Inconsistent boost, right after 3" exhaust install, BOV, MBC or WG ?? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-03-2010, 12:16 PM   #1
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Default Inconsistent boost, right after 3" exhaust install, BOV, MBC or WG ??

So I finally have a 3" exhaust on the car. The big one as recommended and no CAT or resonators. It's great but I've been having issues lately.


Its a BEGI log, with a 2560 and SGDP I have a brass MBC and it was set to 12psi before I had the exhaust. I set it to about 8 when I dropped the car off in case they drove it, and because I was worried about overboosting with the new exhaust. I slowly moved it up to 12 in increments and it was good for a day or 2.

Now though I sometimes get boost building normally and other times its 5000 rpm before I get 5 psi. I've looked for a pattern but I have not driven it enough to find one. It feels a bit boggy when it doesn't build properly.



In my searching I see that it could be a stuck wastegate, so as in stuck open so it wont let boost build?? Is the new 3" and associated flow putting more pressure on it and its cracked open a touch meaning I should add preload?? Or is it something with the MBC. I have the MBC plummed post intercooler as I was taught here.

What about my BOSCH BOV. It used to make a cool howling owl sound just around a vacuum of 3 just under light loads now it wont talk to me.

I'll search for how to test each one properly but for now I'll engage the minds of all y'all.
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Old 09-03-2010, 12:20 PM   #2
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plug your BOV and see if its stuck open.
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Old 09-03-2010, 12:25 PM   #3
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Plug the inlet side coming from the line before the TB - correct?

I knew I should have bought some hemostats after I lost my old set!!

I'll also do some searching on BOV's, I occassionally like the sound of the atmo ones but it could also get old.
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Old 09-03-2010, 12:26 PM   #4
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check bov, check wga, check mbc to make sure each is hooked up correctly and is getting a proper vacuum/boost feed. then check that both the bov and the wga aren't "sticking". lubericate the bov if it needs it (if yours comes apart)

if all are hooked up right check to make sure the wga isn't loose. if it is tighten the arm.

start there
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Old 09-03-2010, 01:19 PM   #5
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When i used to have a BEGI kit, the wastegate flapper door had gotten stuck on the inside of the downpipe once. It took me forever to figure it out. I would disconnect the wastegate from the flapper arm and make sure it flaps properly. Mine was getting stuck on its way back (shutting) on the inside of the separated gases dp. I had to take a dremel to the inside and smooth it out a tad and loosen and slightly shift the dp to get it to work properly again.
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Old 09-03-2010, 01:32 PM   #6
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/\ that would make a LOT of sense based on his description of the problem
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Old 09-03-2010, 01:33 PM   #7
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Please no, not unbolting the thing ......oh the humanity


It only started once the 3" was installed so perhaps just more preload.


*** Update: Took some things out of the way and tried the WG, and yes the WG is catching and it seems to catch through the whole range of motion. I guess I'm taking of the DP and we will see what needs to be modded.

Last edited by Landrew; 09-04-2010 at 03:22 AM.
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Old 09-06-2010, 01:18 PM   #8
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+1

My BEGI divorced exhaust mani had the same problem. Required a bit of work with grinder to clear wastegate.
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Old 09-06-2010, 01:32 PM   #9
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Quote:
Originally Posted by Landrew View Post
Please no, not unbolting the thing ......oh the humanity
We understand
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Old 09-07-2010, 11:55 AM   #10
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So here is what I found:

IMG00317-20100906-2237.jpg
You can just see some scrape marks on the seperator.
IMG00316-20100906-2237.jpg



I wish I had of known that this was a potential problem since I would have taken care of it before installing !! I took the time to port the WG so I would have certainly been able to set this up.

I'm going to grind it while in the engine bay as you see it hanging there. I will thin out the seperator and take of a mm or 2. Should I also grind the perimeter weld on the ID of the DP WG hole? I'll test fit it and see where and or if it contacts that hole?



PS - when you guys do the clutch is it that much easier to do with the BEGI DP off?? I've never done it and it's going to start slipping soon so if I can save an hour or 2 now I might as well do it.
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Old 09-08-2010, 02:17 PM   #11
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Quote:
Originally Posted by MeOughtta View Post
+1

My BEGI divorced exhaust mani had the same problem. Required a bit of work with grinder to clear wastegate.
I did not know we made a divorced gas exhaust manifold, that can catch a wastegate....
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Old 09-08-2010, 02:22 PM   #12
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Quote:
Originally Posted by Landrew View Post
So here is what I found:


You can just see some scrape marks on the seperator.




I wish I had of known that this was a potential problem since I would have taken care of it before installing !! I took the time to port the WG so I would have certainly been able to set this up.

I'm going to grind it while in the engine bay as you see it hanging there. I will thin out the seperator and take of a mm or 2. Should I also grind the perimeter weld on the ID of the DP WG hole? I'll test fit it and see where and or if it contacts that hole?



PS - when you guys do the clutch is it that much easier to do with the BEGI DP off?? I've never done it and it's going to start slipping soon so if I can save an hour or 2 now I might as well do it.
yup you're most likely going to need to remove the downpipe and exhaust to take the tranny off and swap the clutch. look for samnavys write-up for the clutch it helps a bunch.
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Old 09-08-2010, 02:23 PM   #13
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Quote:
Originally Posted by Landrew View Post
I'm going to grind it while in the engine bay as you see it hanging there. I will thin out the seperator and take of a mm or 2. Should I also grind the perimeter weld on the ID of the DP WG hole? I'll test fit it and see where and or if it contacts that hole?

PS - when you guys do the clutch is it that much easier to do with the BEGI DP off?? I've never done it and it's going to start slipping soon so if I can save an hour or 2 now I might as well do it.
You got an old downpipe, sorry about that. Usually those caught on the upper side, and not the divider. However, you can *barely* trim the flapper door in addition to grinding the divider down. If you need some more help, feel free to give Corky a call.

We do clutches with the downpipe in and out. Which ever is easiest for you...
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Old 09-08-2010, 02:24 PM   #14
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Old 09-08-2010, 02:32 PM   #15
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Thanks to all for the diagnosis and repair tips.

I saw that clutch writeup and with the included pics it is extremely helpful.
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Old 09-08-2010, 03:26 PM   #16
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Landrew,

Others have already commented on how to deal with the "sticking flapper". Sorry if you already mentioned it but what is the current PSI rating on your actuator? The reason I mention this is because I had all the issues/symptoms you mentioned. I discovered the problem was, in addition to the BEGI "sticking flapper" issue, when I added the 3" exhaust the wastegate actuator was definitely not strong enough to hold the flapper closed no matter how much preload was added. I replaced the actuator from a 6 psi to a 9 psi and the inconsistent boost went away. Just a thought. GL
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Old 09-08-2010, 03:51 PM   #17
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I'm not sure which one I have. I will find out

It has about a mm of preload. Perhaps I will add 1 or 2 full turns tighter on the rod as I'm re-assembling everything.

I may do the clutch as well since the DP is loose so it will be a few days until driving it as I have house projects and a motocross trip in the rockies this Fri/Sat !

never enough time...........
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Old 09-20-2010, 05:44 PM   #18
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Did some grinding and dremeling of the SGDP and put it all back. It works great now.

Thanks for the diagnosis - twas spot on.
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