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Old 08-06-2020, 10:12 PM   #1
Savoelh's Avatar
Join Date: Jul 2018
Drives: Scion FRS
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Missing: BOOST - if found please post here -

I have found an interesting / complex problem with my turbo frs.

The set up: 10.5 to 1 iag block with precision 5558 turbo as well as an external waste gate by precision. Tuned by delicious

The problem: Not boosting past a certain threshold
The specifics: 91 will not go past 3 psi. E85 with not go past 6 psi (at 1 time it would go up to 7 or 8 but that doesn't happen anymore)

I have a boost controller but I have bypassed it by running the vacuum tube from the turbo to the top of the waste gate. Here I get the same psi as I do when everything is hooked up correctly.3 psi on 91 and 6 psi on e85. My waste gate spring is 6 pounds.

Here's the interesting part. When the vacuum tube is routed into the boost controller / waste gate correctly BUT the vacuum tube has been removed from the turbo the car boosts just fine. I have seen it go up to 16 psi on e85.

What I have done: I have replaced the boost controller as well as the ECU. My tuner is 100% sure that it has nothing to do with the tune. My mechanic says it's not a vacuum leak.

Things you should know: The last thing I did to the car before all of this happened was that I replaced the auto transmission with a manual. That means both front chassis wire and the dash wire harness were replaced. So was the ECU and the abs module. The vacuum tube from the brake booster was routed into the intake manifold as that's how I saw it done on a stock version of our cars.

The question: why does the car boost just fine with the vacuum tube disconnected from the turbo but poorly when everything is routed correctly? Any ideas on what to check next?
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