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Thread: BOV Troubleshooting

  1. #1
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    Default BOV Troubleshooting

    Troubleshooting

    Blow-off valve troubleshooting
    Proudly installed your shiny new blow-off valve without as much as a cheeky sideways-glance at the instructions, but somehow it's not working properly? Had your race-car-mechanic uncle expertly fit it for you but still ain't gettin' that BOV whoosh? Forum gurus offering you pearls of wisdom like: "Errr, what'd you fit a BOV for?", "GFB BOVs are #*@%, get a brand X that'll fix it!" or "Everyone knows you can't vent-to-atmosphere on a [insert your car]"?

    Don't worry; GFB will have you sorted in no time. But before you ask us, take a good look at this troubleshooting guide; in almost all cases we'll only give you one of the answers that you'll find below anyway. Blow-off valves really are incredibly simple devices and there are very few things that can go wrong with them, so this guide should be all that you need.

    Almost 100% of 'faulty' blow-off valves reported to us turn out to be installed or adjusted incorrectly, and can be rectified very easily; we've almost never found a problem with any of the valves that have been returned. The troubleshooting guide below lists the eight most common BOV problem scenarios and how to fix them. Go on; show your uncle how it's done.

    The 8 most common blow-off valve problems:


    1. Engine doesn't run smoothly (atmo BOV fitted)

    If the BOV piston is slightly open at idle as a result of insufficient spring preload, it can cause some or all of the following symptoms:

    - Engine stalls after venting, or when pulling to a stop

    - When revs fall back to idle, they drop below normal idle rpm before stabilising

    - Engine idles roughly or revs hunt up and down

    - Significant black smoke or backfiring from the exhaust on deceleration

    - Noticeable hesitation when changing gears

    The solution is to increase the spring preload by turning the spring adjustment clockwise a turn or two at a time until none of the above symptoms are evident. The aim is to have the piston closed just before the engine returns to idle after being revved.



    2. BOV doesn't vent or make any noise

    On vehicles that originally had a black-plastic Bosch factory diverter valve fitted, it's really easy to confuse the inlet and outlet of the valve; Bosch valves are usually installed by manufacturers in the opposite orientation to most aftermarket types so try switching the orientation of your GFB BOV around. Trace the hoses that connect to it; the boost pressure from after the turbo should enter the base of the GFB valve, and the dumped air vents through the side outlet(s). Just about the only way a BOV can not vent at all , is if it's installed backwards. Once you've checked that the BOV is oriented correctly, try reducing the spring preload slightly by turning it anticlockwise a couple of turns at a time until it starts to vent.

    3. A fluttering sound is heard rather than a 'whoosh'

    Turn the spring adjustment anticlockwise to reduce spring preload. Some cars, Nissans in particular, will always do this under normal driving conditions, but should still vent cleanly when driving hard – this is nothing to worry about. Pulsar GTiRs and other cars with multiple throttles, porting or ‘big' cams (or any mods that reduce manifold vacuum) will require a softer spring (GFB 6116).

    Also, check that the vacuum hose is connected to the engine inlet manifold after the throttle, that the hose is of at least 3 mm inside diameter, is less than 1m long, and that there isn't anything else tee'd into it e.g. a boost gauge or a MAP sensor. T-joins, accessories and small diameter hoses will slow the reaction time of the valve.

    Case study: A customer arrived in a 180SX with a GFB Valve that wasn't working properly, and the car was running like a dog. A quick inspection showed a single, small diameter vacuum connection on the manifold led to the fuel pressure regulator, which was tee'd to the boost controller, which was tee'd to a boost gauge, which was tee'd to the blow off valve. The boost controller didn't work because it should not be connected to vacuum (hence the idle was bad), the tuning was out because the fuel pressure reg was not receiving a clean signal, and the BOV had no chance at all after the signal had been weakened by all the ancillaries before it.

    4. BOV doesn't flutter, it makes a 'whoosh' sound

    If a car is making a fluttering noise when venting at high rpm and boost, then the BOV isn't doing its job. Contrary to popular belief the noise is actually emitted from the turbo, not the BOV itself.

    With an aftermarket BOV fitted some non-detrimental low rpm fluttering does characterise certain cars (as mentioned above) particularly where a larger turbo and intercooler have been fitted.

    When properly adjusted, the valve should make a clean PSSSH or Whoosh sound when venting a significant amount of boost. If you want your BOV to flutter a bit, try increasing the spring preload by turning the adjustment clockwise. It's perfectly safe to set your BOV up to cause some low-rpm flutter, as long as it vents with a whoosh at high-rpm and boost.

    5. BOV leaks boost

    First of all, there are usually three things that lead people to suspect a BOV leak: an audible whooshing sound under boost, a drop in boost pressure, or their mechanic concludes that it is after performing a smoke test.

    If there is a suspected BOV related air leak for any of the 3 reasons above, check the o-ring in the base is installed and lubricated, the grub screws are securely tightened and any hose clamps are tightened. Then check that the vacuum hose on the top of the BOV is connected to the manifold with no other accessories tee'd in (see #3 ). A BOV relies totally on receiving boost pressure from the manifold to stay shut when the throttle is open – if the hose is not connected or suffers a pressure drop for any reason, the valve can open under boost. It's extremely rare for the workings of a BOV itself to actually leak – leaks usually turn out to be elsewhere in the inlet system – like a loose intercooler plumbing connection or a blown manifold gasket.

    Audible whooshing sound: there needs to be a significant leak for it to be audible whilst driving. If the valve installation is checked as described above, look for loose intercooler hose clamps, gaskets etc.

    A drop in boost pressure or loss of power can occur for a number of reasons depending on the car. As above, there needs to be a significant leak (the equivalent of a 1 cm / 1/2” diameter hole) for there to be a noticeable boost drop. The only way a BOV can leak this amount is if the top of the valve is not receiving full boost pressure. If it is, then unless the valve is physically jammed open, the leak will NOT be coming from the valve. In the case of late-model Audi and VW engines, sometimes the ECU can reduce boost as a safety measure if it detects something is not right (e.g. if the valve is not adjusted correctly).

    6. A smoke test indicates that the BOV is leaking

    A smoke test isn't a valid means of testing a blow-off valve for leaks – it doesn't replicate on-boost conditions. The problem with a smoke test is that the plumb back port is pressurised – this never happens when you're driving the car. On a Deceptor Pro this will cause smoke to escape from the trumpet. However, this doesn't mean that the valve leaks. Also, the smoke test does not pressurise the manifold, which the BOV relies on to remain shut under boost. As stated above, it's pretty difficult for a BOV to develop a serious air leak of the sort that would noticeably affect the way the car drives.

    IMPORTANT! Note that almost every turbo car has a boost controller. The way a boost controller works is by leaking air, but the leak is nowhere near large enough to drop the boost pressure. Consider this if you think a pin-hole-sized leak is coming from your BOV and is causing your 4psi boost drop.

    7. BOV cracks open under boost

    If you know that your BOV is cracking under boost i.e. you've seen it crack open in the dyno, read on.

    If you suspect that it's cracking under boost, it's more likely that scenarios 5 and 6 will help you solve your problem, so you're better off starting there.

    Check the vacuum connection. The valve relies on the manifold connection to provide boost pressure to keep the piston shut under boost. If there is a leak or kink in the hose, the pressure will be uneven on the top and bottom of the piston, causing it to open.

    Mounting the blow-off valve before an intercooler can also cause this, as it is possible for a poorly designed or ‘bargain basement' intercooler to have a pressure drop large enough to cause the valve to crack open. If possible, measure the boost pressure in the BOV vacuum line and near where it is connected to the intercooler piping. Pressure drops across intercoolers will usually occur at higher RPM.

    Case study: A customer had a Mach 1 that was opening under boost on his WRX. It was mounted immediately after the turbo, and a large aftermarket front-mount intercooler was installed. Sure enough, at about 5000RPM at 17psi the valve opened. Testing showed a significant 4psi drop across the intercooler, which started at 4500RPM and increased rapidly. So the bottom of the valve was receiving 21psi, whilst the top of the valve dropped to 17psi by 5000RPM, enough of a difference to cause it to open. In this case, the customer swapped to a much-needed new intercooler (besides causing the BOV to crack open, a 4psi drop across the intercooler would seriously affect performance) which fixed the problem.

    If the valve is receiving full boost pressure to the vacuum hose with the throttle wide open, it is not possible for the valve to open, unless it is physically stuck. This can be easily checked visually, or by pushing the piston up and down by hand.

    8. Your Audi/VW's check engine light is on

    If the valve does not make a whoosh noise, or a fluttering sound is heard when shifting gears, check the orientation of the valve. Note that a GFB valve MUST be installed in the reverse direction to the factory diverter. If you are unsure, simply swap it around anyway and try it.

    If the valve does actually make a whoosh sound when shifting gears, increase the spring pre-load 1 turn at a time until the check engine light turns off. You will find that the valve does not vent as much at lower RPM, and you may hear a flutter in this case. Rest assured this does NOT damage the turbo.

    There will quite often be some low-rpm flutter with an aftermarket BOV installed, since they work differently to a factory valve in order to allow them to vent to the atmosphere and make the blow-off whoosh.
    It's like a tornado, inside it's own metal house

  2. #2

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    Top post! Very useful info here
    Thanks rob

  3. #3

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    Quote Originally Posted by rayd View Post
    Top post! Very useful info here
    Thanks rob
    Agree with @rayd this really answers all questions about BOV's lol

  4. #4

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    Awesome post
    I vote this gets sticky'd!!!


    Peter "SLIXX"

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