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Boost Control Problems?

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So I have a HX35w on my L28et, Tial 38mm with a yellow large spring which is supposedly 10psi based on equal intake and exhaust pressures, I have a boost gauge inside the car as well as datalogs of runs with megasquirt. I just welded the original internal gate closed.


I had a few times where I have had my boost go up to 20 psi, according to my boost gauge and MAP sensor. Almost anytime I get on the throttle the boost gauge goes crazy and jumps all over the place. Generally between 0-10 psi. I went back to see my datalog and it shows the MAP to be smooth. How much filtering is done on the MAP sensor? Fluctuations like this dont seem normal, is there a possibility that my gauge is just messed up and resonating or something like that? Both MAP sensor and gauge are both coming off of the manifold.


I cant say for sure if the wastegate is opening because I am generally watching the gauge as I apply the throttle in an effort to keep it in the 10psi range. Im gonna do some more testing today to try and figure that out though. It has its air line going to the manifold as well. The top of the wastegate is just left open to atmospheric.


In the past I have had a couple times where the boost gauge came up steady and held but that was a while back and I havent got it to do it since. Im gonna try to get it to do it again today.


Also, although my car isnt tuned well yet it seems like its way faster to slowly lay into the throttle than just go to full throttle.. I kind of remember something like this on my ZX originally but is that normal or just my engine not running correctly? Again Ill have to go test it some more but if my memory is correct its been way faster when the boost holds consistent on the gauge, the times where its all over the place it doesnt pull well.

Edited by Challenger
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What do you have to control boost? Just spring of the WG, or is there a solenoid controlled by the MS or other controller?


Check the hose that goes to the gauge, there may be a small hole in it that opens at a pressure closer to 10 PSI, then closes and allows pressure to build again to around 10 PSI, rinse and repeat, I've seen such things happen before.


Any leaks in the exhaust, especially pre-turbine? Those can cause some serious boost control issues.


Where is the pressure source for the WG connected?


Some pictures of the set-up may help figure out what may be an issue.

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Its just the spring of the waste gate and the air line going to the manifold.


No leaks that I am aware of. Ill check though.


Ill check the boost gauge for leaks.


Ill try to get some good pictures.


Downpipe, you can see where the exit to the wastegate is.




Welded to the original turbo manifold is one of these. Wastegate bolts directly to the adaptor below and the downpipe.




Hard to see but here it is. As I mentioned, the port on the far side of the wastegate diaphram is open to atmosphere and the near side goes directly to the manifold.




This is from a run I just did, again I wasnt full throttle so I dont know how accurate it is but it went up to about 8+ psi and held till I left off. Seems like it was bouncing around 8 psi so I hope that is the waste gate opening.


Green is MAP and red is TP.



Edited by Challenger
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Subscribed; I have a very similiar setup as far as the Tial wastegate/adapter/downpipe/MS2 and am having a similiar problem as you describe. If I throttle slowly the RPM's increase smoothly and boost builds until I release throttle and it blows off and I've watched my gauge steady around 7 to 8 psi. But when I quickly depress throttle/full throttle and try to wind out the RPM's the car seems to not want to rise past 4k or so. It bucks wildly and makes a loud repeated chirping noise kind of like a chirping tire on a veh with locked diff taking a corner. But while this is going on if I slowly let back out of the throttle and ease into it then it will begin to climb past 4k onto 5 to 6k. Now I have been suspecting the Wastegate and have it controlled on a GReddy Profec B set at 12. I did notice when mashing the throttle, the boost rockets up and when it nears the 12 psi mark it begins this bucking and chatter <- because of this I have been suspecting the Wastegate and want to begin testing but just haven't dove into it yet. If you find out the cause of your problem please share and when I start checking into my car I will do the same.

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It was dropping down to 12.3-11.9ish with the headlights on. The wiring had too much resistance(because of several splices) and couldn't really keep up with the demand. I was referring to the post above mine, your problem just seems a bit different.

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  • 2 weeks later...

Well I turned off the Profec and it's just on the spring and seems to climb and hold better at around 12-15lbs, but I still get the same symptoms during WOT's. But on a side note I did find some small exhaust leaks (2) pre turbo, (1) post turbo that I will need to fix before testing again. If still acting up then I suspect checking the insides of that WG or some Profec tuning.

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