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Old 10-17-2006, 02:20 PM   #1 (permalink)
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Default What Causes Overboosting?

Hi all,

I have 94 Steath TT which has an HKS intake, full exshaust work and a boost controlller.
The car is having a over boosting problem.
When driving normally and not stepping on the gas, the car drives fines. When driving the car hard, it takes the boost upto 20 psi and then I get a fuel cut.
What can cause this?
I have a HKS EVC installed, which I disconnected and replaced it with a Manual boost controller for trouble shooting purposes but still having the same issue.
As it is overboosting, does that mean I have bad turbo's?
The turbos are stock and the car has 150kms on it.
In need of help,

thanks
Manj
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Old 10-17-2006, 03:03 PM   #2 (permalink)
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Default Re: What Causes Overboosting?

It does not mean you have a bad turbo, but could mean your wastegate actuator is not functioning properly. You could also have a poor vacuum configuration.

have you done the vac reduction? I would suggest doing that first, and possibly plugging the wastegate actuators directly up to the y-pipe, eliminating all chance of error (assuming those lines do not have any holes)

If they still overboost, I would look to possibly getting new wastegate actuators, or trying to shim your current ones
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Old 10-17-2006, 03:24 PM   #3 (permalink)
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Default Re: What Causes Overboosting?

I have not done the boost reduction.
And what do u mean by "shim" the current wastegate actuators?
And, the hoses are still original.
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Old 10-17-2006, 03:27 PM   #4 (permalink)
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Default Re: What Causes Overboosting?

Check wastegates for proper functionality using an air compressor and the vac line, also check for boost leaks in the vac hoses, or completely disconnected vac hoses.
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Old 10-17-2006, 03:50 PM   #5 (permalink)
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Default Re: What Causes Overboosting?

In order for me to check the wastegate, how would I test it using Air Compressor and Vac line.
Is this a simple procedure anyone can do. I'm not that savvy when it comes to anything under the hood
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Old 10-17-2006, 04:08 PM   #6 (permalink)
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Default Re: What Causes Overboosting?

If you apply 7psi or more to the vac/boost line going to the wastegate it will operate (you`ll see the arm moving), another way to test is to just link the two pipes that go the stock boost control solenoid, if it`s all the piping is stock this will give you 6 to 7 psi of boost only, if you get more it`s either a sticking wastegate or the vac lines are routed wrong.
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Old 10-17-2006, 04:28 PM   #7 (permalink)
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Default Re: What Causes Overboosting?

sounds like something just isnt hooked up right
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Old 10-17-2006, 04:31 PM   #8 (permalink)
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Default Re: What Causes Overboosting?

keithmac: As I have a Manual Boost controller, does that mean I have eliminated my stock boost control solenoid?
And, I am not clear on how to measure the boost. You stated, "if you get more, its either a sticking wastegate or vac line"
Would I need a special tool to do this?
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Old 10-17-2006, 06:18 PM   #9 (permalink)
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Default Re: What Causes Overboosting?

Take the MBC off and link the pipes together, this way the wastegates are going to see the actual turbo pressure. Drive the car carefully on boost and check to see if it stays at 6-7psi using your boost gauge, if it stays at 7psi you know the wastegates/ piping is ok.

If it overboosts you know you have a problem with the pipes/ pipe routing or wastegates sticking, take a look at the sticker under the bonnet and make sure the vac lines are connected exactly the same as the diagram (it is confusing until you find your way around it!). After you`ve got consitant boost at 7psi you can re-fit the MBC and tune it in.
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Old 10-17-2006, 06:20 PM   #10 (permalink)
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Default Re: What Causes Overboosting?

On my setup I have the Y-pipe nipple piped to the boost control solenoid (you`d be to the MBC) and out to the 2 wastegates with the unused stock lines capped off.
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