1.8t Issues (incorrect airflow detected) solved now i think!
Had a good time off over the Easter break, doing as is seems what many did, getting those anoying jobs done that got left all winter. Wanted to just post up some of my own findings after spending hours searching forums etc.
The job was to replace my lower front bumber, due to damage in the winter, and fit a new timing belt. Firstt time pulling the front end off the Audi so was a bit scared! lol Sorry I dont have any pics or procedure but its all out there if you search. I can say that to change the timing gear you dont need to remove to radiator etc, you can just drop the front panel forward and it gives enough room
Soooo, while i had the front bumber off I had an opertunity to get at the secondary air pump. This I believed has been the source of an engine managment light on and a vagcom code talking about incorrect air flow. Low and behold the pump was full of water, very common it appears. Ahh thats what caused the engine management light I thought. So I pulled if off and started the search for a replacment. While doing this it seemed the best fix was to remove it, and the related secondary air system. after many hours of searching i decided to do it. Just to give a bit of understanding of what this system does...
There is a large air pump, it draws air from the airbox, after the filter and blows it through a valve at the back off the engine direct into the exhaust chamber before the O2 sensor. This tricks the engine management into thinking the fuel supply is too lean and it adds more fuel, this excess fueling assists in heating the Cat up to operating tempreture faster therefore assisting emmisions
Sorry thats as much as I could simplify what I found from many different sources.
Now because this pump injects air into the exhaust flow there must be a way to block the reverse flow when the pump is not in use (it only operates for a very short time when the cat is cold) this is done with a valve, on the BFB engine(Quattro) it sits at the back of the engine just under another black plastic valve. This is operated by a vaccum that is solanoid controled, the operating solanoid sits under the inlet manifold.
When this valve is faulty it allows hot gas from the exhaust to flow through the secondary air pump, this is how the pump gets full of water as the hot air condenses inside the pump, and into the air intake, crucially after the filter. im sure you can imagine this is not good. Hot exhaust gasses going direct into the MAF. along with all the crud from the now faulty air pump.
This also was causing other issues that I now realise were caused by this valves failure.
The engine revs would die down very slowly after releasing the pedal and also seem to blip between gear changes - obvious now as the exhaust was pumping air into the intake
Sound of the exhaust blowing, and very boomy on the motorway - again there is basicly a hole in the back of the engne letting exhaust gas out!
So the fix..
Remove the silver valve off the back of the engine, Tricky as its held by 3 allen head bolts you can only get to with a allen key, remove the vaccum hose from this valve and block it
Remove the pump and any loose pipes this leaves behind
You will need to block the hole in the back of the engine where the valve was removed. Forge make a plate just for the job, They charge what seems a ridiculous price for it (£20 plus postage) but is does the job quickly and easily.
You will also need to block the hole in the airfilter box left from removing the air hose to the air pump. This must be blocked as it is post filter so unfiltered air will be getting drawn in
There is a lot of discussion in various forums about adding resistors in place of the pump that was removed. And also removing the solenoid valve that operates the valve. I didnt go there and as yet dont have any issue or any engine managment light.
The car now runs much smoother, much quiter, its quicker and so far has performed about 3-5 mpg better. I have added some images of the parts removed. You can see the black soot inside the valve from the exhaust blowing through it
Hope this helps someone find an answer. please feel free to ask any questions as I am sure I didnt cover everything.