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Discussion Starter · #1 ·
So, I drove my car on Saturday (1998 A4tq auto) and no issues except the CEL was still on (have a slight exhaust leak, so the O2 sometimes trips the CEL). Go to drive to work yesterday, and the car had a rough idle and almost stalled. Seemed to not make much boost, I can normally hear the turbo by around 2500rpm, I couldn't hear it until around 3500rpm. Go to leave work, and it actually stalls twice and almost stalls two more times. Felt like I had to keep giving it throttle to keep it alive. After it warmed up a bit (I let it idle while I locked up the gate at work), it seemed to be fine. Got home, left the car running, decently smooth idle, VCDS plugged in, just the same code for the O2. Cleared it. Shut the car off, counted to 30, re-started, rough idle. No codes. I was looking at some of the channels, and it seems like it is having to give itself excessive air flow, 10-13 (% or degrees?) of throttle to maintain an idle, then after I rev the engine and let it "warm up" again, it drops to 4-6 (% or degrees?) and has a decent idle again.

Car has 127k on it, as far as I know original O2's. I replaced the 4-pin coolant temp sensor last year or so, just replaced all four plugs & coil packs and the POS a few weeks ago. Also replaced a couple breather lines.

The fact that it behaves itself after it sits and warms up a bit leads me to believe it is a sensor or O2 issue but I'm not sure what values to even look at or what they should be showing in VCDS.

Help please!
 

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O2 seems possible. Is the maf clean?
 

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Discussion Starter · #4 ·
Throttle body I replaced with a new one not too long after I bought it, so about 3 years ago? I thought about that too, but the stepper is controlling for the ISV function (my AEB is cable with the stepper only for ISV duty).

MAF, no clue, I've never done anything with that.
 

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Thats why i always run a vac/boost gauge in all my turbochargered cars. VDO vision matches the older VAG stuff and you can easily notice vacuum leaks and boost leaks having one. O2 sensors are wear out items,every 100K i replace the pre cat O2's on all my cars. Im a bit anal but i never needed a tow EVAR,lol!
Flaky coil driver could do this too....
 

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Make sure your per cat o2 is tight. Mine would like to back out now n then. Let me know if you need a aeb throttle body. I can pull the one off mine that's sitting.
 

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Discussion Starter · #7 ·
I have a VDO gauge laying around that was going to go in the CQ when I had a 20vt for it. So I could put that in the A4. I'm seriously considering trying to tighten the DP flanged connection bolts, as I believe that is where my leak is, and order a new pre-cat O2. Seems the most likely, as it would cause excessive fueling which would require excessive air intake, hence the high throttle position I saw. Seems plausible...
 

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Its not too far fetched.
 

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Wow. That's is odd. I have a friend who works at Autozone and he price matches stuff like that for me.
 

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Discussion Starter · #12 ·
Okay....I replaced the O2 hoping that it was bad. Old one was sooted up pretty good, but that is probably because the car is running rich, you can smell it. Car is still acting up with brand new factory Bosch sensor, it's loading up I'm guessing. Throttle body angles are definitely corresponding to my foot, and it is trying to maintain proper throttle to keep the idle steady (no load, it does a decent job, under load, like backing up my inclined driveway it is REAL surgey and rpm drops way low then climbs back up the drops, climbs, drops, etc). I data logged a little bit, grabbed O2, MAF, throttle, timing, injector time, engine load. Cleaning up the data so I can better graph it.

I just recently changed plugs, coil packs and the POS. I used NGK or Bosch plugs, Beru coils and a "generic" POS. Car has run perfectly fine in the 1-2 months since I did this. I thought about putting my old coils packs back in (at least three of them anyway, pretty sure #2 was bad due to the misfire code on that cylinder) and eliminating that. I can't put my old POS back in, tried that already, definitely toast.

The only air leak I hear is a very slight exhaust leak at the cat to DP flange (I did also find a slight coolant leak, which I hopefully fixed as a bolt was loose on the coolant manifold on the back of the head).

If I left-foot-brake and brake-torque the engine, some times it will load up, other times it will build boost. I will see some black exhaust (unburnt fuel) puff during some of this, not very often. I noticed it when I backed up the driveway too. My son commented that the car was stinky and that it "sprayed" him (he's 6 years old). Guessing he got sooted on. Sniff at the tailpipes confirms rich running. It does not smell nearly as bad as the A6 did when it's MAF died, and it doesn't run quite as terribly, but it does remind me of similar behavior. I'll try to post the graph when I get it done.
 

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Did you connect the boost gauge? Is the ECU chipped or flashed? Did you check for vacuum leaks? If you want to find common leaks,look at all the hoses and lines directly under the intake. They all rot and melt and car will idle high and run like poo. Connecting your scanner aint gonna help you so start looking for vacuum leaks and rotten/green wiring and connections. If all else fails,id find a known good ECU and swap it in. Takes 5 min and ive seen ECU's take a poop on these cars. I actually have one,sitting on a shelf.
Have you ever changed this????? If you havent,chances are its melted http://www.bestpricecarparts.com/ShopBy ... se&mode=PA
 

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Oh and from what i remember,and people on here can correct me,motor should run fine with the MAF unplugged. Unplug the MAF and drive it around. See if the problem goes away.
 

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Discussion Starter · #17 ·
ECU is stock, I have replaced that breather tube as mine was more of a "vent hole" than a tube. I have not gotten the gauge installed yet, but it will build boost if it gets a moment of good running.

Unless a critter ate a vac line, I don't see how the car can go from running "perfect" last Saturday evening to doing this on Monday morning (I did not drive it at all this past Sunday) if it was a vacuum leak. Not saying it can't have one, but I wouldn't think a vacuum leak would just appear.
 

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My 98 aeb would not run without the maf. Well would start but not much of anything else.

My current thoughts...

Plugged cat.
Big vac line on back and or under intake Mani cracked or blown off. Factory clamps suck.

Timing chain tensioner failure between intake and exhaust can. Those plastic guides like to break causing some funky running behavior.

How do plugs look? Failed injector??
 

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Discussion Starter · #20 ·
I didn't check the plugs, but I ran it until it warmed up (coolant temp at 90+ C). Interestingly, it ran decent (no stumbling/bogging, ran it up and down the street a couple times and it seemed to build boost, but I couldn't really accelerate due to the short street and number of little kids that live on the street). Then, the temp gauge finally showed it was fully warmed up, and the oil temp was up to normal levels too. At that point, it was really stumbling, I'd rev it up and it would almost stall. Still no codes, but as soon as the coolant went from being in the 86C range to over 90C it started acting up. I also noticed that I hear compressor stall on the turbo, so I'm guessing I need to finally install the 710N that I have sitting.
 
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