2004 Jetta MAF sensor issue?

Hello,

I have a 2004 Jetta with 297,000 km. I have an intermittent problem that I can't figure out. It started out as a odd hiccup in 5th gear while cruising around 2200 RPM. The engine would momentarily misfire and puff black smoke out the exhaust. Slowly the issue started to increase in frequency and affect driveability. I was travelling in Alberta and replaced the MAF with one from NAPA. The apperent misfiring greatly decreased (didn't stop altogether) but the car started setting code 16485 errors.

When I got home to BC I replaced all the intercooler o-ring, cleaned the EGR, valve and intake flap. I also inspected the vacuum lines and replaced any that seemed suspect. The car is driveable but seemed to lack full power and fuel mileage was down. I picked up a used MAF from a 2003 TDI from Pick N Pull and swapped out the New Napa MAF. To my surprise the power was restored, the engine revved quickly, fuel mileage returned. During a 400 km drive the occasional misfire during cruise persisted (fewer in frequency but more noticable.) Interestingly, no error codes were thrown.

The Napa MAF has a lifetime warranty so I returned it and got a replacement. I installed the replacement and the occasional misfires (puffing black smoke) increased but felt less intense. The engine set code 16485 once again. I pulled over and installed Pick N Pull MAF again. The misfires went back to being stronger but much less frequent and during the 300 km drive no codes were set.

Why does the apparent misfire only occur under light throttle or slight de-acelleration while cruising? Why would a New MAF set an code 16485 while a used one set no code?

Sounds like you used a generic code reader, not VCDS?
Did you check the code at Ross-Tech.com?
http://wiki.ross-tech.com/wiki/index.php/16485/P0101/000257
I web-guessing the sensor was not a real Bosh unit.

Interesting notes in the Ross Tech wiki, says this code may be triggered by mechanicals, which is unusual.

If NAPA sold you the correct MAF, I'd run it and troubleshoot from there.

The 2003 MAF may not even be talking to the ECU, or who knows. Symptoms don't seem like a MAF problem anyway.

The code was read using an Xtool vag 401 reader that I keep in the glove box when I'm travelling. With the Pick N Pull MAF (Genuine Bosch) the car runs great and accelerates quickly. The only problem is the occasional misfire (black puff of smoke). This occurs only during light throttle or slight de-acelleration typically in 5th gear. No engine codes are set using the Pick N Pull MAF.

When the NAPA MAF (Non Bosch) is installed the apparent misfires (hiccups) occur more frequently and a 16485 (improbable MAF) code is set. The car doesn't seem as perky as with the genuine Bosch MAF. For some reason I don't think the car likes the NAPA MAF sensor.

It's not a turbo issue because with the Xtool I can see requested and actual boost are pretty much equal.Unfortunately I don't have VCDS so I can't data log. I don't believe that it is a mechanical issue either.

Don't have a specific answer, just a general caution, FWIW: "implausible MAF" most often means "the airflow is not what I'm expecting" as opposed to "the MAF is putting out a bad signal".

Meaning: it's possible the new MAF is fine, just more sensitive to a problem elsewhere... which is causing an airflow pattern that the ECU thinks is implausible. The junkyard ECU could be dulled by years of dirt/oiled filter etc and so not as sensitive to an actual issue.

Only mention this as a suggestion to look beyond the difference in the two MAFs when tracking this one down.

Could just be one smoke detector is smarter than the other... and the fire is elsewhere.

Vince Waldon said:

Don't have a specific answer, just a general caution, FWIW: "implausible MAF" most often means "the airflow is not what I'm expecting" as opposed to "the MAF is putting out a bad signal".
Meaning: it's possible the new MAF is fine, just more sensitive to a problem elsewhere...

Click to expand...

S/A what Ross-Tech indicated:
Excessive restrictions in the Intake/Exhaust System
Dirty air filter, clogged snow screen, clogged CAT

Truck240, Did you checked if the intake manifold if clogged?

I've done the basic troubleshooting. I've replaced all the intercooler seals. I installed a new air filter, fuel filter, took down pipe off and completely punched out the cat converter with a metal bar. I examined all the vacuum lines. The car runs great with the pick n pull MAF! It has great power, revs quickly, good mileage and most importantly no codes. The only issue is a infrequent hiccup or shudder while in 5th gear and cruising. I'm stumped as too why a not just one but two new MAF sensors throw a code and performs worse. I would think TDI hot film sensors should behave the same.

Try driving with the MAF unplugged.
If it runs as good as the PnP one then the PnP one may not be correct for a diesel.

A local parts store here lists the same non Bosch MAF for gas & diesel.
Rock Auto lists 3 different ones (2 different gas engines)

wonneber said:

Try driving with the MAF unplugged.
If it runs as good as the PnP one then the PnP one may not be correct for a diesel.

A local parts store here lists the same non Bosch MAF for gas & diesel.
Rock Auto lists 3 different ones (2 different gas engines)

Click to expand...

The PnP sensor is a OEM Bosch from a 2003 Jetta TDI. The car is running great with it installed! I hooked up the Xtool and the PnP Maf reads well over 900 m/g. When I install the new NAPA MAF the car hiccups and throws code 16485 code. The NAPA MAF is in a TDI housing but I suspect the sensor itself is not a Bosch but a cheap substitute. I've read stories of problems with non OEM or genuine Bosch sensors but always thought it was nonsense.

When it comes to buying a mass air flow sensor, nothing but a Bosch oem makes more sense.

Camshaft has a worn lobe.
New maf sensor is picking up the variations between cylinders pulses better.
Misfires under part load are very common with a partially worn lobe on the camshaft. The lifter may not be worn through yet but it will be shortly.

Could very well be a worn cam lobe or two, check your cam.

Also the lower turbo piping, the piece that plugs into the turbo itself. It a rubber hose with two metal ends that have the nubs that snap together. Check the nubs on the end closest to the turbo. I just found that I had one nub worn through that was allowing a slight boost leak. That could be the cause of your occasional black smoke.