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Discussion Starter · #1 ·
So just recently, my car stop making boost.... I should say that it is 100% stock; no mods; daily driver and I'm not nearly as tech-savvy as most of you are, so maybe you have some ideas or answers...

The car is not making boost, is slow and when giving it gas, it tries to go, but is slow and makes the "whooooshh" sound. Indy mechanic pulled the code for TCV "signal too high" and is going to replace the valve.

My question is if the failed TCV is causing the wastegate to stay open, would the car make the air-leak sound I'm hearing at WOT? Is the excess air dumped to the atmosphere or back into the exhaust? If back into the exhaust, would I still be able to hear it as the "whoosh" sound? The sound is right in the general area of the turbo and is just when it's at WOT, not at idle or old-lady driving around town. He looked at all the piping and hoses for leaks and they all looked good.

Thx.
 

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This sounds very similar to what I'm experiencing as well. I haven't had any codes pulled, so you're at least a step ahead of me there. I don't know the answer to your questions about the wastegate, but I'm thinking replacing the TCV will probably do the trick. I'll likely end up doing the same.
 

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Discussion Starter · #3 · (Edited)
This sounds very similar to what I'm experiencing as well. I haven't had any codes pulled, so you're at least a step ahead of me there. I don't know the answer to your questions about the wastegate, but I'm thinking replacing the TCV will probably do the trick. I'll likely end up doing the same.
Thx. I know if the code says the TCV is no good and that explains all the symptoms I'm having. My main question is if the TCV is shot and it's causing the wastegate to dump or bypass all the air into the exhaust, would it make the sound of an airleak in our cars that you could hear in the engine bay?

Just trying to rule out if there is an actual boost leak hiding somewhere that is in addition to the TCV.... Other than that, I guess I'll just wait until the TCV gets replaced and see if that fixes it 100%.
 

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I don't believe so, but I'll let someone else chime in. I assume what you're hearing is just the turbo during a slight period of overboost.
 

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My main question is if the TCV is shot and it's causing the wastegate to dump or bypass all the air into the exhaust, would it make the sound of an airleak in our cars that you could hear in the engine bay?
The wastegate does not dump air, it dumps exhaust. Normally, the wastegate is closed and exhaust passes through the turbo, spinning it, which creates boost by compressing intake air. When this is not needed, either because you aren't on the pedal, or boost has reached a maximum value, the wastegate opens, dumping exhaust directly out of the manifold. The exhaust will sound different when this happens.

There is a second valve called the CBV - Compressor Bypass Valve, which can open to recirculate intake air. This is a passive valve, not controlled like the TCV/wastegate, it opens when you lift off the pedal quickly but it can also open in severe overboost conditions. It will cause a whooshing noise.

If you have a TCV code, take care of that before diagnosing anything else. A non-functioning TCV will generally hold the wastegate closed. But in general, if you don't have a check-engine indication, there is no boost leak on the intake side.
 

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Discussion Starter · #6 ·
The wastegate does not dump air, it dumps exhaust. Normally, the wastegate is closed and exhaust passes through the turbo, spinning it, which creates boost by compressing intake air. When this is not needed, either because you aren't on the pedal, or boost has reached a maximum value, the wastegate opens, dumping exhaust directly out of the manifold. The exhaust will sound different when this happens.

There is a second valve called the CBV - Compressor Bypass Valve, which can open to recirculate intake air. This is a passive valve, not controlled like the TCV/wastegate, it opens when you lift off the pedal quickly but it can also open in severe overboost conditions. It will cause a whooshing noise.

If you have a TCV code, take care of that before diagnosing anything else. A non-functioning TCV will generally hold the wastegate closed. But in general, if you don't have a check-engine indication, there is no boost leak on the intake side.
Thanks for the explanation... that's what I like about this site. I come here for great info from guys who know tons more about this stuff than I ever hope to. :beer:

The TCV is getting replaced on Monday so hopefully that will fix it.... Car is just embarrassingly slow without the turbo.
 

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Definitely check the CBV once you get the TCV issues sorted. The stock CBV is just plastic, so it's only a matter of time before it fails.
If it were to have gotten stuck open, you'd be losing all of your boost pressure and getting the whoosh noise.
If it were stuck closed, you'd be getting turbo sputter noise
 

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Discussion Starter · #8 ·
Definitely check the CBV once you get the TCV issues sorted. The stock CBV is just plastic, so it's only a matter of time before it fails.
If it were to have gotten stuck open, you'd be losing all of your boost pressure and getting the whoosh noise.
If it were stuck closed, you'd be getting turbo sputter noise
Is the CBV controlled by the TCV?

CBV makes sense based on the symptoms. I guess I'll know more on Monday....
 

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The vacuum hose to the CBV crosses over the engine under the timing belt cover and connects into the intake manifold.
You might also want to check and make sure that hose is still hooked up to the manifold.


This is the hose, as seen in the boost gauge installation guide.
 

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Is the CBV controlled by the TCV?
No. As I said, the CBV is passive, it is not controlled. It is normally closed, and opens when the boost pressure is higher than the manifold pressure. This occurs during overboost situations, and when lifting off the gas under boost. It is there as a safety valve, limiting boost pressure, and also to limit damage from rapid pressure changes.

If the hose that MNIWT mentions pops off, or the CBV itself is damaged, that will cause trouble, but you'd almost certainly get a check engine indication because of the air leak. However, if the CBV is merely opening too early, it can lead to other issues. It's not worth thinking about until the TCV issue is resolved though.

I disagree that it will cause this trouble if stuck closed though. If that happens, the car will run strong when you're on the gas. In fact, some people slap a plate on the CBV flange to block it off, for just that reason. Then they install a sneeze valve to blow off the overpressure during shifts, leading to other compromises however.
 

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I disagree that it will cause this trouble if stuck closed though. If that happens, the car will run strong when you're on the gas. In fact, some people slap a plate on the CBV flange to block it off, for just that reason. Then they install a sneeze valve to blow off the overpressure during shifts, leading to other compromises however.
Stuck closed wouldn't be the problem that the OP has.

The car might run stronger with it stuck closed, but it's damaging to the turbo and other components of the car to have boost pressure stored without a way to recirculate and/or vent. When the throttle closes, all that air is running up stream and the CBV opens to prevent it from smacking back into the turbo.
As you said, the people blocking off the CBV are adding blow off valves to vent it away. BOVs sound cool, but it's more efficient to have a bypass valve to recirculate some of that air. People putting BOVs on our cars often get CELs because they're venting too much air and the system detects a leak.

The Forge CBV has a stiffer spring, and the Turbo Smart dual port CBV has adjustable stiffness, so both are nice upgrades for a tuned car running more than stock psi.
I'm running the Forge CBV, with a Greddy BOV. Low boost shifts, air is recirculate, high boost and it's venting out.

I've worked on a few P1s that had CBVs stuck open with no CELs, so it's worth checking. But like tmtalpey said, fix the TCV first.
 

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The Forge CBV has a stiffer spring, and the Turbo Smart dual port CBV has adjustable stiffness, so both are nice upgrades for a tuned car running more than stock psi.
The spring doesn't hold the valve closed under boost, the line from the manifold is what does that, by feeding boost pressure to the back side of the valve. In fact, that's why the valve opens when you lift off. When the throttle flap closes, the cylinders are sucking on a closed intake, that low/negative pressure reaches the valve and WHAP the CBV opens instantly just like you want.

The only thing the spring does is hold the valve closed while boost is building. The maximum boost is pretty much the same with either spring.
 

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But still, stiffer spring is wanted with higher boosting cars.
Correct - because the CBV needs to be held closed for a longer time / higher target pressure. However, if the turbo has not been physically modified, there is little reason to increase the spring. There's a fairly wide margin of safety.

One more thing to add...

The stock CBV is just plastic, so it's only a matter of time before it fails.
Actually the stock CBV is a rubber diaphragm, and while the pressure cap is plastic, it is under no force. Stock CBVs fail either because of a torn membrane, or physical trauma to the pressure side.

Aftermarket (Forge) CBVs are aluminum cylinder-type valves and while very well made, can stick if debris enters the system. So they are certainly not immune to issues.

however, I will note I love my Forge CBV. It makes a bit more noise than the stock unit, but delivers a much snappier boost response. I use the lighter spring in mine, btw.
 

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Discussion Starter · #15 ·
Just an update:

Work got in the way this week so didn't have this looked at until today... anyway, new TCV and all is good again.

Car is a dog no longer. Easy fix but man, they did bury that valve....
 
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