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Old 05-10-2018, 12:44 PM   #1
nathanlevang's Avatar
Join Date: Apr 2016
Location: Santa Clarita, CA
Thumbs down Boost Fading Issue? 1st pull reaches full boost, very slow to boost thereafter

Hey guys! So let me preface with how the car was before the issue arose.

I had the engine running pretty well with my MBC set to boost at around 14psi. Turbo worked well, car reached peak boost easily, not really any problem.

The other day, I noticed my boost gauge wasn't reading where I was used to it, was only reaching about 7psi and when I was able to get it to peak boost, it would fall off to about 7psi after a few seconds.

I then found a pattern. Every first pull would reach 14psi quickly. I found this phenomenon where I would have to wait to pull again in order to hit full boost, we'll call it my "turbo reset." I couldn't hit any boost for ~30 seconds, otherwise, the turbo wouldn't "reset." Let's say I try to hit boost very soon after the first pull. Every subsequent pull within ~30 seconds of my first one would either boost slowly or only reach about 7psi.

For diagnostic purposes, my car is running stock boost pressure. I've since taken apart my CBV and the spring and diaphragm seem to be in good health, and I don't suspect any boost leaks since I just finished overhauling the entire vacuum system and I've taken apart, cleaned, and put back together all the turbo piping.

My mind wants to say that the internal wastegate is sticking inside the turbo, or the CBV spring is worn out, but I want to know if there are any other tests I should be performing. A boost leak makes sense, but if there was a leak somewhere, the boost pressure wouldn't be so inconsistent.

Thanks TB fam
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Old 05-10-2018, 01:11 PM   #2
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Join Date: Sep 2005

seems like the wastegate valve does not close fast enough. If you are using an MBC there should be a small venting/bleeding hole in the MBC body. Maybe that's clogged? Or too small? If the pressure in the wastegate actuator canister can't bleed away fast enough pressure in that canister will prevent the wastegate valve to close. This hurts fast spool up.
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Old 05-10-2018, 01:11 PM   #3
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Join Date: Sep 2012

I would go directly to that wastegate and have a look. I would ALSO look at your boost controller as it may be sticking.

Does this happen at stock boost? Without your MBC connected? I would isolate your MBC and see if you have boost issues on wastegate. If not, it's probably your MBC sticking. If you still have boost issues, then it might be the wastegate actuator leaking/sticking/etc.
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