Hi everyone I'm new here and just start to learn to tackle and understand cars.
Well my 2003 Audi A4 1.8t (190) with 98K on clock, is having jerking and power loss issues once I accelerate harder sometimes even during smoother drive. Starts fine, fuel pump turns on and I can hear a buzz, dunno if that particilar pumps buzz is bad, but can hear from saloon. Idle run is no problem, giving gas on idle no noticeable backfires. The engine light flashes but goes off but no codes left when I plug my OBDII to read any.
The car had full service like little over 2 months ago. Before service engine light came on - Secondary air pump intermittent fault and I didn't do anything with that as I prioritized to fix coolant housing and rocker cover gasket. Now I plug my OBDII it showed me P0411 Secondary air system incorrect flow detected, but no other codes. Im not sure if my car jerks and looses power because of SAP or it could?!
Just few days ago changed fuel filter to tackle my issue, but for sure wasn't that. My oil dipstick funnel is cracked and not sure does that affects engine vacum or pressure?!
At moments the car runs fine gives the boost as should and after short moment jerks again, ease of pedal and press it more smoothly it somehow stabilizes. Checked O2 sensor with scanner, but only showed me one, on idle 750rpm O2 sensor was .7-.8
I know can be several problems but i'm trying to narrow it down. Should i bring it to pro diagnostic test to see what they say or is something simple like coil pack, wire?

 

A cracked dipstick tube will present a vacuum leak but I don't think it would cause that much of an issue. Hook up a better OBD code reader and see if there are some more error codes stored in the ECU. But since you said you had an O2 sensor issue (and with the oil dipstick tube leaking) it's possible that you may have a glazed spark plug or two. Before throwing any more parts at it, I'd recommend a new set of spark plugs. A glazed plug will cause that bucking on acceleration. Check out this little write-up I did on glazed spark plugs.

https://www.audiforums.com/forum/gen...k-plug-218806/

 

Thanks Dave for the tip, spark plugs been changed like 2 months ago but worth a look and same time can check the coils.
I checked the codes and only code I had for the SAP system flow, dont think that is the cause as it shuts of like in 1min in cold start.
Should on live data be 1 or 2 O2 sensor readings? I had only one, so it does indicate the O2 sensor is faulty?, but can that cause this the car to jerk?

 

I could if the 02 sensor is sending bad readings to the ECU. There should be two O2 sensors. One before the cat and one after.