I need some expert help here please...

About 2 months ago, I'd noticed the loss of power with my 2001 A4 Avant V6 2.8L.

The car starts and runs smooth, but it was evident that the snap under acceleration was no longer there one day. I used the Memoscan U280 scanner and came up with these codes:

16814- main catalyst efficiency below threshold (16816 closest)
16684-random misfire detected-any cylinder
16685-cylinder #1 misfire detected
16687-cylinder#3 misfire detected
17831-secondary air injection system (B1)-Flow too low

I cleared the codes and it seemed to fix the issue (probably coincidence) but the issue came back within a couple of weeks.
I scanned again and now came up with:

16804- main catalyst efficiency below threshold Bank 1 (16806 closest code)
16814- main catalyst efficiency below threshold Bank 2 (16816 closest code)
16684-random misfire detected-any cylinder
16687-cylinder#3 misfire detected
16688-cylinder#4 misfire detected
16712-knock sensor #1-circuit-low input (bank 1 or single) (16711 closest code)
17734-Cylinder #2 knock control limit attained

Going up hills or acceleration at this point is weak. Under heavy load you can hear the car knock/ping so I pull off the accelerator.

I suspect the cats, knock and such are results of the cylinders misfiring. Fix these issues and the others go away ( I think!) Cats are a couple of years old with new O2 sensors replaced at the same time.

So started with cylinders 1 & 3 to have misfire, but now 4 and cylinder 2 misfire is gone, but the "knock control limit attained" exists now...I do not know what that means.

Can someone please point me in a direction to remedy this issue? I need fast help as this is my only source of transpo and don't want to "F" this thing up.

Thank you!!!

 

See my post about "glazed" spark plugs in this section. You might want to check them if you haven't already. However, it almost sounds like you have a clogged exhaust or something..

 

Thanks Dave, I'll have a look

BTW, I was a huge Porsche 928 guy some years back...miss all of my cars.

 

My Landshark is languishing away awaiting me to restart the restoration again. I just haven't had the time to do anything to it. I currently am trying to get my 87 5000S Quattro back together and getting an 88 944S back up and running. My 87 944 is going to get parked soon to undergo some body rust (rockers) work. Along with some other items. Too many projects, not enough time and money.

 

Dave...pulled #1 plug and found this. Is this what you mean?

I think I probably used too much 87 octane gas now that I think back. I'm pretty sure this can foul the plugs. Went out and bought new factory recommended Bosch plugs tonight and will swap in the morning and clear all codes again. The fouled plugs are NGK.

FWIW, I lightly scraped off some of the crud on the plugs and wire brushed them. Seems to have made a nice difference by just doing that, though new plugs are a necessity after viewing these...whether this is the problem or not. These plugs were installed 6 months ago.

Keeping my fingers crossed and make sure I only use 92 octane.

Much thanks for an easy fix (if this is what it turns out to be), instead of the headaches ahead I was dreading.

BTW, new cats and o2 sensors were installed a couple of years back.

 

Yep.. That thing is glazed. Looks just like the ones I pulled from mine the other day. It's damn near impossible to get that crap off since it has basically been melted to the electrode and surrounding ceramic insulator. I'd go ahead and replace them all. Also make sure they are gapped about .032".

You might also want to reset the ECU. This will dump any crappy data that it may be trying to use to run the engine. To do this, remove both terminals from the battery (be sure to have the radio code just in case you need to re-enter it) and touch them together. This will discharge any power storage devices in the electrics including the ECU. Wait an hour or two and then reconnect the battery. Turn the ignition on but DO NOT try to start the car. Let the ignition switch set in the ON position for about 15 seconds. Then proceed to start the car. It may stumble for a few seconds but it should recover. The ECU will forget all the bad "memories" from before and if you have truly gotten the O2 sensor issues and the exhaust is not leaking anywhere between the engine and the downstream O2 sensor, it should be fine again.

I use 87 in my A8 all the time with no issues with octane.

 

Well, I'd changed all the plugs and it ran/runs fine....until I get on the throttle under heavy load and then BAM...stuttering and loss of power at WOT. I have to pull off the throttle and pretty much nurse it to get up hills motoring along the freeway. This seems to only be apparent at speed.

So after changing the plugs, clearing codes and running the hell out of the car for a couple of days, these are the codes that showed up.

16684-random misfire detected-any cylinder
16685-cylinder #1 misfire detected
16686-cylinder #2 misfire detected
16687-cylinder#3 misfire detected
16688-cylinder #4 misfire detected
16689-cylinder #5 misfire detected
16690-cylinder #6 misfire detected
16826-Evaporative Emission Control System-Leak Detected (Small Leak)
17551-
17819-secondary air injection system (B2)-Restricted Flow
17831-secondary air injection system (B1)-Flow too low

I'm beginning to think this is an air restriction issue. Seems like at speed, the car just can't get enough air to create more power. I can even hear the pinging and that is when I back down off the throttle.

I looked up the code 17551 and could not find in the book, but online some have mentioned the MAF. There is no reason why all cylinders are misfiring now, no cat issues, etc. As I'd thought before, these things are all result of one issue...and it seems to be pointing towards air intake.

So I'm wondering if the "secondary air restriction" is a result of a dirty MAF???

I gotta fix this, car feels like its gonna die when I hit kickdown switch.

Ideas?

 

There is a secondary air pump in (or under) the air filter box with the MAF. There is a hose that runs from the that pump to the engine around the back to the metal piping that injects that air into the engine exhaust. Check that hose for kinks or some type of collapse. Mine had a couple of small holes in it where it clamps to the metal pipe giving me a lean condition. I was fortunate to have enough hose left to reach when I snipped the bad part of and reclamped it. I've not had any issues with that since.

Being that you're getting the error on both banks, it leads me to believe that it is something to do directly with secondary air pump. Check the fuses for it. I also found this video on YouTube that may help.
I think if you get the air pump issues fixed, the other errors should go away. If you get it fixed and they don't, you could test the MAF by unplugging it and driving it and see if the problem goes away. The ECU can run the engine just fine without it. If it runs fine, you can try to clean it and see if it that fixes it. If not it's time for a new one.

Anyway, I'd check the secondary air pump first. Here's a good video showing the components and operation of the system pretty well too.

 

Thanks, but its not the air pump. I have no CHECK ENGINE light and I can hear the pump work. I checked the air lines to the pump also and they are fine.

I even replaced all the vacuum lines from the air reservoir to around the engine. Cheap fix, but it did not fix the issue.

However, just by chance the other night, I was leaving a friends house and started the car from cold as it had been sitting for several hours. It was 30 degrees outside and I do not usually go heavy on the throttle until the engine has warmed up. But being frustrated with this whole thing, I got on the throttle hard and the car had its power back it seemed. I tried it several times and it had spunk until it warmed up.

Is it possible the auto choke would cause this problem???

 

Have you tried a new coolant temp sensor? There are two circuits in them from what I can tell. One for instrument gauge/idiot light and the other is the sensor the ECU uses to tell what the coolant temp is for "choking" purposes. It doesn't actually "choke" like the old fashioned carb system but rather puts a little extra fuel in the mix for cold starting. I had some quirky issues with my A8 last year (hard starting and what not). It's a fairly cheap part and a snap to replace. It's held in with a plastic clip (get another one with the new unit as they will get brittle over time and I snapped mine getting it out) and really only takes 10 minutes to replace if it's not buried too deep. It's usually located in one of the metal coolant pipes near the firewall.

Here's what it looks like to help you find it. This one comes with the clip too.
2001 Audi A4 Coolant Temperature Sensor | AutoPartsWarehouse