The car is a 2001 A4 1.8T fwd, automatic. Miles: 153k

After driving the car for a few months while showing the O2 sensor code, I finally decided to change it out. Simple enough, huh?

Disconnected negative battery cable.
Disconnected O2 sensor cable.
Unscrewed O2 sensor.
Assembly, as they say, is the reverse of removal.

Hooked battery back up and cranked car.

Runs like total crap. Won't idle. Won't drive.

Put old O2 sensor back in. No dice. Same thing. (Note that prior to removal, it was running decently, but with lower efficiency)

Put NEW O2 sensor BACK in. Still crap.

The symptoms, as best I can describe them are: car doesn't seem to take throttle sometimes. when it does, I can get it to hover at around 2500 rpms, but when I try to go higher it stalls out. Quick throttle pulses stall at around 3k rpms.

I was hoping it was bad gas (I had just filled up prior to the O2 work). Drained the tank and went back with new. No luck.

Next I was hoping it was a fuel filter. No luck after replacing.

Next I checked the fuel pressure. Plumbed a gauge into the pressure line feeding the fuel rail under the hood. Solid at 55-60 psi.

Checked the air hoses leading to the intake manifold. No cracks.

Had the wife actuate the throttle while the air supply hose was off. butterfly valve seems to follow throttle position appropriately.

I'm out of guesses. Before I shoot this thing in the head, any ideas?

I should also mention that I took the car to a reputable shop. They spent four hours on it, replaced the spark plugs with some that are apparently made of unobtanium ($70 worth!), and did a "seafoam treatment". Charged me $500 and I left with a bit of a limp. Anyway, they showed me the old plugs and they were sure enough carboned up. This should have been my first clue that fuel delivery wasn't the problem, but I wanted to check it anyway, just for my own (in)sanity.

While I'm at it, I'll give some more hints related to the way it drives currently. Seems to idle pretty rough. When in gear, I can feather the throttle and find a little sweet spot where I get some response for a moment. When it starts to go dead, I floor it and start feathering again. Lurch forward stall lurch stall. The cat is getting sure enough hot. Smoking and stinking when I shut it off. I'm thinking cat, but I'm not spending that kind of money, not knowing for sure. And the gurus at the shop were non-committal on whether they thought the converter was the problem.

In Park/Neutral I can get the RPMs to about 3500 without any problem. Beyond that it stalls out. Flattening the accelerator gets a further stall until I let up, and it will rev up to 4k very momentarily then drop back down....

Here are the codes, in the order in which they come back after clearing them:

17887 (within a minute)
16514 (about 5 minutes)
16684 (~15 minutes, after a short 1 mile drive through the 'hood)
16688 (showed up with 16684)

Again, any ideas?

Thanks,

Jason

'01 A4
'07 Jetta 2.5
'86 325es

 

The first code you've listed doesn't show up on the list that I have.

The second code is an error for the primary O2 sensor (presumably the one you replaced). I would inspect the engine harness side of the plug for that O2 to make sure that it hasn't suffered any trama.

Third code is a random multiple misfire. This can happen for many reasons: vacuum leak, failing coil, MAF, fuel pressure/injectors...

Fourth code is a misfire detected on cylinder 4.

The first thing I would try aside from inspecting the O2 plug is to unplug the MAF. Doing so will default the ECU to its tables. If it runs better with the MAF unplugged, you may have found your culprit.

 

Thanks. Quick question though: Doesn't the car start out running from the tables when cold? It has the same (if not worse) symptoms at first start-up.

Gosh. I can't believe I'm going to ask this, but I'm afraid I have to: Where is the MAF? I see the throttle control harness at the start of the intake, and the manifold pressure sensor down in there at the bottom of the flexible hose that runs from the intake toward the heat exchanger (?). Is the MAF over near the air filter??

 

Never mind. I see in the airbox. going to run out and disconnect it. stand by.

 

OK. So, unplugging the MAF makes it drivable. Not only that... I can hardly remember it EVER running that good. I guess I'll try to clean this one first, then order a new one.

What are the chances that it's NOT the MAF, and the action of unplugging it and sending it to the tables is just bypassing the problem? (Look... not asking for a guarantee.. just a gut feel.. do you think it's the MAF given the reaction that I have described above?)

Thanks a TON.

 

From everything you described, it sounds like the MAF is bad. I tried the cleaner on mine with no effect, but it may fix yours. I picked up a reman MAF on amazon for $75 with no core charge.

 

I can't thank you enough. Tell you what... I'll send you half of every dime that I get back from the "reputable" shop that I took it to, who spent $500 of my money and couldn't figure it out. Live and learn I guess. Thought about going back and seeing if they'll "cut me a deal" on a MAF. Nah... I think I'm done with 'em. Moving on.

Thanks again.

 

No problem. We're all here to help and learn.

I would not go back to the shop you took it to. It sounds like they didn't have a clue. Get yourself a MAF and come here next time when you need help.

 

FYI: $500 is a lot for plugs and Seafoam. I Seafoamed mine myself the other week for $11 and Denso Iridium plugs are $8.50 a piece.

 

Based on the bill, the majority of the $ was for "head scratching" time. Drain the tank and rule out bad gas...plug up vag com and clear/look up codes...etc...

I guess I'm going to chalk this one up to "stupid tax".