Random hissing noise

[CodySvo]

Okay so when I get into boost sometimes, but only sometimes will I hear a very loud hiss and boost wont go past like 12 or so. I've checked every hose for leaks or cracks but found none. I've also noticed that my brakes have been really spongey and I have to pump them and when I do my vacuum goes up on my boost gauge. What could be causing this, my car will randomly feel likes it's got a boost leak at times but it only does it sometimes and it does it until the car has warmed up to. If the car hasn't warmed up yet and I take off with it at around the N it just bogs and pops when I give it gas. I know it's nothing major because I just had the motor rebuilt.

[svono50]

Your brake booster has a check valve on the inlet to the body. I wonder if that is leaking and letting the positive pressure to get into the booster housing. As for other locations of boost leaking, it could obviously be almost anywhere a vacuum line runs. Finding it could be a long project, since the suspect hose may only leak above a certain pressure, but be fine under vacuum.

[CodySvo]

Alright I'll start with the check valve, quick question what is the stock psi the wastegate runs. Mine is 15 and I wasn't sure if this was stock or not?

[SVOeric]

15 psi is about right for a stock SVO. SVO Boost gauges are also notoriously inaccurate.

[CodySvo]

Well I have a aftermarket boost gauge, that's how I can tell that when I pump the brakes the vacuum goes up on the gauge.

[SVOeric]

OK, somewhere between the stock boost gauge (assuming it's still hooked up) and the aftermarket, there is a T. This is a pretty common place for a hose to come unhooked, or loose.

Do you know if you can hear the leak from the outside of the car ?

[svono50]

The stock wastegate spring is set for 10psi, so if you connected the boost signal directly to the wastegate you would see 10psi. The factory boost solenoid 'masks' the signal to the wastegate to give you the 15psi you are seeing, which is perfectly normal. You will likely see a quick spike over 15psi then settle to that value, which is quite normal as well. If you ever hear your dash buzzer go off on acceleration on say a cold night, that is your overboost warning, which is triggered at ~17.5psi, again not too uncommon once you start opening up the exhaust and intake a bit.

With your vacuum moving around when you pump the brakes, I would start with the booster's check valve and then move onto the booster diaphram itself. Another side effect of a bad check valve is a very hard pedal when you hit the brakes when in boost or move quickly from the gas/boost to the brake.

[CodySvo]

I no longer hear the hissing anymore, replacing the check valve seemed to do it. But I'm not running the factory boost solenoid I'm just running wastegate pressure and that's all. The only thing I've done is add a k&n instead of the factory airbox, well I also don't have a catalytic convertor installed either as I don't need one. And on the aftermarket gauge I just used the line that was hooked into the factory gauge for the aftermarket. So the factory gauge is no longer hooked up just straight to the aftermarket.

[SVOeric]

It's good you're not hearing the hissing any more.

BUT
unless the stock waste-gate actuator is stuck badly, you'd be stuck at 10 psi, unless there is something there to bleed off the boost until it hits 15 psi.

There is some kind of boot valve, or bleeder there somewhere.

[CodySvo]

Nope I've just got the boost source going to the wastegate nothing else. What could cause it to boost up to 15 psi, it's not causing any problems and when I hammer down on here now she will boost past 15 a little then settle back to 15psi. Could it be the hairline crack in the exhaust manifold somehow, I don't see how but maybe??

[SVOeric]

nothing, unless you have some custom waste-gate actuator that's regulated to 15 psi instead of the stock 10 psi

[CodySvo]

As far as I know I don't, I mean it's looks like it's stock to me lol. But looks can be deceiving.

[CodySvo]

Well I figured out what was causing it to boost up to 15psi, the plumbing for the turbo was wrong this is also what caused me to have the random boost leak feeling. The hoses that used to go to the boost controller were just unplugged and there was a hose running from the t coupler on the compressor housing to the t coupler on the intake part of the turbo which was causing problems. I guess whenever the mechanic took everything out he unplugged them and then put them back like that because before I just had both blocked off.

[svono50]

Glad to hear you got your issues sorted out and it sounds like it is running good now. Whenever someone works on your car, you almost always have to go over things just to make sure they were put back the way they were.

[CodySvo]

Oh yea she runs great no random hesitation anymore. The boost was at 10psi like it should be until I installed my boost controller and bumped it up to 15. Quick question how would I install a dual piston bov that has a 1 inch nipple on one end, on a factory intercooler?