Boost leak? Exhaust leak? I'm stumped.

[fishsticks]

So I just threw a bunch more parts at the truck and now I seem to have caused myself an issue. I have a hiss that happens under load. I've read the thread about the Spec-U exhaust noise but I don't think that's my issue. This is a very clean hiss, like a vacuum leak but the engine idles and runs great. The noise definitely seems to come from the front of the truck, rather than underneath. It's easily louder than the BOV.

I have pressurized the exhaust and sealed the (small) leaks I had there. No change.

I've triple checked the flange bolts, sensors and plugs.

I've pulled the belt off and driven the truck a couple thousand feet. The noise changes in intensity a little, but is still very much there.

I had my intake manifold off to install new injectors during the upgrade. I've checked the PCV, fuel return and bypass valve/BOV vacuum lines.

Parts installed were Spec-U headers, full exhaust, 75mm pulley and 80lb injectors.

Before I pressure test/smoke the intake, does anyone see anything I'm missing?

Video of the sound.

https://www.youtube.com/watch?v=Rv8o0G0Y1CQ

(Ignore the crack in my windshield. This truck sees a lot of miles. )

 

[Gadget]

Do you have a boost gauge or an old vacuum line that used to go to a boost gauge?

G

 

[Gadget]

Check the hoses on the recirculating valve.

G

 

[fishsticks]

No boost gauge. It's on my list along with a decent AFR setup. I've been reading boost/AFR from the OBD port, which is annoyingly slow to refresh.

I'll check the valve.

 

[Nashman]

disconnect the bov vacumme line and then put some tape over the out let this will tell you if the valve is leaking


hks ssq don't tend to leak but the crappy cheap ones do

 

[Torspd]

Sounds like the recirculation operation to me. Have you sprayed any silicone or carb cleaner around the engine bay? Have you used a vacuum hose to your ear to probe around the engine for the source?

 

[justin13703]

Yeah that sounds like the recirc valve to me. Since it's a centrifugal supercharger it is always pushing through a certain amount of air at a certain rpm and if you're at partial throttle that bypass valve will be open and you'll hear all that excess air escaping.

Try this, under very light throttle, in 2nd or 3rd gear, run it up until you start to hear the hiss. Then hammer it. If it is the bypass valve your hearing, when you go to full throttle it should snap closed and the hiss should go away until you let off the throttle again.

 

[Torspd]

Good idea.

 

[fishsticks]

Sounds like the recirculation operation to me. Have you sprayed any silicone or carb cleaner around the engine bay? Have you used a vacuum hose to your ear to probe around the engine for the source?

I have probed with a piece of vacuum line and couldn't get any sound. I didn't have carb cleaner around so I pulled the vacuum line off the BOV/recirc valve and started spraying soapy water on joints looking for boost leaks that way. Didn't find anything.

Yeah that sounds like the recirc valve to me. Since it's a centrifugal supercharger it is always pushing through a certain amount of air at a certain rpm and if you're at partial throttle that bypass valve will be open and you'll hear all that excess air escaping.

Try this, under very light throttle, in 2nd or 3rd gear, run it up until you start to hear the hiss. Then hammer it. If it is the bypass valve your hearing, when you go to full throttle it should snap closed and the hiss should go away until you let off the throttle again.

I will try this today as well as checking the clamps on all the hoses. I'm assuming if it is the bypass valve making this noise that it's not normal. I went from an 80mm pulley to a 75mm. I'm not pushing that much more air.

Thanks all for the ideas.

 

[Torspd]

Sounds just like this one to me.

http://youtu.be/7hgKiiSo4jc

 

[fishsticks]

Well, early day today, so I tried playing with the throttle on the road a bit. Noticing that if I'm below 2K RPM the hiss doesn't really show up, even if I floor it. About 2200ish though there it is. Flooring it doesn't change it much.

I disconnected and plugged the hose off the charge pipe to the recirculation valve. Took a test drive and no change, so I think that part of the system is eliminated. I pulled the upper intake back off too just to double check my work there. Reassembled and no change.

I think I'm going to proceed with pressure testing the intake, as I'm at a loss. I rechecked everything I've touched at least once. I've had cars/trucks with large primary headers, tuned Y/X/H pipes etc and nothing on the exhaust side has made a noise like this. The fact that I can't figure this out is infuriating.

 

[Torspd]

Recirculation valve.

Here is a good perspective.

https://youtu.be/WScTqvwZDZ8

 

[fishsticks]

Recirculation valve.

Here is a good perspective.

https://youtu.be/WScTqvwZDZ8

But I effectively removed it from the equation by plugging its feed off the charge pipe. There was no change to the sound even when it had 0psi on either side.

 

[justin13703]

I'm willing to bet all you're hearing is the bypass valve. It is normal for them to hiss very loud with this type of supercharger. As the rpms go up, that supercharger starts blowing a huge amount of air. If you're not at wot, the engine is not using that air which will make it exit out of the bypass valve, creating a hiss or "whosh" sound.

With that vacuum line removed, and the valve not seeing any boost pressure, that air will easily push that valve back open and escape there. That supercharger is pushing an excess amount of air when you're at partial throttle, and it will find the easiest way to escape. Which will still be that valve.

Just to clarify, do you have the mk3 system? If so it actually uses a blowoff valve but the idea is the same.

 

[Gadget]

That is correct. Both the recirculation and blow off valve need boost pressure from the vacuum line to stay shut. With the vacuum line disconnected the boost pressure will override the spring and vent.

 

[Nashman]

is this a popet style one bosch ect because thous will leak but the hks should stay shut in less you are pushing so much boost you are over riding the spring and need the extra help from vacumme

 

[fishsticks]

I don't think I'm explaining myself clearly. I didn't just remove the vacuum line. I removed and plugged the feed off the charge pipe (the 1" hose). If I'd have undone one more clamp I could have had the bypass valve riding shotgun in the truck with me. There was no air passing through it. At all. Same sound.

As I stated a few posts up, I have pulled the belt off the supercharger (no boost) and the noise changes but doesn't go away.

The BOV makes a much different sound from an audibly different location when you're in the cab.

This is not normal. I'll keep looking.

 

[Nashman]

Sounds like a fun time lol good luck with the battle :starwars:

 

[Torspd]

No belt. Missed that part earlier.

 

[justin13703]

I don't see where you said you pulled the belt off the supercharger? I saw where you said that you took the belt off and drove a couple thousand feet which to me sounds like you pulled your belt off, drove it, then put it back on.

If there is no belt on the sc then yes it very well may be a vacuum leak but you will have a hell of a time finding it using soapy water. That will find where air is escaping. For a vacuum leak you need to use something like carb cleaner so that when it gets pulled into the leak it will make the idle change. They're not fun to find, you'll just have to narrow it down to the general area by ear and then spray around with carb cleaner until you find it.