So it's probably the sensor like he said.

 

"LOL" actually the expansion tank gets fuller "hence the term expansion tank" when the engine heats up so that kinda kills the theory that it is low when the cap is on. I will however agree with the idea that the sensor is probably faulty.
Thanks for your input guys.

 

It does get fuller.... but it gets even fuller if you then take off the cap when it's hot (because the compressed air which was pushing the liquid elsewhere is let out).

BE CAREFUL IF YOU UNDO THE CAP WHEN LIQUID IS HOT. If hot enough, it will flash boil You do not want to be in the line of scalding steam. (It also can be bad for the car.)