Some info on boost leaks, what they cause and how to diagnose them:
(note: if you know the causes/symptomes skip to the test guide at the end of this post)
The most common boost leak is intercooler, intercooler piping, turbo leaks. This is because these areas are under high pressure, hence loose piping is more likely to pop off or split. Its also the longest air piping in the engine bay, plus by far the most common area for 'inventive' fixes by alot of car performance enthusiasts. These are all positive pressure boost leaks which generally don't cause damage to an engine
Max boost pressure could be the same with small or even medium leaks therefore do not use this when attempting to diagnose boost leaks. Use the tips below:
Boost leak after turbo (e.g. loose intercooler piping)
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Positive boost leaks simple make the turbo work harder to compensate the lost air, this results in over fueling. Over fueling occurs due to AFM reading more air than is entering the engine.
Symptoms:
higher RPM required for same boost pressure, fuel cuts, flames, pops, less power, poor on boost performance, black spark plugs, bad fuel consumption
The best way i've found to identify positive pressure boost leaks is to take note of turbo RPM spool up. I.e. At what RPM the boost pressure maxes out. If its higher up the RPM range than normal, its likely you either have your timing out, or a boost leak. The reason this works is as follows - if the turbo is made to work harder for the same boost pressure, its peak boost point will be shifted up the RPM range, quite simply, it has to flow more air to get the same boost, anyone remember reading up on turbo flow maps?
Boost leak before turbo (e.g. loose AFM)
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No boost pressures changes will be seen however severe the leak, all that will be noticed is the symptoms of a very lean engine. People with their engines running lean may never know they have a air piping leak as its not the most obvious cause. The reason the engine runs lean is simple. Air will be sucked in not forced out the leak as the turbo/engine sucks it in. This means unmetered air is entering the engine bypassing the AFM. This causes the AFM to underread the airflow resulting in less fuel added than actually needed. Fueling will be lean which could lead to DET, DET will mean to the engine dying very quickly.
Symptoms:
engine starting problems, poor engine running, pinking, pinging, deting, power black spots (i.e. certain rpm ranges have reduced power), pistons popping out the engine bay (worst case).
As for indentifing boost leaks before the turbo, these are very hard to diagnose, best thing to do with these is to check all the pipes by hand or perform the guide detailed below. These pipes are not under big pressure differences so loose piping doesnt result in lots of air leaking like in intercooler piping however it is still a leak no matter how small. Its also alot more damaging than a positive pressure boost leak as it could cause DET hence a big engine rebuild bill.
Air leak guide: 'How to test for boost leaks'
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(by far the most accurate sure way to test for boost leaks)
- Detach boost pipe attached to the turbo compressor and block it by putting a can or tin of appropriate size then tightning a jubilee clip around to create an air tight seal.
- Remove idle bypass pipe (attaches to a T piece on your intercooler pipework)
- Block idle control unit (attached to the side of the manifold) to prevent air escaping there. I use a large bolt and screw it into the pipe. Note: if you've detached the bypass pipe at the T piece which also goes to the cold start bypass, you will need to ensure this is also blocked.
- Power up compressor and build up pressure, insert the end of your compressor gun into the idle bypass pipe. Wrap lots of tape around the gun and bypass pipe, this will provide an air tight seal. You will need to use alot of tape over a wide area as the pressures will be high in the pipe work.
- Using your compressor gun, allow pressure to enter the pipe work, if everything is correct your pipe work should pump up and maintain its pressure inside.
Note: If you have a very bad leak it'll never build up any pressure at all. Either quickly run around the car after putting air into the system to listen/feel for the leaks or get another person to do the listening/feeling while you do the pumping. This should allow you to identify the leaks.
- Fix any leaks that are found and repeat the test, pressure will leak out the system as the manifold and valves will never be completely air tight however it should maintain pressure for at least 30 seconds if your pipe work is good. Identify and fix every leak
Note: If you find a leak between the next step and the previous but can not find any leaks in the pipework, it's likely your turbo itself is leaking. This could be a split turbo casing or faulty turbo seals. If in the next step your sure you have no boost leaks, you may need to investigate the turbo itself.
- Now remove the tin/can blocking the turbo compressor pipe and use it to block off the air flow meter. If you now do the same test you should be able to identify leaks before the turbo (WARNING: DO NOT ALLOW PRESSURE INSIDE TO EXCEED 10psi, AS THE INLET PIPES COULD EXPLODE!!!! THEY WERE NEVER DESIGNED TO TAKE POSITIVE PRESSURE)
- Fix air leaks and repeat.
- If you are sure you have no leaks but can not maintain boost pressure, try rotating your engine a little, as if the inlet and exhaust valves are both open (wild cam profiles) and air is getting past your throttle butterfly, air could be escaping down the exhaust. You could try blocking off at the butterfly to test this
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Originally Posted by sideways danny
this is where you need a second human being! Took the wheel and arch liner off, and got my dad under there listening, and you could hear the air coming out with each push of the compressor handle. Felt with fingers and added a bit of soapy water, and there it was, right down the whole side of the wing-mount 
it seems to drive fine all through the rev range but if i give it the berries and lit go ov the throttle while at max boost it jumps around like a kangaroo?and on idle it seems to hunt but ive had the on board diagnostics read and there are no faults?im running out of ideas now
it doesnt seem to be as fast as wat i normaly is!
. before it used to hit 10psi.
