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No or low boost in lower gears?

 
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GregSC  



Joined: 19 Jul 2008
Posts: 99
Location: Lovely, KY

PostPosted: Fri Oct 02, 2020 12:15 pm    Post subject: No or low boost in lower gears? Reply with quote

Hey all,
I've recently done a lot of work to the exhaust system and turbo on my 931, and while it currently runs better than before in my ownership, as I continue to shake it out after the exhaust and turbo refresh and intercooler install it has recently displayed some unusual behavior.
When I first start the car up for a drive I've noticed that it takes a couple of minutes of driving before the boost will come on or before I can hear the turbo spool up (pretty distinct sound now that the exhaust leaks seem fixed and turbo rebuilt). Additionally, while I'm in first or second gear, even when having been driven a while, the situation doesn't get much better. I can hear the turbo spool up, but boost is minimal, like 5 or 6 psi, when it is set at right under 10 at the moment. In 4th or 5th gear no problem it seems, boost showing right under 10 psi on an older VDO analog gauge.
Any ideas or experience with this? Is it a boost leak somewhere perhaps? I did rebuild the wastegate, perhaps it is sticking, or my rebuild is not up to par? Any help would be greatly appreciated as always!
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chuck21401  



Joined: 20 Feb 2005
Posts: 322
Location: Annapolis, MD

PostPosted: Tue Oct 06, 2020 8:43 am    Post subject: Re: No or low boost in lower gears? Reply with quote

GregSC wrote:
Hey all,
I've recently done a lot of work to the exhaust system and turbo on my 931, and while it currently runs better than before in my ownership, as I continue to shake it out after the exhaust and turbo refresh and intercooler install it has recently displayed some unusual behavior.
When I first start the car up for a drive I've noticed that it takes a couple of minutes of driving before the boost will come on or before I can hear the turbo spool up (pretty distinct sound now that the exhaust leaks seem fixed and turbo rebuilt). Additionally, while I'm in first or second gear, even when having been driven a while, the situation doesn't get much better. I can hear the turbo spool up, but boost is minimal, like 5 or 6 psi, when it is set at right under 10 at the moment. In 4th or 5th gear no problem it seems, boost showing right under 10 psi on an older VDO analog gauge.
Any ideas or experience with this? Is it a boost leak somewhere perhaps? I did rebuild the wastegate, perhaps it is sticking, or my rebuild is not up to par? Any help would be greatly appreciated as always!


Seems like a problem with the wastegate would result in too much boost rather than too little. And if the wastegate is sticking open...then you'd never get to 10psi in the higher gears right? But then again that's a big spring pushing that valve shut...I'm not sure what would have to get in the way to keep it open.

So I'm thinking boost leak(s). Many threads have discussed chasing leaks on this board...I'm guessing that you are reading through those. Maybe it's something simple like the big O ring between the intercooler and the throttle body? In this diagram I believe it is 24.



With my car the boost spools up in all gears. Of course it also depends on how much throttle is being applied. I have the gauge that reads vacuum and boost. At the same time I can run through the gears without seeing positive boost on the gauge if I keep the RPMs down/light throttle.


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GregSC  



Joined: 19 Jul 2008
Posts: 99
Location: Lovely, KY

PostPosted: Tue Oct 06, 2020 11:30 am    Post subject: Reply with quote

Hey Chuck, thanks for the reply! The more I think about the symptoms the more it seems like a boost leak, at least from the little experience I've had with it. This past weekend I did a cursory check for obvious leaks but didn't see anything that jumped out. The large orange o-ring and seal seemed securely in place. I did use an old fuel hose with appropriate fittings on the ends as an extension to connect the original boost control line from the wastegate to the intercooler. Perhaps there is a leak at one of the fittings or in the line under boost. I think I'll try to re-plumb that with new line and some metric to hose barb fittings to see if that makes a difference. Hoping that's it instead of something harder to reach or that has to be deconstructed. Really don't want to take the exhaust system apart again!

And yeah, definitely giving enough throttle to induce boost, 4000 + rpm. I can hear and feel the difference in the engine when it's working by itself and when boost is present.
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