kjmiles

I've looked around the forums for awhile to see what I could find and while it gave me some new theories, I figured I'd post. One day (and I hadn't heard it before, although my wife drives it more than me), I got in and started the car up. Sounds normal. I backed out of my driveway (the sound happens on both backing up and acceleration, so I don't think it's the turbo) and heard this kind of "dut dut dut" sound which I thought was coming from the exhaust. I stopped, waited idling for a minute, then revved up to a few 1000 rpm with only the normal sound it's made over the past 12 years, waited again, and then started driving. On acceleration, the sound started up again. So I took my foot off the gas and it went away. Experimenting, I noticed that the sound didn't happen when I only had my foot lightly on the gas, but under load or when I put it down farther it came back. So I took a trip to my mechanic (Audi/VW mechanic for the last 8 years). We drove together and then he drove. He said it wasn't "a death noise" and I scheduled an appointment. I had gotten a misfire code in cylinder 1, by the way, which was cleared and now the MIL is back on again (not sure if it's the same code or not) as well as a catalytic converter code (I think that one haas been taken care of). He said he thought it was a subtle sound and hard to diagnose and he suggested that I wait to see if it gets louder. Well, I don't think it has gotten appreciably louder, but I definitely notice that there is either no turbo boost (I chipped the car a few years ago via APR) or a lag in power on acceleration. Other than that, everything else is "normal" as far as I know. There were suggestions in other posts about a PCV check valve gone bad or maybe cracked/broken/disconnected vacuum hose or hoses. Like I said, this was not something that seemed to happen over time, so I'm hoping that's good.
Thanks in advance for answers/theories!
KJ

lyleswk

"Dut, dut, dut" is a new way to describe a noise Anyway, the best thing to do is download the codes again. I suspect you will find another misfire code which will hopefully be cylinder 1 again. This is generally the coil pack. The cat code wouldn't cause misfire issues unless it is completely clogged.

kjmiles

Huh, the coil pack?! I never would have suspected that. Can you say more about why you'd think it was the coil pack? Would this happen rather suddenly - like one day it's fine and the next there's this sound? I tried to be descriptive with the sound. The sound is directly related to the acceleration, meaning that it increases in frequency as you push on the gas and then it eases off when you're "cruising." I'm assuming the MIL is for the cylinder 1 misfire again. I can get that on Friday. I was driving the other day and there seems to be boost in the turbo, but I think it's being overrun by that sound, which is why I thought maybe the turbo wasn't kicking in. If it is the coil pack, is this something I should replace immediately? If I bring this theory to my mechanic, can he easily check it to see if it's the culprit?

lyleswk

If he downloads codes and you get the cylinder 1 misfire again, then you could do one of 2 things:

1) swap coilpacks around and see if the misfire "follows" the coilpack.
2) just replace it with new.

1) above will isolate it better and potentially cheaper in the event it is not the coilpack.

If it is a different code, let us know.

kjmiles

So it was NOT a cylinder misfire code. Maybe that's good news? It was a catalytic converter code. He said it was coming from the pre-cat oxygen sensor. The last time I was in, he gave me an option of replacing the cat or putting a bypass on, which I chose the latter. That was the only code there was. I asked him if it could be the coil pack and he said that usually presents a flashing MIL accompanied by a lot of bad things (loss of power, bucking, dead car, etc.). I'll have to check my maintenance records on the last time I had my plugs changed as I was doing some research and someone reported a strange sound due to a bad plug. I also asked him if it could be an emission problem (I keep going back to the PCV check valve) and he doubted that. Could it just be some kind of good old fashioned exhaust leak? But I'd be stumped as to why it would only happen when the car is moving both forward and reverse and not just idling/revving? It certainly has that sound like there's a hole in some component or a disconnected vaccuum line. Other than the loud, annoying sound (which is directly related to acceleration), it drives fine. This did happen about 2 days after the left lower control arm was replaced. Not sure if that could have anything to do with it.

kjmiles

After about 2 hours of research and video watching, could it just be that either a piece of the cat or a more significant chunk has come loose and is either being wacked around or is causing a little back pressure in the exhaust? I'm not sure which side it is, but what would be a recommendation on a new cat? My mechanic quoted me a stiff $1,500! I assume this is for a new Audi part. This could be the beginning of the end if that's the case. I'm not averse to putting money into the car, but yeesh. Maybe an alternative can be had for less. And if it were that much, it sounds liked be better off replacing the exhaust. I was looking at the 034 Motorsport cat. That's $330 and I've already spent $400 tricking the sensor. Love to have input.

lyleswk

Did he put it on a lift and look for leaks? I had to replace the flex portion of the exhaust which is just after the turbo but before the cat. Other than that and given that you have a cat code, it may be that the cat has come apart. $1500 is way too much for that. When my cat on My Mercedes went out, I went to a reputable muffler shop, and they cut the old one out and fitted a new one. Has worked fine for 3 years and was about 600 if I remember.

kjmiles

As it has been like the tundra here in MA recently, I'm also getting a pretty good exhaust or something smell in the cabin when the heat is on. Not sure if this is from the cat breaking up, an exhaust leak, or some other emission control problem (as I mentioned earlier, could the sound and/or smell be coming from the a broken PCV? Where is that located, by the way?). Anyway, I am considering a high flow catalytic converter from 034 Motorsport. Anyone have an opinion? Since I've already paid my mechanic to put the bypass for the O2 sensor in and it seems like the cat is breaking up, this seems logical. It seems like a 2-3 hour job? Should I go to a "reputable" muffler shop or have my mechanic do it? Who is considered "reputable" in the world of muffler shops? It seems like they do this stuff all the time and they would have more expertise in replacing a cat than my mechanic. Thoughts?

kjmiles

Oh, and I did have the flex portion of the exhaust replaced 2-3 years ago. In fact, the last time I was in, my mechanic had a whole box full of new flex joints sitting on the floor!

kjmiles

Ok, well here's the update if anyone is listening. The MIL turned off this morning (getting a pre cat O2 code checked by two places today - read on). I quick went to get an inspection sticker, which I got. Then later on in the day, the light came on again. Woo hoo! Little victory there. I went to a custom exhaust shop after that and they said my exhaust system is in excellent condition and no leaks. Not sure what's causing the problem. Suggested a wheel bearing. I laughed. No. Could be the cat they said. Maybe, I said. Need to schedule a full day appointment. No one wanted to drive it to hear the sound it's making. Doesn't anyone listen?? Then I decided to call my friend Jeff who owns Wheel Dynamics in Natick. Highly recommended as I've gotten tires from him for years and he does other work as well (like, say, wheel bearings, and suspension, and window tinting, and etc.). Great guy. So I call because if anyone will actually listen to the sound, it's him. So we drive and he hears exactly what I hear in minutes and diagnoses it exactly how I do - a thumping noise on acceleration and under load and goes further to say it's slightly at idle and more on the higher gears under lower RPM. We checked a lot of things and he actually used a device to listen to the cat from underneath the car. He doesn't think it's that - he doesn't hear anything that would indicate a piece has broken up. He does see my oil leaking from the back of the engine. Possibly a head gasket. He also asked me about the differential fluid being changed. Not to my knowledge, I said. He scanned the code and got bank 1 error - the pre cat O2 I think. Same as what the exhaust place got and the same as my mechanic got the last time I stopped in. In the end, he couldn't figure it out, but it was a good try! He passed me on to someone else in Framingham at Eurotech (Porsche) as the guy who owns it (owns the same car as me) has a friend who is an Audi genius. So I went over and the owner drove it and didn't know what was wrong either. He is going to contact me after discussing with his friend and I'll make an appointment with him to see what's what. Stay tuned!