Testing/replacing the mk4 and mk5 MAF
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Introduction
The mk4 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" is slightly different than "hot wire" MAFs found on many other cars. Instead of measuring how much electrical current is required to maintain the hot wire's temperature (an indicator of air flow), the VW 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 will deal with testing the MAF.
The mk4 jetta's MAF is outlined in yellow in the picture below. All VW
MAFs are located immediately downstream of the airbox.
The MAF commonly fails in the MK4 cars. It can produce a sudden reduction in car power but the normal failure is gradual. 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.
The solution is to replace the faulty MAF. This can occur around 40,000-100,000+ miles. It is covered by an extended warranty for 7 years or 70,000 miles for pre-2002 cars. Also note that 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 have to re-oil the aftermarket 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. Due to the MAF design, the mk4 TDIs are very sensitive to contamination and oil, and for this reason alone, oiled aftermarket air filters are not recommended. In fact, the stock TDI air filter is the same one used on the VW 1.8T, vr6, and even the 240hp R32, so there will be little to no power gains from not using the stock air filter setup for almost all TDIs. Why do they do this? Because it's cheaper to make 1 airbox and 1 air filter instead of 10 different parts. (They also sell a cold weather air filter so technically it's 2 parts)
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.
Parts (click links to compare current prices)
VAG-COM (optional but recommended for testing)
MAF removal tool http://www.metalnerd.com/cat05.htm
(if you decide to remove the MAF), a dremel can be substituted to cut slots in
the screw
MAF : check with your parts supplier to see if there is a new
part number
ALH engine: VW# 038 906 461c
From
worldimpex , from
mjmautohaus , from
tdiparts
BEW engine: VW# 038 906 461b
From
tdiparts , from
mjmautohaus , from
worldimpex
Old part numbers for the ALH engine: 06A 906 461 discontinued MAF, 06A 906 461X
discontinued remanufactured MAF - note the "x" suffix.
Procedure
Testing the MAF
The first test you can do is to unplug the MAF electrical plug. Do not remove the MAF from it's housing, just unplug the plug connector. This sets the ECU to replace the faulty signal with a default value. If the car immediately runs better and has more power or is unchanged, the problem is most likely the MAF. Do more tests or replace the MAF. 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 for a test drive. Note that this will set a check engine light, it will go away after you reconnect the MAF and start the car a few times.
Another test you can do 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.
The next test should be with a VAG-COM cable.
Go into engine measuring block 003 and start datalogging.
In a high gear 4th or 5th gear, give full throttle from 2500 rpm up to 3500rpm.
It would be best to find a hill so that you can load the engine and not exceed
the speed limit, of course! Also give your full attention to safely
driving the car and not datalogging, have a helper take care of that!
Save the the datalog file where you can find it. Open the file in windows explorer. Graph MAF actual vs rpm in excel. The MAF is okay if the actual value is at least 850-900 mg/h at 3000 rpm. If the power falls off as you pass 3000 rpm and the reading from the MAF falls off, this is normally an indicator that the MAF is faulty.
A last test you can do is to measure the voltage of the MAF from pin 5 on the MAF connector. There should be 0-0.3 volts key on engine off, 0.7-1.2 volts at engine idle and 1.5-2 volts at 3000 rpm. However, note that a weak MAF can pass this test.
You can try cleaning the MAF if it fails these tests, but cleaning a hot film type MAF is normally not a permanent solution. It will often work with different MAF types, but it couldn't hurt to try. Remove the MAF, spray with MAF safe cleaner spray and replace.
Replacing the MAF
First, check the prices on MAFs. A rebuilt MAF should be just as good as a new MAF because they completely replace the MAF element. A rebuilt MAF will only reuse the old plastic housing.
Unplug the electrical connector and use the 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.
Installation is the reverse of removal. Here is a picture of the security screws replaced with phillips screws.
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