951 Boost Testing

Boost testing is a fairly big subject to attempt on the fly so a little planning may be needed to get organized.  Generally, there are two parts; the first is just finding out it you have leaks at all. The second is locating the leaks and getting them under control. 

As far as I can tell, there are two ways (tests) to begin. You will probably do one of the other.  Each of the tests requires an adapter which is nothing more than a device that allows you to apply compressed air into the intake or cold side of the boost system. Once the system is "pumped up," you monitor the pressure with a gauge and see if you have leaks.  Choosing the particular test and its' adapter is maybe the first step to think about, so here are some comparisons.

Copper Cap Test vs Intake Test 
  • The copper cap test adapter (aka Boost Test Adapter) is a 2 inch copper sweat cap -this comes from the world of plumbing. It typically has a simple schraeder valve threaded into it. A pressure gauge and compressed air source are also needed. I think the best pressure gauge is the old school fuel pump/vacuum gauge. Its big and scaled to the pressure you will be using - plus its cheap and readily available.
  • The Intake Test utilizes a "locally made tool" that inserts into the j-boot in place of the AFM.  This "J-boot Adapter" must have some sort of in-line valve for shutting off the source air pressure. A pressure gauge is also needed for this test.
  • The Intake Test is documented in the test point 13 of the DME KLR Test Plan and as such; is the Porsche method for evaluating for leaks. The test is stated in one sentence, "Check all connection points after the air flow sensor for tight fit and leaks, build up 0.5 bar pressure in the intake with a locally made tool and check for leaks." The Copper Cap Test is informal, not documented and (I believe) originated somewhere in the 951 community.
  • The copper cap adapter plugs into the rubber sleeve of the inter-cooler inlet, in place of the #1 inter-cooler tube.  CAUTION: be sure the adapter is secured there with a good clamp so it doesn't inadvertently eject when pressure is applied.
  • The Intake Test adapter inserts, in place of the AFM, into the J-boot. Because the Intake Test pressurizes the J-boot, AOS and crankcase where seals or gaskets might be harmed, the test pressure is limited to a maximum of 0.5 bar.  
  • The Copper Cap Test does not apply pressure to the J-boot/AOS/crankcase so higher test pressures are often used. These are advantages in that higher pressure more closely emulates everyday boost levels and leaks become more apparent under higher pressure.   
  • Since the copper cap adapter inserts downstream of the turbo, there are some parts of the boost system that do not get pressurized and therefore those must be evaluated separately (perhaps via visual inspection of those components or by doing the Intake Test). In this respect, the Intake Test is more comprehensive than the Copper Cap Test.  
  • Both tests are just general starting points that answer the first question - Do I have leaks in my intake/boost system? If leaks are present, it is up to you to go forward with other tests or ways to isolate and identify the leaks and thats the part that doesn't get talked about much in the forums.
Sketch of the Intake/Boost System

So to start, I came up with this sketch, its crude but this is a good exercise to get all this plumbing in my head.

Next I devised this list of where boost/vacuum leaks can occur. The original idea was to use this to develop an end-to-end flowchart that tested everything in some kind of logical order. That fell by the wayside though when the chart got big and complicated.

So then I fell back to a few basic things to get me started. This is just my approach, you may come up with something much better. 

Boost Testing - Philosophy?

Generally, I want to do the easy stuff first.  I learned this the first time I got serious about boost testing. I had symptoms of a slight stumbling under max boost. I pulled the intake, I replaced every hose, I made up adapters, in-line regulators, secondary monitoring gauges - I spent all day tinkering on this little problem that was on a car that was already VERY fast. In the end it was a dumb-ass plastic tee that was cracked on one of the vacuum lines. It should have been an easy 10 minute fix. 

So now my rule is ... Do the easy stuff first. I get an eyeball on every inch of accessible tubing, every clamp, every hose and every damn thing I can see, gets a good visual inspect.  Next I bring out the vacuum gun and pull vacuum on each of the two small lines that connect to the fitting between intake runners 3 & 4.  I do the same on the small tube that runs from the bunny ears (small temperature vacuum switch under the back of the intake that sort of looks like two small bunny ears).  And last, I pull the BOV and ensure that I cannot just blow air through one port (j-boot side) to the other - it should only open when there is vacuum applied to the diaphram. 

Now that covers a lot of stuff that I can put on my "probably not a problem list." 

So, moving on... I chose to do the Intake Test.

1) The ideal result from this test is that your test gauge slowly rises and when the pressure supply is cut, the pressure holds or very slowly leaks down. If that happens .. I'm done. Note I have my test gauge "Tee'd into the line that goes to the BOV.


Will the intake system pump-up and how fast does it leak-down? These are are the questions I am looking to answer in this test. Determining the test results is a judgment that you must make because there is not a specified allowable leak-down rate. I suppose the implication is that there shouldn't be any leakage at all.

Personally, I have seen rates as low as 0.5-bar-leaking-down-to-zero-in-4-minutes and leaking so badly that I can't even pump up the system to 0.5 bar. The first I consider as passing (meaning no leaks found) while the second means Big leaks found.

If it is leaking, I have a spray bottle of soapy water ready to help me find leaks in the hoses/connections that are accessible.  This might be the point where you realize your j-boot or turbo outlet hose is split or your throttle body seals are gone. These can be easily fixed but other problems may yet exist so once any sort of repair is made a second test is a good idea to verify the fix or to know if there is more work to be done.

Some Considerations:

  • The turbo has significant capacity for air volume and can overcome minor leaks but remember all leaks contribute to turbo lag.
  • The turbo seals may leak during this test but perform adequately when the turbo is spinning and has oil pressure.
  • Use the spray bottle - a loose clamp can have a leak that is significant but easily overlooked. The soapy water will make it obvious.  



If the intake system will not even pump up (using the j-boot adapter and your shop compressor) then leaks are almost certainly serious. Perhaps even serious enough to affect your engine starting and/or idle. The leaks shown above are the kind you may find without having to remove the intake manifold. My troubleshooting weapon of choice is to isolate parts of the system and then repeat the pump-up test.  Pinching off hoses can help eliminate problem areas although many times the hoses are so hardened, its impossible to pinch them off.

Even if you're able to repair some leaks in the accessible areas, theres a good chance that you will repeat the pump up test and still see your monitoring gauge bleeding off rapidly. So, to look deeper, you will need to pull the intake and investigate. This is almost always the case for me. 

So basically; I intend to pull the intake manifold, pressurize the plumbing under the intake and then use soapy water to find any leaks. The problem is that with the intake removed there will be several openings that will not allow pressure to build. This same situation occurs weather I chose to use the copper cap adapter or the intake test adapter.  So, what to do? Another adapter and some plugs are needed, of course. The plugs fit into the hoses that would normally connect to the intake ports between intake runners 1&2 and 2&3.  And the throttle body block-off adapter simply blocks off the throttle body flange. 

 Note: here I use the small vacuum port on the bottom of the throttle body as a port to connect my monitoring gauge. 

So finally, I have the boost system pressurized and accessible. So out comes the trusty spray bottle and I focus on the area under the intake manifold.

Some leaks can be fixed by tightening of clamps, replacement of hoses or even swapping out venturi orings. In most cases, I find that the ICV is blowing bubbles and you will again have to judge whether to replace it or not. If it is leaking so badly that you cannot even pump up the boost system, then it has to go. 
ICV Notes:

  • I have seen very strong running 951s with leaking ICVs 
  • New ICVs are expensive $250 to $425 
  • I would never buy a used ICV that has not been leak tested.

 In my case: a variety of small leaks are found and fixed. This is typical!

For this car the real problem was the ICV.


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