Strange Vac/Idle issue, what could it be
 

OK this started like a week and a half ago, strange and intermittent problem. How the car ran all the time before

800rpm idle always
around 1bar (14.5-15hg) at idle, 19 decel in gear


How the car runs now, and keep in mind, this is not a constant issue, just comes and goes. Still builds full boost, and at the same rate as before.

Idle will vary from 1000-1300rpm
around 18hg at idle, still 19 decel in gear

Also when this issue comes in, I can let off the gas and the car keeps going as if I am still lightly on it (keeps accelerating). If I shut the car off when this is happening and fire it back up it goes away for a bit and then comes back.

Sometimes when I am slowly spooling up the turbo and the gauge still reads in vaccum, I can hear something that sounds like an air leak, no idea what it really is, I've checked the lines and everything looks intact and tight.

I'm thinking that maybe the sticking throttle issue can be related to the EGR valve that may need to get cleaned? I know I have to re-check vac lines for some reason I think I missed something. Takes on this problem?

i have no idea what year your car is, but most mazdas ive worked on always seemed to have an egr or intake related problem, especially when an idle problem is involved. when i say intake related problem, i mean something like a MAF or a tps issue. the latest one i worked on was an mx-6, which was having an idle issue like you are. It turned out to be the tps AND the coolant temp sensor. but again every car is different, so i would look into your IAC as well.
i also thought i would mention that in my eyes, mazdas are known for carbon buildup issues so maybe cleaning out the IAC and egr would be a good and cheap start...

Yea the EGR seems to be a common issue. This is a 2004 Mazdaspeed MX-5 with a few mods.

I know where the EGR valve is, no idea on the IAC, and I should probbably use some maf cleaner too and clean my air filter.

the iac is usually on the throttle body...it usually has at least one coolant line going to it and most of the time, some sort of electrical connector as well...iac is short for idle air control valve and i wouldnt be surprised if that was your problem...i think it can also be called an idle speed control or something of that nature...

I'll have to look into that, but I want to take off my bumper and check all the couplers for the FMIC as well, as well as re-check all the lines under the hood, just based on that sound I hear when the turbo spins in vaccum, it definately sounds like my friend's evo when we didn't tighten one of the couplers down all the way for the FMIC, every time he would go into boost it sounded like a duck.

Not always though. It can also be just a plain motor that will allow a certain amount of air to bypass the throttle body on idle. It will adjust the idle per the ECU which receives a signal from the coolant temp sensor so it knows the engines temperature and how to set the idle.

Put some tape temporarily on my bov-intake return line where i found a burn spot, and re-tightened all of the clamps. Gonna see on my long-ish drive to class tomorrow if the issue has gone away.

I'm going to guess that the problem stemmed from a boost leak from that leaky line. Replace that, and then boost-leak test the entire car to 5PSI more than you'd normally run at any given time.

Well even after taping that line, still having the issue. The sound of a boost leak seems to only happen when the engine is still cold, once it warms up it goes away but the leak (or whatever is happening) doesn't. Could be that the BOV needs to get cleaned and lubed, could it possibly be because my air filter is dirty? Maybe TB needs to be cleaned?

It only happened twice on my way to school today vs. about 10 on my return trip on Monday but the issue is still here regardless.

I am not an expert on those cars but it seems that it could be a vac/air leak somewhere. I hope you get it figured out soon.

you have a bov? they do need to be lubed sometimes IIRC. they can get stuck open.