Idle problem, vacuum leak?
 

My car idles like shit, it bounces from like 600rpm to around 850, just does that constantly. (goes buh-rump, buh-rump, buh-rump) Anyway, I think I have a vacuum leak somewhere thats causing it, and I *think* its coming from the back of the TB. It seems to be coming from the little white cylinder thing with two nippes on it for vaccum hoses, one hose goes down to the UIM, and the other goes underneath the TB to a little vacuum diaphragm on the front of the TB. I seem to have a leak coming from thay little white cylinder (thats where i can hear the sucking sound the loudest) but I dont know exactly where its leaking. Could someone tell me what that thing is, and how/if there could be a leak there.....and if there was a small leak, would that cause my idle to do what its doing? Thanks

Also if I brake hard to a stop it almost wants to stall, but it usually brings itself back up. My TPS is adjusted properly also.

Please?

Why would you think It's coming from there? If you know where It Is coming from than why haven't you placed the air hose(s) back on? That thing Is a Water Thermo Valve. So In your case, you have two coming out of It (3 If you had Automatic Transmission). I suspect you are not leaking from there b'c you have Identified the hoses coming out of It and you probably put them back on to their original place.

If I were you, I'd start the car up. Then on the next place I suspect the air leak to be give a mild shot of Starter Fluid (aerosal can) and check for an Increase of revving has occured. Doing this will allow you to zero In on where the leak Is (somewhat). Please don't do this for extended periods of time. Better yet, I'd start over and check ALL vacuum connections before I preceed to the following step.

Yes, I have checked all vaccum lines, even took the UIM off to see if everything was ok under there. All my vacuum lines are on, and not cracked or anything. On the very end of that water thermo valve, it seems to have some sort of a cap. I can like spin it, I thought maybe it screwed on or off, but after turning it for like 10 minutes It doesnt seem to do anything. Its not cracked or anything either. I did spray some carb cleaner, and the idle did seem to smooth out when I sprayed back there, but i cant find the leak for the life of me :(

Well you have removed the Intake off completely, right? Therefore I'd say you are familiar with the vacuum hoses and the like. I'd rule out vacuum leaks and possibly trouble shoot elsewhere. Electrical, I dunno know.

Coincidently, I have my entire Manifolds off aswell, am preping for some porting, and I can turn the cap of the Water Thermo Valve. Not freely but with some effort.

Hmmm....I couldnt get the cap off after like 10-15 minutes of trying. Maybe I should play with that some more.... Yes, Ive had the intake (upper and lower manifolds) off before, Actually many times, I could probably do it blindfolded. I would try and look for another cause, but I can actually hear a leak somewhere, so thats gotta be it.

just read this old post while searching for a solution to my problem.
i currently have the same EXACT problem with my 91 na.

my idle jumps from 600-800 but steadies out perfectly if i unplug the vacuum hose from the water thermo valve (the hose not thats not going underneath the throttle body to the double throttle diaphram).

my idle also slightly dips when i push the brakes down...just like the circumstance in this post.

did anybody ever solve this?
i think its something with my water thermo valve or maybe an electical problem?

my vacuum lines are plugged into the water thermo valve just like its done on page f1-35 on this link:
http://208.62.166.66:82/guest/shop_m..._non_turbo.pdf
i did notice that in the illustations on the following pages (f1-36 and f1-37) mazda shows the vacuum lines plugged differently into the water thermo valve... they are switched around in some pictures with the check valve on the line that is going to the double throttle diapram... whats up with that??
btw.. ive got a 5speed 91na

hoping to hear from you guys.. im stoooped hard