The mk4 and newer MAF is a "hot film" sensor immediately downstream of the air filter box that senses how much air passes through the sensor. The hot film sensor on your VW is slightly different than "hot wire" MAFs found on many other cars. A hot wire MAF measures resistance to an electrical current required to maintain the hot wire's temperature (an indicator of air flow). Your VW hot film MAF measures the difference in temperature across an electronic membrane's upstream and downstream sides. See 1000q: MAF FAQ for more details and technical information about the MAF, this page is focused on testing and replacing the MAF.
The mk4 Jetta New Beetle / Golf MAF is outlined in yellow in the picture below. If you
buy a new MAF element only it's just the part outlined in yellow, if you buy the
housing it's also the hard plastic cylinder. Prices vary so click the
links below to compare pricing. All VW
MAFs are located immediately downstream of the air intake box.
The mk5/mk6 MAF sensor location is shown below. It's also downstream of the air intake box.
The MAF is a common failure part on mk4 VW TDI Jetta, New Beetle, and Golf. It might produce a sudden reduction in car power but the normal failure mode is a gradual loss of power over weeks. If power is restored by shutting the car off and turning it back on, you are in limp mode and the MAF is probably not the main contributor to the power loss. The MAF could be slowly failing and contributing to power loss but if you suddenly go into limp mode, you have other problems. See 1000q: limp mode for more details on diagnosing limp mode. If you have constant low power it could be the MAF, see 1000q: constant low power or low revs for other causes.
The solution is to replace the faulty MAF. It is covered by an extended warranty for 7 years or 70,000 miles for pre-2002 cars. If you over oil an oiled-type aftermarket air filter, excess oil and dirt will get onto the hot film and damage the MAF. If you re-oil any aftermarket oiled air filter, just put a very thin bead at the top and let it migrate throughout the filter because any more will probably damage the MAF. The mk4 TDIs seem to be very sensitive to contamination and oil due to the MAF design.
For this reason alone, oiled aftermarket air filters are not recommended. In fact, the stock TDI air filter is the same exact filter used on VW's 1.8T, vr6, and even the 240hp R32. Dyno tests show that there are little-no power gains from non-stock air filters. Only the 90% percentile of the most highly modified TDI would probably see a noticeable benefit. Why do they use a filter with so much excess capacity? Because it's cheaper to make 1 airbox and 1 air filter for all their cars instead of 10 different parts. (They also sell a cold weather air filter so technically it's 2 filters). Side note: the R32 Golf does use a larger air box and cold air intake snorkel but it uses the same exact filter.
Lastly, a lack of power could also be caused other things so make sure that the engine air intake filter, snowscreen, and intake manifold are relatively clean and operating well. See 1000q: intake and snowscreen cleaning and 1000q: intake manifold cleaning or 1000q: intake manifold cleaning for pumpe duse for more details on these procedures.
VW has gone through a number of MAF part numbers, the latest part number for
ALH TDI engines ends
in "c". The housing is very different but it's not known if it's
more robust than earlier designs because the element is the part that fails.
The newer mk5 BRM engine MAFs look similar but have a different part number.
Parts (click links to compare current prices)
Ross tech VCDS cable (optional but strongly recommended for testing, used to
be called vag-com)
MAF element only removal tool from kermatdi or http://www.metalnerd.com/cat05.htm (if you decide to remove the MAF element), a dremel can also be used to cut slots in the screw
1 MAF sensor: check with your parts supplier to see if there is a new part number
ALH engine mk4 Jetta, Golf, New Beetle 1998-2003: VW# 038 906 461 c (newest part number)
from kermatdi(check part numbers)
Old part numbers for the ALH engine: 06A 906 461, 06A 906 461X (remanufactured - note the "x" suffix)
BEW and BRM engine mk4-mk5 Jetta, Golf, New Beetle 2004-2006: VW# 038 906 461 b, Bosch#0 281 002 531
from mjmautohaus from kermatdi
Note - if your car has an aftermarket chip tune, the MAF requested by the car's computer may be off because the chip tuner changed it.
If you have really low power the first test is to unplug the MAF electrical plug. Do not remove the MAF from its housing, just unplug the plug connector. The ECU will then replace the faulty signal with a default value. If the car has more power or is unchanged, the MAF is definitely bad. If power is significantly worse, the MAF is good or marginal.
You can also try driving around with the MAF plug unplugged to see if it makes a difference. Most gasoline cars will run horribly with the MAF unplugged but a diesel car will run OK but slow for a test drive. It may set a check engine light which should go away after you reconnect the MAF and start the car a few times or can be cleared right away with a VCDS.
Another test is to warm up the car to normal operating temperature and put the transmission in neutral. Rev the engine up to redline in neutral. If the MAF is good, it should go all the way up to the rev limiter at 5100 rpm, if the MAF is bad, it will definitely not rev that high. However, a marginal MAF can also rev that high so to be sure, do more testing with a VCDS cable. The point of doing these tests first is because they will reveal a really bad MAF.
The most reliable and accurate method of checking the MAF is with a VCDS cable.
This will show a MAF working at only 85%. The above tests will only show a major
MAF problem. Through VCDS, check
MAF actual vs. specified (by the car's computer) at idle, high rpm, and high load.
A low MAF actual could also be from servere intake clogging or possibly a
malfunctioning EGR recycling too much EGR gas from the exhaust. Start the engine, plug
in the cable, and
click on "engine", then click on "meas. blocks" in the
screen below. Mk5 and mk6 cars are slightly different but the same test applies.
You should see the screen below. Hit
"up" until you see "group 3". Below is an example of a
bad MAF (engine is at full throttle) on a mk4 ALH TDI engine - the
specified value and actual values are way off. Bad MAFs don't go over 400-500
because the signal degrades. Also check for faulty
wiring or a bad plug.
You can also click "log" to view a graph. Below is a good MAF - the specified value and actual value are close and both
can reach 800+. Above 3000 RPM, it should reach about 850. Some
spikes at the beginning of each test run are normal due to turbo boost spikes.
Now do some tests while driving - have a helper operate the computer while you concentrate on driving, don't get distracted while driving or else it could result in a crash! It would be best to find a hill so that you can load the engine and not exceed the speed limit, of course!
In 2nd or 3rd gear, start at a low rpm and give heavy acceleration to a high rpm near redline. Note the MAF actual vs. MAF requested. You can also do this test in 4th and 5th gear. What you want to see is MAF actual and MAF requested match or be almost an exact match. This means that the sensor is delivering what the car's computer expects. If they are off or MAF actual plateaus then the MAF sensor is bad - replace it. A good ALH MAF sensor should be able to read up to around 850-900 mg/h. I'm not sure the BEW sensor range but it should be about the same. The BRM MAF sensor should go a little higher because the engine has more power. The CBEA and CJAA 2.0L TDI engine MAF sensor should go even higher due to power.
Below is an example of the MAF reading on a BRM engine.
Again, it's possible for an EGR malfunction or clogged intake to displace or restrict air. Temporarily blocking off the EGR with a block off gasket will set an error code but it will let you know 100% if the gasses are causing the low reading. Test the EGR valve function before doing this test since if the valve works it's probably not the problem.
While cleaning a bad MAF with MAF cleaner spray from the local auto parts store won't hurt anything, cleaning a hot film MAF normally doesn't fix it. Just make sure to use a no residue cleaner. It sometimes works with hot wire sensors but the MAF used in your TDI uses a more advanced and more sensitive hot film. It's worth a try.
First, check the prices on MAFs. If available, a rebuilt MAF is just as good as a new MAF because they completely replace the MAF element. A rebuilt MAF will only reuse the old plastic housing. If you purchased the element with a housing then just loosen the spring clamps and phillips screws, remove, and replace. When replacing the housing with the element already in it, you do not need to purchase the security bit MAF removal tool.
If you purchased just the element, you need a bosch MAF removal bit (linked above) to remove the security screws. You can also use a dremel to cut slots into the screw heads and use a screwdriver to remove them. Harbor Freight also sells a set of security/special bits that should include a special torx bit to fit in the special screw.
Installation is the reverse of removal. Here is a picture of the
security screws replaced with phillips screws.
This article only shows the data logs from a ALH engine. If you have logs from a BEW engine or need more advice on testing the MAF, please post in the myturbodiesel.com forums or search below: