Volkswagen Repair: 1999 Passat - P1136 Lean Condition - Check Engine Light, air flow meter, mass airflow sensor

Question
I have a 1999 VW Passat Sedan; 1.8L Turbo with standard transmission. My check engine light recently came on the day after I had the oil changed.  I brought it to a mechanic who provided the diagnostic code P1136.  They found some debris blocking the air intube to the airbox.  This was cleaned, the air filter replaced and the mass airflow sensor cleaned.  The code was reset but now it's been tripped again - two days later.

Any help?

Answer
Hi Steve;
Debris in the intake tube to the airbox WILL NOT restrict the intake of air enough to cause this condition.  If you ever took out the filter box you would find an alternate air intake path.  This is a problem common to turbocharged engines.  Have you noticed any loss of oil?  When you check the dipstick right after an oil change it will be close to the marking "crosshatch" pattern.  After several hundred miles when you check it again, is it low on the dipstick?  This would indicate that oil is seeping into the intake pressure tube, and diluting the hydrocarbon based fuel that should be in there instead.  Once in the combustion chamber, that is.  Also, the density of the air is changed by the addition of atomized oil, and the ECM is further fooled.  The amount of unburned HC coming out of the exhaust ports is high so the ECM leans your engine out even more, and the cycle continues.  Hence, a lean condition.  So, there are only 2 explanations for this, and one is a bad "air flow meter," but it is my experience that with a 1999 Passat, it's usually the turbocharger that is at fault.  You won't like the cost of replacing that item.  If you want to try to do it yourself, be prepared to teach the entire neighborhood your entire repertoire of "cuss words."  The exhaust system is an absolute nightmare to remove at those flanges, and the turbo is completely surrounded by STUFF!  Anyway that is what to expect, one of 2 things, air flow, or turbo.  Sorry.