Can’t get temp right

Dorado318

have a medium egg and I start what looks like a good fire then when I add the diffuser it won’t come up to temp.  Same problem when. Trying to run hot over 450 no diffuser 

lousubcap

There are some useful posts within this related thread.  I would give it a look and then see how it goes.  FWIW-

Newbie needing guidance

1911Man

IME, when adding any diffuser (either made by BGE, or from CGS) you need to give the egg time to get to it's set temp. Depending on how you set the vents, will determine what temps you'll see. 
I've found, with my medium BGE, that I have to almost completely close both the top and bottom vents to get in the 250-275 range with the CGS stone in position (full circle) in the woo ring.
On the other end of the spectrum, when grilling, I you have to open things up more to get the higher temps. I often grill at around 450F (up to about 500 at times) without trouble.
If you have an older egg with the rotary style top cooking cap, get either one of the new style from BGE, or one of the Smokeware caps (I have one of those). The Smokeware cap does make things easier. 
I've also found having the Kickash basket, with the bottom coal grate removed, also makes things easier and allows more air flow from the bottom.
Best way to learn the vent positions is simply by using the BGE and noting the positions for the temperatures.
Also, I usually have the vents open during the preheat stage before smoking on the BGE. Then dial things back once the flavor wood, stone (and pan) and meat has been added. I look at the dome thermometer about every 15-30 minutes to make sure it doesn't get away from me. I did have one cook that had overflow into the coals (rendered turkey fat) that caused things to go a bit sideways. Which is why I got the woo ring and such (shortly after that).