Need advice to locate Boost Hiss
08-04-2002, 02:15 AM
#1
1st Gear
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Need advice to locate Boost Hiss
Hello, I need some help to diagnose a new noise with the 86 951. Bascally Stock 951. I started noticing a loud hissing noise while under full boost. I have verified it only occurs under max boots and not present with high RPM. I still have 1 bar of boost and the car runs strong. I've checked all the hoses and connections and they appear to be tight and right. I tested the blowoff valve and it too appears to be working correctly. Is there a way to pressurize the system with the engine off to search for the leak? any suggestions? I think I would loose performance if the wastegate were leaking.
TIA!
TIA!
08-04-2002, 03:25 AM
#2
Instructor
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I had the same thing happen. It would occur while reaching full boost, but would quit about halfway to full boost(no performance loss noted). I had added a Tial wastegate and manual controller. The problem ceased without me ever actually figuring it out. I did have the intercooler pipe to throttle body boot blow off a couple of weeks ago, so maybe I was leaking around there. Sorry can't be more help.
Chris '86 951 48k
Chris '86 951 48k
08-04-2002, 02:02 PM
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Hey frankie, i had the same thing, has it got to a point where it's dumping boost and blowing huge plumes of black smoke? I found my problem using an air compressor with a regulator set at 15lbs., charge the system with 15lbs. of pressure and start looking for the leak or leaks, most of the time it's the intercooler boots
08-05-2002, 12:47 AM
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Burning Brakes
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Frankie,
Sounds like your car is ok, thats the sound of the turbo taking in air, the fact that you mentioned it only happens at full boost supports that fact. Years ago, before I started tuning these cars, I went thru the same thing! That same hiss, it haunted me forever, but its normal. Another thing that might perplex new owners, that hissing sound at idle, it seems to come from the throttle/hardpipe area...its normal also, its thair making its way past the idle screw in the throttle body, the aluminum pipe seems to amplify the sound.
Hope this helps.
Sounds like your car is ok, thats the sound of the turbo taking in air, the fact that you mentioned it only happens at full boost supports that fact. Years ago, before I started tuning these cars, I went thru the same thing! That same hiss, it haunted me forever, but its normal. Another thing that might perplex new owners, that hissing sound at idle, it seems to come from the throttle/hardpipe area...its normal also, its thair making its way past the idle screw in the throttle body, the aluminum pipe seems to amplify the sound.
Hope this helps.
08-05-2002, 02:25 AM
#6
Race Director
Get one of those flourescent dye kits that you put into the oil. Run it around for a while and make it hiss under boost a bit. Then pull into garage and turn off the lights. Turn on the flourescent black-light and the leak should be quite obvious! 
08-05-2002, 03:57 AM
#7
Racer
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Frankie,
You might want to check if your idle adjustment screw is missing or not. It's located on your throttle body. My vehicle had a missing screw and you could hear and also smell the positive crankcase pressure when the engine hit boost. It's a pretty common problem to have the screw work itself loose and disappear. A sure sign of this is that the sound deadening material on the underside of your hood, directly over the missing idle screw, is missing some material (presumably blown away by the boost pressure exiting the throttle body where the screw should be).
The funny thing about my car is that even with the screw missing, I was able to get 2 bars boost and then the engine would cut off. Apparently the screw's diameter is really not enough to fully affect the turbo's boost performance. I'm sure I have other issues with the vehicle that's causing this cutoff.
Hope that helps.
*****
You might want to check if your idle adjustment screw is missing or not. It's located on your throttle body. My vehicle had a missing screw and you could hear and also smell the positive crankcase pressure when the engine hit boost. It's a pretty common problem to have the screw work itself loose and disappear. A sure sign of this is that the sound deadening material on the underside of your hood, directly over the missing idle screw, is missing some material (presumably blown away by the boost pressure exiting the throttle body where the screw should be).
The funny thing about my car is that even with the screw missing, I was able to get 2 bars boost and then the engine would cut off. Apparently the screw's diameter is really not enough to fully affect the turbo's boost performance. I'm sure I have other issues with the vehicle that's causing this cutoff.
Hope that helps.
*****
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08-05-2002, 05:43 PM
#9
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Alpine951
The idle adjustment screw is on the top of the throttle body/housing. There should be a hole on the throttle body where the screw will thread into. It's actually a round thin cast protruding upward from the throttle housing.
It is part# 951.110.919.00
Hope that helps.
*****
The idle adjustment screw is on the top of the throttle body/housing. There should be a hole on the throttle body where the screw will thread into. It's actually a round thin cast protruding upward from the throttle housing.
It is part# 951.110.919.00
Hope that helps.
*****
