So I have just solved a lot of problems with my evo the past couple days. I pulled a p0300 code earlier today so I changed my plugs and it runs fine.

Now for the problem...I have a boost gauge in bar that goes to 1.5....I'm pretty sure that's close to 22psi and I max out my gauge. Earlier today I got into it to see if everything was running fine. I watched my boost gauge go up and when I hit about 5k in 3rd my boost dropped down to 17 very fast. It still runs fine anyother time. I'm using the APS twin vent BOV. Any ideas?

 

It really depends on how fast you mean. It is normal to see a boost drop, but if its like 4900 22lbs then 5000 17 lbs., there may be a boost leak. This can be in a number of places so I suggest doing a boost leak test. A leak in the exhaust manifold, or a loose turbo clamp could also cause this issue. The last thing I can think of is if you tighten the wastegate actuator it'll hold your boost a little better. Good luck!

 

It's pretty much exactly like that. It just plummets. I'm not really sure how to do a boost leak check or what all I would need for it.

 

look at all the vacuum hoses, they are a common problem
I find that 22psi peak is some way high

 

I'm not sure how much I'm boosting. My gauge pegs so I just know it's above 22. I'm not sure if it fixed it or not but I had an IC hose slipping off the clamp. It was still on just not very well.

 

What boost controller are you using?

 

Boost hose likely the culprit. Especially if you still have the stock piping. The factory couplers wear out over time. Your prob going to keep blowing it off. You can try hair spray but you might just want to replace the couplers with silicon.

 

Could it be the ECU is detecting knock and lowering the boost suddenly? Can you do a datalog of a 3rd gear pull?