Trying to fix a heater and wondering if I just have a fuel problem? The thing is only a few years old. Same with the fuel.

I studied this question for years, and have never got a good answer, other than a recommendation to replace fuel every year.  
I used to store my heater kero outside, and I also used to have a heater that performed maginally.  What would happen was that the wick performance would degrade pretty rapidly with use, in that it wouldn't heat up, or burn at the same rate over time as when with a new wick.  Possibly due to colored kero (I now only buy clear), or old or not great kero purchased to being with (here in the NW, not a lot of kero is sold, most burn wood), or perhaps due to moisture saturation (there was not a separate water layer, but some water is soluble in kero.  Anyway, this performance loss seemed increased if my kero was a year older or more.  

I now use a different heater and it performs much better.  I also only buy clear (undyed) kero.  I also keep my kero stored in an out building that stays dry.  
I don't seem to notice any performance loss with kero that's 1.5 yrs old.  Beyond about 2 yrs old (and that is stored dry and not at temp extremes) would make me nervous due to potential wick performance loss.  Wicks are the weak link.  For pressure burning (like in a Petromax lantern), this wouldn't be as much of an issue.  
Again though, much I've what I've experience is not hard measured fact, because there are many variables, but rather what I've become comfortable with, and the  risk that I perceive.  

Another thing I've noticed is that a wick will not work for me after being left unused over the summer, even if burned dry at the end of the season.  This has never worked for me, even though there are those that say they are able to re-use wicks from season to season.  I've tried an the performance is very poor.  Poor perfomance means low burn rate, low flame, lots of smell, and probably more carbon monoxide too.  

I wish there was more scientific data on this, but for me now, I try to use what I buy within 1 year.  I do end up with some stored over the summer, and so far it's working good, but that's un-dyed, kept in dry storage, although it does get above 100ºF for spells.  

Kerosene is essentially just very pure diesel. It will last indefinitely when kept in a proper container. The only fuel that simply will not stand up to long-term storage is gasoline. I suspect the shelf-life of gas is even shorter now with the increase in ethanol. I keep four 275 gallon tanks with diesel, kero, gas and the fourth is where I keep WMO. I keep the contents of the gas tank in-cycle with my ATV and yard equipment. Its always getting refreshed every two weeks.

About the only thing you can mess up in a kero heater is the wick. If you get water/condensation in the tank it's time for a new wick. They are a minor chore (and icky, nasty, dirty) to replace, but that's the first thing I do when trying to rehab a kero heater.

Thanks...,..

Have an A.A.S. In Diesel Tech so figured the fuel should be good for this aplication at least a few years.

Thing is, I only have two days to fix...

i have a torpedo style 55,000 BTU heater and its had the same fuel in it for 8+ years. i filled it once 8+ years ago and only use it a maybee an hour a  year.  it burns just fine.

i use it when i change my oil in the winter.

Kero and diesel will last for decades if stored in a sealed metal container.  I've used 5 yr old kero and 20 yr old diesel with no loss in performance.

Bad kero will smell bad.  I don't know how to describe the smell of bad kero, other than to tell you that is smells "old."

Ops