Samz91tt:
First of all, you will need to supply allot more information about your test conditions, your equipment that you are using to test with, and if you have a factory service manual at your disposal.
You will need to follow the steps REQUIRED in the service manual, using the specified or equivalent equipment to perform this test.Secondly, you will need to perform a visual inspection of ALL the joints leading from the Turbocharger outlet, to the I/Cs, and to the upper plenum. Thirdly, you will need to perform a visual inspection of ALL Components of the system:
Turbocharger
Intercoolers
Ducting
By-Pass Valves Ensure that all of the components are crack free and in servicable condition.
Are the Intercoolers clear and free of obstructions and any other visual damage?
Do the turbochargers show any evidence of leakage (air or otherwise)?
Housings intact and undamaged? Lastly, you will need specific equipment to perform a leak test:
Block-off Plates / Plugs
Suitable source of conditioned / REGULATED compressed air
Manometer Isolate the upper plenum by installing a plug or block-off plate at the hose connecting to the Throttle Bodies.
Install a plug or block-off plate at the I/C Inlet.
Pressurize that part of the system to 15 psi. and maintain.
Trace along the the ducting and all joints isolated in this part of the system with your manometer probe.
Have someone else monitor the manometer for any movement.
Movement on the manometer indicates location of the leak.
Install block-off plates / plugs between the I/C inlet and turbo outlet.
Pressurize that part of the system to 15 psi. and maintain. Repeat above procedure while having someone else monitor the manometer for movement (as above).
If no leaks are found, remove all the plugs and block-off plates. Remove the Upper plenum and install a block-off plate between the upper and lower plenums.
install a plug onto one of the throttle bodies, install an adapter to the other TB to provide the pressurized and regulated airflow.
Pressurize the upper plenum to 15 psi. and maintain.
Repeat above procedure while having someone else monitor the manometer for movement (as above). In the above described test, you have isolated the induction system into three systems. Performing the testing in this order will isolate the most probable location for the leak (Ducting between the TB's and I/Cs) in decending order.
However, I should advise you as well to complete the entire test and isolating each system regardless if you locate a leak.
You may find a leaking intercooler hose, that was a major source of the leak, DON'T stop there as there may very well be additional areas that are leaking that are still contributing as a leak path. You should also pay particular attention to the By-Pass valves located on either side of and in front of the A/C condensor. These By-Pass Valves incorporate a spring loaded diapham set to prevent over-boost conditions. When this valve opens, it sends a great deal of pressurized air into the intake tract. If the valve is stuck, it could explain the "hissing" noise you are describing. Ensure ALL of the hoses on these valves are tight, in leak free, servicable condition. If you follow the above procedure, and cannot locate any leaks, AND you are able to maintain 15 psi in each isolated system WITHOUT having to keep adding air, (i.e. the system is leak free). then the culprit of low boost pressure is likely the by-pass valve(s).
Perform an operational / function test on these valves (as detailed in the service manual), repair, replace or adjust as required. If you do not have the equipment, knowledge and necessary skills to perform these above procedures, take your vehicle to a certified technician or mechanic to have this test performed for you. Good Luck and Best Regards,
S2z
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