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#4: 12-02-2010, 07:52 AM
 
 m sprank
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We've been seeing a number of threads and posts on forums about testing for boost leaks in the turbo system with many suggesting the "smoke test" as a good means by which to test for BOV or dump valve "leakage".

That concept of "leaks" and their practical significance is a whole subject on its own but we thought we'd start with our view of smoke tests and what you can expect to learn from using this method.

Below is an extract from our engineers paper on this and we hope you find it educational. As usual, we don't believe we know everything, this represents our view and we would love to hear from anyone that might disagree with these points.

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Smoke testing for leaks:

When chasing boost leaks, the humble and effective smoke test is often used. Unfortunately, the results may not always be what they seem.

If you have a GFB valve fitted to your car, a smoke test will very likely return a “false positive” for a boost leak. There are a few things about the common smoke test that make it difficult to determine if a detected leak is really there.

To begin with, let’s look at a typical turbo system.

(Should be an image here but having trouble uploading, will have another attempt)

When you are driving, you floor the throttle and reach peak boost. In this situation, where you sure don’t want any leaks, the intake manifold and intercooler (and associated piping) are effectively at the same pressure (give or take a small amount for losses through the intercooler or throttle).

Therefore, in the diagram above you’ll see that the GFB valve is subjected to the same pressure top and bottom. This is essential for a GFB valve, as is the case for most valve types, because the pressure from the manifold at wide open throttle (WOT) counter balances the boost trying to push the piston open. Were the vacuum hose (poorly named in this example because it is currently in boost) removed from the top of the valve, it would simply blow wide open.



During a smoke test, the operator will typically remove the air filter and insert a large plug with a hole through the middle where the smoke is injected.

Here’s the first problem: this test does not typically pressurise the intake manifold, therefore the top of the valve is left un-pressurised, and will therefore blow open under a relatively small amount of pressure – this is the first case where a false positive can result.

The second problem is during this test, the turbo’s intake side is pressurised, which in real life never occurs. At worst, there may be a small vacuum in the intake pipe as a result of flow restriction at high boost/RPM.

A GFB Hybrid-type valve that features two outlets, one for recirc and one for atmosphere (and any other valve of this type on the market for that matter), will typically show a leak through the recirc port to the atmosphere port. Since during the smoke test the recirc port is pressurised (where it normally wouldn’t be), a second false positive can occur. Hybrid-type valves are simply not designed to hold boost pressure on the recirc ports, hence the leak.

As a separate issue, it should be noted that although a smoke test can point to a leak, it doesn’t actually give any kind of indication of the size of the leak. Sure, a leak is a leak, right? Well yes, but even leaks that are detectable by smoke test shouldn’t necessarily be assumed to be the cause of boost loss without further investigation.

If the symptom that warrants the smoke test in the first place is a loss of boost pressure noted on the car’s boost gauge, keep in mind the size of the leak you are looking for – a pinhole leak (or even a vacuum hose popping off the manifold) that can be detected by a smoke tester won’t even register as a boost drop on a gauge when driving.

If boost pressure drops when driving according to a boost gauge, in reality you’re going to be looking for a leak equivalent to a 6-8mm hole or larger.

Note that a smoke tester pumps a miniscule amount of air in comparison to a turbo, so therefore if it is able to build pressure in the system any existing leak is highly unlikely to be larger enough to be the cause of boost loss.

If on the other hand the smoke tester is simply unable to build pressure in the system at all, then that would indicate a reasonably large leak that could well be the cause of boost loss.



So, what can you do to eliminate the “false positives” during a smoke test? First thing to do is ensure the top of the valve gets the same pressure as the bottom – this will ensure the valve stays closed as it would when driving at WOT. The second thing to do is remove the recirc hose from the valve and plug it – this will prevent pressure leaking out through a path that is never pressurised in real life.


Finally, if you’re chasing a boost drop and the smoke test keeps nagging that the blow-off valve is the problem, there’s one sure-fire way to prove it either way – it’s very simple, requires no tools and you’ve probably already performed it without even knowing! This test is relevant to any GFB valve that you can hear when it vents (through the filter if it’s fully recirculated, or to atmosphere).

Here’s the test. If a GFB valve goes “whoosh” when you shift gears, it’s about 99.9% certain that it’s not the cause of a boost pressure drop.

Bold statement, I know - here’s the reasoning behind it. If the valve vents normally with a “whoosh”, it means the piston is not physically jammed, and the vacuum hose is connected properly, since without the vacuum hose the valve would not stay shut under boost, nor would it vent when the throttle is closed.

Since the piston is not jammed and the vacuum hose is connected, this also means that the piston cannot possibly open under WOT conditions, because the pressure on both sides of the piston is equal, which therefore cancels itself out (no matter how high the boost is) – in fact, a GFB valve is capable of staying shut under WOT conditions even if the spring were removed entirely!

Put simply, if a GFB valve vents normally when you shift gears, it is not physically possible for it to leak enough air to cause a measurable boost pressure drop. It’s either working or it’s not.

So the smoke tester is a very useful tool, however the results of such testing should simply be regarded as highlighting areas of interest for further investigation, rather than the definitive answer to a problem.