I have a sour beer when its not supposed be.

[Gretas Brew]

I made a spiced pumpkin ale recently using the BIAB method for the first time. I was pretty excited about going all grain for the first time after years of extract brewing. So it was frustrating when I tasted it for the first time after 3 weeks of bottle conditioning and it tasted pretty sour with none of the expensive pumpkin I put in. Does anybody know what I could have done to cause that so I can prevent it next time?

[MaltLicker]

If it's so sour that all residual sweetness and even things like pumpkin flavor is gone, then I would suspect some infection from a bug like acetobactor or similar that just keeps eating sugars until everything is gone. 

Soak everything you can with a hot PBW/oxyclean solution, and then a sanitizer of choice, such as StarSan.  If you have any doubts, toss it and replace it.   

Take stuff apart that you don't usually break down, such as ball valves.   I break valves down after every brew, and every time I get hoppy wort juice on my kitchen counter that would be stuck in that valve and potentially ruin a batch if I didn't clean and sanitize it out of there. 

[Rep]

After completing the ideas MaltLicker proposed I would suggest you brew a simple all grain with no adjuncts.

Brew an Amber Ale, Stout or Porter.  Get your new all grain procedures nailed down with beers that will allow for some mild off flavors. 

Good luck

[jhough]

Gretas  need more input before we can help !!  How did you brew, sparge, volume ,carb, condition . Would love to give some help , but , need some variables .
 

[Gretas Brew]

Sorry I have been away from the site for a little bit. Well jhough I use the BIAB method with no sparge. I only have a 8 gallon kettle so i can't do a full boil like i should with BIAB. I bottle condition. In hindsight i don't think i sanitized my bottle like i should have. I won't make that mistake again.

[Maine Homebrewer]

I chew a kernel or two of every grain, taste the pumpkin or whatever else I add to the mash, taste the mash, taste the part I boil for decoction, taste the runnings off the sparge, taste a spoon or two during the boil, taste the sample I take for O.G., taste the sample I take for F.G., I'll even taste a fingertip of a yeast if I've never used it.. I taste it all.

For you to be surprised at not tasting the pumpkin leads me to think I take way more taste tests than you.

As for the sour part, hold an unopened bottle or two up to the light and look for rings in the necks.  If you don't see then they the sour flavor is probably caused by something other than an infection that, as most do, leaves rings in the necks of bottles.