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So I unplugged my mass airflow sensor and my car continued to run, witch it should have stalled out? So must be a bad sensor?
No. The reverse, or sort of orthogonal/unrelated. When you unplug sensor, it runs open loop--meaning no computer intervention based on sensors. Thus, the default for the running characteristics is in order, car is running close to 14:1, and the MAF isn't trying to correct (much) for a fault somewhere else. For example, if running dogged out when you unplugged MAF, it might suggest a decent sized vacuum leak. You would probably see lean codes in this arena too. Meantime, remember/know that SAI is pumping air into the exhaust side of the motor, not the intake side. The extra air is alongside a richened mixture when cold that causes secondary combustion in the header area to heat the catalytic converters up faster so they can then clean up the exhaust.
Bigger picture, MAF is an unlikely place find an SAI issue. First look at combi valve vacuum hoses as your most likely prime suspect. The combi(nation) valves are the basic feed to the exhaust for the air injection, along with their other EGR function when engine is at operating temp. Could also be air pump issue, kink in air pump line on either feed or pressure sides, break in pressure side feed lines to combi valve(s), or combi valve(s) themselves. There is one combi valve for each head. If it were an FSI motor, you also would be thinking about carbon fouling, but for 2006 you are still port 4.2. Triggering only occasionally isn't surprising to me. System self checks for SAI periodically, and only when it sees two sequential errors does it kick the CEL light on. Part of the whole OBDII system for self check and smog purposes.
Also, spin up air pump using VCDS to be sure that is in order electrically. A guess is that will check out, since unless you rarely drive it the light should turn on more than every two weeks with an outright non-running pump. If you disconnect the pressure side you can also confirm it is really blowing some actual air. FWIW, on my D3 an air pump (W12 has two) ate water from the air box and stopped; operator error (me) when I didn't catch that the air box drain line was kinked when I reinstalled it at some point. On one C4 I had a weak combi valve (at high miles). On C5 4.2 it used to regularly develop vacuum hose leaks on the lines that switch the combi valves, until I changed out all the German nice looking but long term junk braided vacuum line to some good NAPA line off the reel. On a 4.2, in general the vacuum lines on the motor either run the SAI or run the secondary throttle cutover flaps in the intake.
No. The reverse, or sort of orthogonal/unrelated. When you unplug sensor, it runs open loop--meaning no computer intervention based on sensors. Thus, the default for the running characteristics is in order, car is running close to 14:1, and the MAF isn't trying to correct (much) for a fault somewhere else. For example, if running dogged out when you unplugged MAF, it might suggest a decent sized vacuum leak. You would probably see lean codes in this arena too. Meantime, remember/know that SAI is pumping air into the exhaust side of the motor, not the intake side. The extra air is alongside a richened mixture when cold that causes secondary combustion in the header area to heat the catalytic converters up faster so they can then clean up the exhaust.
Bigger picture, MAF is an unlikely place find an SAI issue. First look at combi valve vacuum hoses as your most likely prime suspect. The combi(nation) valves are the basic feed to the exhaust for the air injection, along with their other EGR function when engine is at operating temp. Could also be air pump issue, kink in air pump line on either feed or pressure sides, break in pressure side feed lines to combi valve(s), or combi valve(s) themselves. There is one combi valve for each head. If it were an FSI motor, you also would be thinking about carbon fouling, but for 2006 you are still port 4.2. Triggering only occasionally isn't surprising to me. System self checks for SAI periodically, and only when it sees two sequential errors does it kick the CEL light on. Part of the whole OBDII system for self check and smog purposes.
Also, spin up air pump using VCDS to be sure that is in order electrically. A guess is that will check out, since unless you rarely drive it the light should turn on more than every two weeks with an outright non-running pump. If you disconnect the pressure side you can also confirm it is really blowing some actual air. FWIW, on my D3 an air pump (W12 has two) ate water from the air box and stopped; operator error (me) when I didn't catch that the air box drain line was kinked when I reinstalled it at some point. On one C4 I had a weak combi valve (at high miles). On C5 4.2 it used to regularly develop vacuum hose leaks on the lines that switch the combi valves, until I changed out all the German nice looking but long term junk braided vacuum line to some good NAPA line off the reel. On a 4.2, in general the vacuum lines on the motor either run the SAI or run the secondary throttle cutover flaps in the intake.
thanks for all the info I’ll just bring it to my mechanic to figure it out! I seen this video so figured that’s an easy fix that I can handle!
03-20-2018, 11:15 AM
Secondary air injection system insufficient flow
