Hello! I have a lovely a4 with many of the issues that come with a vehicle this old. It has just shy of 150,000 miles, and i am slowly tackling each issue as it comes up. I recently had a CEL come on that would slowly clear itself after about 50 or so miles, and now is stuck. I have 9 codes coming from a scan:

P0452
p0174
p0171
p1106
p0170
p0173
p0174
p0141
p1250

Apparently I'm running a very lean fuel mixture at the moment. The car runs fine, has great mileage and power. The light is really annoying though. I have changed the throttle body gasket and egr gasket as part of a valley pan gasket change, and plan to change all of my vacuum hosing as well as my crank breather tubes. One of them has already been replaced after shattering in my hand like an egg shell.

I guess the question would be, is there anywhere i should look for specifically? I'm not ase certified by any means but do know my way around the engine.

Thanks for the help!

 

id start with replacing all the hoses above the intake and throttle body. all the hoses on my 96 were pretty shot and after replacing all of them, i have not had any more problems with vacuum leaks.

 

Awesome thanks I'll start there I've got 5 ft of super thick black hose hopefully it helps

 

That's a low-mileage '96, mine has probably 90,000 more miles on it than yours!

Vacuum leaks big enough to throw codes often make a hissing noise, so listen for that while idling. Your P0452 is for Evap pressure low, and that might be due to the leak detection pump being defective, or perhaps the vacuum line that "powers" it may be leaking, maybe at the pump in the left-rear wheel well.

On a '96, besides changing vacuum hoses, change your coolant temp sensor if you have not already done so, and clean the MAF with something such as CRC MAF cleaner spray. Also, I was really surprised on my car to see how dubious the joint was between the plastic intake duct and the throttle-body. That rubber seal relies more on luck than anything else to seal, and it is down-stream of the maf, so any air leaks there will result in lean codes. I suggest Permatex or Loctite silicone sealant at that joint.

 

Yeah it was quite a find its in pretty great shape considering it's age!

The intake duct you are referring to, is that the intake plenum where the breather hoses plug into? Because if so you are totally right there is nothing holding that on to the throttle body at all it just sits there i can actually visibly see a gap on the seal. I can post a picture of it later.

I have also looked at the maf and it looks brand new, would brake clean work to clean it out or should it specifically be maf cleaner?

And in regards to the evap pressure low, the car does not have a/c any longer, I'm pretty confident it needs a recharge. Would that cause the code to show up?

Thanks for the help!

 

Update:

So i replaced my valley pan gasket, again, accidentally forgot to take off the old gasket and had puddles of oil showing up every morning. Im dumb. I also removed the oil check valves and sprayed them with a healthy amount of brake cleaner. Engine doesnt click while starting anymore.

So i took off the intake manifold, and on the back of the throttle body, there is a black gasket that just sits there, it is removed in this picture:



Here i have inserted the gasket into what audi calls the "intake plenum", this is a very hard part to find and i have found very little info on it.



I sealed it with silicone and put in place. The engine feels like it is running smoother now. It is quieter, but i still have a CEL on. So the search continues!

I also sprayed the inside of the MAF with brake cleaner. The part looks brand new, hardly any dirt or dust on it. It would be hard to believe that it is failing.

As far as the plenum goes, i found a milky build up inside of it as well as inside the passenger valve cover a bit and in the passenger breather hose as well. Replaced breather hose (14$ on ebay), and now doing an oil change and coolant flush today. Next step is to change coolant temp sensor and O2 sensor.

I was wondering if anyone knew which O2 sensor i should change? Guy at firestone said it was one on the Cat, but i see a total of 3. Do the codes tell me anything about which one could be bad?

 

Update:

I messed up. So i tried replacing an oxygen sensor with one i found online, it had the proper plug to fit into my system but had 4 wires instead of 3. I plugged the new O2 sensor in, started the vehicle, and it ran fine for about 3 minutes, it then started to idle really rough, but would occasionally switch back and forth every few minutes. I took it for a quick drive to the gas station, and when i tried to turn it back on, it would not start, and I heard a very loud clicking coming from behind the dash (turned out to be a fried fuel pump relay).

I replaced the relay and fuse, but the car still will not start. I hear the starter engaging and the car has power, dash lights, head lights, etc still work. I think i fried the new relay as well because now it is clicking really loud.

At this point i believe the ECU is fried, i read that the O2 sensors do not pass through a fuse before communicating with the ECU. I will be replacing that as well as ALL relays and fuses soon.

There is a great article on 12v.org about O2 sensor replacement, and I now have some nice BOSCH 3 wire sensors that apparently compatible with my system (hopefully).

Can anyone confirm the lack of fuses between O2 sensor and ECU? This seems like a really risky design flaw to me...also, are there any other electronic fuses/relays i should be looking at before attempting to put everything back together? I really dont want to fry anything again...

 

Also found the codes:

P0452: EVAP Emission Contr.Sys.Press.Sensor Low Input<-- not sure about this one
p0174: Fuel Trim,Bank2 System too Lean <-detecting lean fuel in exhaust
p0171: Fuel Trim,Bank1 System too Lean <-- same
p1106: O2 Sensor Circ.,Bank2-Sensor1 Voltage too Low/Air Leak <--possibly broken 02 sensor
p0170: Fuel Trim,Bank1 Malfunction <--reached maximum amount of fuel adjustment
p0173: Fuel Trim,Bank2 Malfunction <--same
p0141: O2 Sensor Heater Circ.,Bank1-Sensor2 Malfunction <--also broken o2 sensor
p1250: Fuel Level Too Low<-- non CEL code, probably was running on an almost empty tank

 

Similar problem with P0170 & P173. Problem was that the breather hose on drivers side was broken. Used 2 layers of large diameter heat shrink to reseal it but these hoses are so brittle that they easily break when working around them. Great if there was a metallic hose in stead.