plwsm

I think it's the atmospheric pressure inlet for the vacuum system. Basically, the way the turbo (and EGR) vacuum regulation works is that the ECU varies the duty cycle to the control solenoid valves. These switch between vacuum and atmospheric pressure to control the vacuum to the turbo and EGR valve. I don't suppose it will effect the running if it gets disconnected, but it will probably suck in dirty air into the vacuum system which could eventually cause the values to stick. Taking in 'clean' air after the filter should prevent this.

Hi, Sorry for the late post but I only just found this forum (my first post) I had a MAF go bad on me a few months ago. Initially, I used a multimeter to measure the output voltage at several RPMs and recorded the following voltages - 900 RPM = 1.85V (should be approx 2.2V) 1500 RPM= 2.35V (.. 2.9V) 2000 RPM = 2.56V (.. 3.4V) 2500 RPM = 2.86V (.. 3.6V) 3000 RPM = 2.98V (.. 3.8V) (obviously this is going to be dependant on the exact fault - your MAF sensor may behave differently). The above was measured on a 1.9 TDi engine - I assume that the bigger cc engine air intake would give a proportionately higher mass air flow for the same RPM?. I found it easier to measure the voltage using a pin to push through the wire insulation on the harness to the MAF (pin 5 on a Bosch MAF sensor). I found there wasn't much of a voltage drop between the MAF ground wire and the battery negative, so I used the battery for the meter common. Still not convince that it was the MAF, I made up an OBD cable and used VAGCOM to check 'specified MAF' against 'actual MAF' and this clearly indicated that the MAF was bad. On a motorway run, specified MAF was around 650mg/R wereas actual was about 490mg/R. There were no DTC fault codes reported. A new MAF sensor fixed all the problems. One final note, only this weekend the galaxy had similar problems (lack of power). This time I had a fault code for Charge Pressure Control Deviation (P1550). This turned out to be a loose/ corroded vacuum pipe on the charge pressure control valve for the turbo. It is definitely worth checking the vacuum pipes (and the air filter) before investing in a new MAF if you don't have the diagnostics. Regards Phil