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Discussion Starter · #1 ·
Car in question is a 2002 1.8t 5mt wagon, 122k miles. Car is stock.

The story begins a couple months ago when I got the car. I'm the 2nd owner and got a bunch of service records with the car. On my way home from buying the car, a mere 10 minutes after signing the paperwork, the CEL came on. I noticed the temp gauge didn't come up to normal unless it was sitting at idle for a while, so I wasn't too worried. Tried to scan it with my generic obdII scanner when I got home and it wouldn't communicate with the car. Great. A quick stop at Autozone and they told me it threw P0420, Catalyst System; Bank 1: Efficiency Below Threshold.

Since it wasn't getting up to temp (at least according to the dash) I started with a CTS. That didn't help, so I put a new thermostat in. Now the temp gauge comes up to 190 and stays there. I cleared the code and hoped for the best. Couple days later the CEL came back on.

By this point I got a cable from ebay and got vcds lite running. The P0420 code was still there, but now it had a P1128 friend.

Next, I got the coil recall done at the dealer where they put 4 new coils in. Again I was holding my breath that it would take care of the CEL, but no luck again.

My next repair was replacing the 4 main factory vacuum lines with silicone. A couple of the original hoses were questionable, but they all seemed to hold vacuum when I tested them on a MightyVac. That didn't seem to help the CEL either, but now the P0420 is gone and only P1128 is hanging around.

I checked through the service records and found that the MAF was replaced twice. The last one was done about a year and a half ago, so I decided to give it a cleaning today. Before cleaning, I was getting 2.3 g/s at idle reading measuring block 2. After cleaning I got 2.8 g/s, so it seemed like an improvement. At 2500 rpm it was 6.3 g/s before cleaning and 8.3 g/s after cleaning. I cleared the code again and went for a ride. Drove about 30 minutes to have some dinner and after dinner on the way home the CEL came on again :mad:

The P0420 still hasn't come back, at least not yet. I'd like to read block 32, but vcds lite only reads to 29 or something without registration. I'm saving up for a hex-com cable/registration, but won't be able to get that for a little while.

I don't see any records of O2 sensors being replaced. Would this be a good next step? Should I start with the rear sensor, or just do both?

Are my numbers in block 2 at idle (2.8 g/s) and 2500 rpm (8.3 g/s) good enough to rule out the MAF?

Anything else I should look at? Does it just need to adapt a little bit since I cleared the code again? Maybe a longshot, but my flexpipe is coming apart too so I'm on the hunt for a downpipe. I don't think it is leaking yet, and I don't know if that would cause the issue anyway.

Thanks for reading through all of that. And thanks to PW for the great information I've found so far while trying to fix this.
 

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Are my numbers in block 2 at idle (2.8 g/s) and 2500 rpm (8.3 g/s) good enough to rule out the MAF?
Monitor the MAF while doing a red line run in 2nd gear (get a friend to ride along and monitor that, while you drive). If the peak reading is in the range of 135 g/s, the MAF is likely okay. If below ~100, I'd replace it or maybe try cleaning it again and see if there is any change. When mine was bad, it was reading maybe 50 or 60 g/s (older 150hp 1.8T).

And look for vacuum leaks - since you've replaced the 4 hoses, check the "L" hose.
 

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17536/P1128/004392 - Fuel Trim; Bank 1 (Mult): System too Lean

Could be MAF, O2 sensor, vacuum leak or fuel delivery.

Any idea how old the fuel filter is?

The pre-CAT O2 sensor data is used by the ECM to keep fueling optimal. The post-CAT O2 sensor data is compared to the data from the first sensor to determine if the CAT is operating properly. The function of the one relies on the function of the other.

http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info
 

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Discussion Starter · #5 ·
Scott - There is a pretty good exhaust shop down the street, I'll stop in and see what they can do for the flex pipe. Thanks for the tip and the link, there were a few posts in there I hadn't seen yet. I'm wondering about the vac lines to the N75 now...

Attofarad - I'll have to watch block 2 or 3, maybe both, during a red line run. I think I can log it and go back to find the peak later. Hopefully I can do that today and rule out the MAF. The "L" hose looks pretty new and I didn't find any holes in it. All the PCV related stuff is pretty new too. The dealer did a valve cover gasket not too long ago and replaced a lot of that stuff then.

Steve - I'm starting to wonder if the fuel filter is the original. I need to double check the records, but I don't recall seeing it. I'm putting together an order for brake parts and I think I'll get a fuel filter too. It doesn't seem to have any fuel related issues and I'm still getting 26-28 MPG in mixed driving (calculated, not just based on the MFA).

Thanks for the replies so far, I'll try to keep this updated as I can get more testing done.
 

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a MAF cleaning is probably in order if you don't know that it has been performed. That's the quickest way to rule it out (if it just needs cleaning), and MAF cleaner spray is pretty cheap.
 

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Discussion Starter · #7 ·
a MAF cleaning is probably in order if you don't know that it has been performed. That's the quickest way to rule it out (if it just needs cleaning), and MAF cleaner spray is pretty cheap.
Did the MAF cleaning this past Sunday, but I still need to do a red line run in 2nd gear to get the max reading from the MAF. Advance Auto has the MAF cleaning spray on sale, I think it was $4.99 for the can. Or if you buy the MAF cleaner at full price you can get a can of Throttle body and Intake cleaner free!

I did check my service records and the MAF was replaced 9/18/2009, about 6000 miles ago. It does have a reman sticker on it, but no indication of who rebuilt it. It was replaced by an Indy shop I'm guessing it came from NAPA based on the Napa gold air filter they also put in it. Do rebuilt MAF sensors have a good reputation overall, or are they as flaky as the originals?

I also confirmed there are no records of fuel filter change or O2 sensor change. If the MAF seems to check out ok on a red line run then I'll order a front O2. If the MAF has low numbers then I'll order a MAF and wait on the O2. Fuel filter will be replaced in either case.

Anything else I should think about?
 

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Discussion Starter · #8 ·
Got a chance to log block 002 to red line in 2nd gear tonight. The highest g/s value I see from the MAF was 108.39 - seems a bit low. If the rough calculations are correct I should be around 136 (80% of 170 hp AWM). Looks like I'll be shopping for a MAF...

Anyone have any experience with the aftermarket MAFs on ebay? This one matches my part number and has a 1 year warranty: http://cgi.ebay.com/ebaymotors/VW-06A906461L-NEW-Mass-Air-Flow-Sensor-Meter-MAF-TRE-/300375151514?cmd=ViewItem&pt=Motors_Car_Truck_Parts_Accessories&hash=item45efc1139a
 

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The cat code was from the bad Tstat. A cold engine dumps unburned gas into the exhaust and the cat can't keep up with it.

Sounds like the seller cleared the codes right before you bought it.
 

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Discussion Starter · #10 ·
The cat code was from the bad Tstat. A cold engine dumps unburned gas into the exhaust and the cat can't keep up with it.

Sounds like the seller cleared the codes right before you bought it.
I figured the cat code was probably related to the engine not getting up to temp. The PO took it to a shop for everything and I don't think he cleared the codes, anything is possible though. All the readiness monitors were still set when I got it scanned the first time.

It would have been nice if the CEL came on during the test drive, I probably could've talked him down another couple hundred. I still got a good deal though, and finding a manual B5.5 wagon with service records was a tough search ;)
 

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Discussion Starter · #11 ·
Installed a new MAF on Sunday afternoon and reset the codes again. So far the CEL hasn't come back on, but I've only put about 50 miles on it.

It does seem to have some hesitation that I didn't notice before, mostly between 2000 to 4000 rpm. It's not there all the time and it doesn't seem to be missing, no flashing CEL anyway.

It almost seems like the throttle input isn't right, while it is hesitating it doesn't seem to make a difference if I have the pedal half down or all the way to the floor. I had a friend follow me for a few minutes, there is no smoke.

Do I need to give it more time to rebuild the fuel trims? Time for throttle body cleaning and adaptation? Can I do the adaptation without full vag-com on a 2002 1.8t?
 

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Cleaning and adaptation are your best bet. No need for a vag-com, the open door, turn key to run (but not start) and wait 2 minutes works quite well.
 
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