Diagnose Boost/Vacuum Leaks

Finally found my boost leak thanks to some advice on the RomRaider forum:




1)  Access to LIVE ECU data via OBD2, VAG-COM or Tactrix cable - more specifically the Fuel Short Term Trim Correction

2)  A spray can of Iso-Propyl Alcohol (electrical contact cleaner), carb cleaner, or pretty much any flammable liquid in aerosol form that will evaporate quickly (so dont try to use use WD40 or similar!)



1)  Open the bonnet

2)  Connect the tools for monitoring live ECU data

3)  Start the car and leave it idling until the engine is up to temp, and the short term trim correction value has settled.

Now follow steps 4-7 for each join in the air intake system between the airbox and the intake manifold.  
NOTE:  If your car has a MAF then only test those sections AFTER the location of the MAF (as you dont want to risk poisoning the MAF with the wrong fluid if there is a vacuum leak - and the trim corrections will only be affected by leaks between the MAF sensor and inlet manifold anyway)

4)  Whilst monitoring the live ECU data for Fuel Short Term Trim correction, spray the flammable liquid over a joint for a second or two.  Then for the next 5-10 seconds keep a close eye on the trim values.

5)  If there is a boost leak then the fumes from the spray will get drawn into the intake, enriching the air/fuel mix, and forcing the ECU to lean out the mix.  This will force the ECU to lower the trim correction to lean out the mix.  The trim value using IPA spray will suddenly drop by 5%-10% for a second or two and then slowly rise back to previous levels.

6)  Wait 30-60 seconds for the fluid to evaporate then repeat the test.  

7)  If you do get a sudden fall in trim values when spraying a joint then repeat the test a few more times to be sure that the joint is really leaking, and try spraying in slightly different areas around the joint to pinpoint the leak.

Thats all there is to it!

Of course this wont help anyone who cannot monitor the ECU - but for those of you that can then its a useful extra test to try and pinpoint leaks before resorting to more exotic measures (e.g. doing a pressure / smoke test).