Photos from Max Clesca's post

How about a Tech Thursday (TT). I'll pick a technical topic related to our cars and elaborate on it. Then we can discuss ask questions on the topic. Sounds good to anyone? I hope so!

Today I will discuss boost/vacuum leaks and how to perform leak detection to find and seal leaks.

As we all know, our cars have a very complex vacuum system on top of a long charge piping system with many joints to fail. That's the joy of owning a German car, they love to over-engineer things that are universally simplified on other cars. The age of the cars and the heat that hoses are constantly subjected to doesn't help the situation as well. We've all had split hoses spring up on us at some point or another... and they can be a pain to find.

First line of defense is good clamps. T-bolt clamps or Mikalor clamps for you guys in the UK is a good way to improve the clamping at the joints and prevent leaks to develop there. These type of clamps spread the load evenly and hold their clamping for a much longer period of time than the conventional worm clamps (jubilee I believe they're called on the other side of the pond).

Second thing that help is to progressively service/replace the hoses in the vacuum system with higher grade reinforced silicone based hoses. PVC system is the biggest offender there and always a good place to start refreshing the vacuum system. Other usual suspects are N249 rat nest of hoses, the EVAP split hose that love to tear open, and the vacuum sources under the manifold.

With all that said, leaks are annoyingly challenging! Some of them don't appear until a certain positive pressure is reached. So you can have a system that looks fine at rest or idle and is a glorified strainer when operating at 10+ psi. So for that reason, the only solid way to test, find, and seal every leaks is to perform a good old pressure test. When doing this you should be pressurizing the system at least 5-7 psi above the boost pressure you run. The reason for this is that your charge piping is consistently seeing higher pressure than the manifold where most boost gauge vacuum source is taken from. So in other words, if you run 10 psi, pressurize the system to 15 psi when doing a pressurized leak detection test.

Ok, now that we've covered the idea behind it, this is what you need to do a professional pressure test. An air compressor with a decent tank capacity --- the typical 10 gal system you find in most home garage will suffice. A basic air pressure regulator with gauge (nothing fancy is needed). Some male/female air compressor fittings. Some PVC cap of some sort to plug into your charge piping system or TIP. 2" ID PVC joints will have the perfect OD for the hoses in the charge system (you can also pressurize at the TIP by disconnecting the MAF and plugging your tester there). You also need a flat nose vice-grip to clamp the PCV system (use the hose going to the head off the PRV valve). Also open the oil cap in case any pressure escapes your PCV clamp so you're not pressurizing inside the crank case. The pictures explains how to make a good tester and are worth a thousand words.


- Pop the bonnet open

- Remove the oil cap

- Clamp the PCV line

- Remove the MAF or rubber junction in the charge piping (closest possible to the turbo outlet)

- Plumb your tester and clamp it down securely

- Open your compressor valve all the way

- Set the pressure on the tester's dedicated regulator

If you've done this properly, the vacuum and charge piping will be pressurized and any leaks will be hissing loudly. The big leaks will be obvious and loud. Soapy water in a spray bottle helps find the very small leaks that might be difficult to locate just by listening. Seal all offending points until the car is leak free and hold pressure steady. Once finished the car will run better than it ever did with the turbo's work going to the motor instead of sipping out of unwanted leaks in the system. Discuss!!!

%d comments
  • Since this is intended to be a topic of discussion, the vacuum system can be simplified tremendously on our cars. Many sub-systems like the N249 for example can be removed/deleted with no adverse effect on the car's operation. This will remove the failure ponts by a percentage and reduce the risk of developing leaks. So if anyone want to discuss simplifying things, this would be a the place to do it.

  • Cars
  • This i need lol. Is it a ball ache to remove?

  • Just so i know. A stock 04 225 quattro should be pushing what psi. I noticed my gauge read 20hg at idle and 10psi at WOT. have not done a boost leak test yet, this should help me out tho!

  • No, just have to know how to reroute some hoses that are needed.

  • Is there any online guides about do you know?

  • Basically what Max Clesca sais, just some helpful visual aids haha

  • Most definitely, just type in audi tt n249 delete. Me personally tho, where I'm located in don't need my SAI pump. So in my case I would delete the SAI/EVAP/N249 all at the same time.

  • It's quite easy to do yourself Tommo Maddocks

  • Thanks

  • I believe around 15 psi (don't quote me on this as it's been close to a decade since my car has seen stock boost levels). If you're boosting 10 psi, chances are something is off somewhere.

  • Fairly easy ya! I also will be deleting the egr valve

  • I need to do the lot lol even if just to tidy the bay up a tad

  • No problemo. The catch can install is a must in my eyes as ive seen how much oil it actually catches (and its alot cheaper to make your own pipes rather than buy forge ones) You can locate it where ever you like, as long as you connect the block breather, crank case breather and TIP connection all together. N249 is simple, remove all the pipework ontop of crank case, connect nipple on bottom of inlet to the top of your DV. Leave N249 plugged in and hidden to keep faults at bay

  • 14psi for the 225s and 11psi for the 180s

  • Thats odd, i always believed standard boost on a 225 was 0.7 bar

  • Thanks guys!

  • Thanks.. ill try match that to some pics (y)

  • My a4 1.8t was a bit of a pain to do, had to take off the intake manifold but once it was off, soooo much less hoses to worry about!

  • its actually 1 bar = 14.5 psi

  • Learn something new every day!!

  • Daniel Knight

  • Where are you clamping the pipe on the PCV thanks

  • Leon Kristopher Davies

  • Your a star mate thank you very much this is on my to do list very soon

  • Adrian Knight you're boosting around 10-12 l

  • Sorry just to chip in Max, when pressurising from the MAF you aren't testing the n75 for leaks, or the DV, as both have connection back to the intake pipe, so won't leak to air but will leak internally. Testing from the charge pipe combats this, so if people are only buying one size fitting then making it for the charge pipe is better

  • Bryn, I see what you're saying, but this type of pressure test is to find atmospheric leaks (outside air coming in under vacuum and inside air leaking out under boost). We have a sealed system and everything that gets vented is recirculated. Therefore, the main purpose is to keep the system atmospherically sealed.

    As for components that can leak internally, it is always best to test those individually. The pressure test can help but that's not what it's for. For example we have a bunch of one-way check valves throughout the boost/vacuum system. Pressurizing on the charge piping as you suggested will not point out leaky check valves (and they're very common failure items). IMO, the only true way to check and test internal beed and recirculating components is to test them individually.

  • What can a stage 2 expect to boost at ?

  • 19-24psi

  • Brian, thanks boosting at 22 and holding

  • Good number for a stage 2

  • yeah I'm pretty chuffed all round

  • 22 is nice!

  • Yeah, it really depends on the mapping company or tuner. Some are more agressive than others. You will find "stage" 2 remaps with boost profile spiking up to 26 psi, so it all depends on what stage 2 you have and they're not comparable (the "stages" are not standardized across the remap spectrum).

  • Max Clesca correct me if I'm wrong.. a 26 psi spike is pushing the limit of the rods, no?

  • No, not really!

    Some old timers may remember the old school GIAC stage 2 tune that pushed 28 psi (circa 03-04). Many (including myself) loved that tune due to how agressive it was. Some still run it today on stock rods. It destroyed a few clutches and highlighted weak components like the stock DV and standard intercooling, but the rods handled it fine. You're not breaking rods until you're well over the 330 ft/lbs of TQ mark at the wheels (obviously provided that the tune and supporting hardware is on point).

  • So the picture I had painted in my head of the limit of the stock rods was off by quite a bit. Glad to know that! I always thought it was ~330tq at the crank, obviously 330 at the wheels is quite a lot more.

  • I second that 2x ;)

  • Cars
  • Ruben Florencio