1990 Atwood Water Hester spewing hot water out of the release valve.

[kismoore]

I can heat water up just fine. After about 15 minutes the tank must get full and the hot water just speed out of the release valve until the tank is empty. I pulled the release valve and noticed the spring inside was ?off?. Put it back on track and it looked like it was going to work fine but, not.
New release valve? Looking to buy parts online.

 

[kdbgoat]

You can get a new pressure release valve at any big box store like Lowes or Home Depot, and at most local hardware stores. It's not an RV specific part. Every conventional water heater in a home has the same valve. Grab a roll of teflon tape for the threads while you're buying the valve. Go easy on the tape when applying, you don't need to build up a thick layer on the threads.

 

[Rene T]

I can heat water up just fine. After about 15 minutes the tank must get full and the hot water just speed out of the release valve until the tank is empty. I pulled the release valve and noticed the spring inside was ?off?. Put it back on track and it looked like it was going to work fine but, not.
New release valve? Looking to buy parts online.

What do you mean when you say hot water speed out until the tank is empty. The valve is at the top of the tank so there's no way it could empty out the tank as far as I know.
Also you said the spring was "off". Did you have to disassemble the valve to see the spring and to fix it? 
These valves are set to lift/open at certain temperatures and pressures and they should not be worked on for safety reasons. Buy a new one.

 

[Gods Country]

My valve was leaking last year.  You can get a valve at any hardware/big box store. 

In my case everything seemed visually good with the valve.  I soaked it in vinegar to remove any deposits, and purchased a replacement while it was soaking.  I rinsed the old valve off, and replaced it, and it actually worked.  The old valve remains on the heater, and I have yet another spare part filling my junk drawer.

 

[catblaster]

Try to match the old one with the new one as far a model number or match up with its BTU rating on the label and the bulb length which is supposed to extend into the top 6 inches of the tank. The thread size should also match since it is not recommended to use a bushing or coupling to install.

 

[RVfixer]

There are three causes of a P&T valve to leak.  One, if the water leaks out very slowly or drips this is an indication that you have lost the air gap at the top of the tank.  The water expands as it is heated and pushes the P&T open a little.  There are several ways the cure that but I just drain a good amount of water out of the tank by removing the drain plug, replace the drain plug and open a hot water faucet and turn on the water source until the water runs smoothly out of the faucet.  The plumbing in the tank will not let the tank fill all the way to the top and the air gap at the top will be reestablished.

Second, if hot steamy water is 'gushing' out the P&T valve that would indicate that your thermostat and emergency cutoff (ECO) have both failed.  The ECO is just a thermostat set at a higher temp.  The thermostat is controlling.  If it fails the ECO is now controlling at a little higher temp.  You likely will not notice the hotter water.  Eventually the ECO fails and the water continues to heat up until the P&T valve pops open and as the hot overheated water gushes out cooling water comes in to cool things off...that is IF you didn't go out for the day, turn the water off at the city faucet and leave the water heater on...that would not be good.  The fix for this P&T leak is to buy the kit containing the thermostat and ECO for your model water heater and follow the instructions in the kit to easily replace the parts.

I have no idea what you meant by "empty the tank?"  Perhaps you meant empty the tank of it's hot water and replace it with cold water??

The third way for the P&T to leak, of course, is just a faulty P&T valve....Replace.