MAF or AMM
Mass Air Flow or Air Mass Meter are two descriptions of the same thing.
The AMM is a sensor that measures the Mass of Air Flowing through the intake pipe from the Air cleaner.
It's function is to determine the amount of Exhaust Gas Recirculation.
(It is simpler to think of it as two temperature sensors. One is kept at a constant temperature and the other is allowed to be cooled by the air flow. The difference between the two temperatures is used to calculate the mass of air.)
A faulty AMM may or may not flag a fault code. (depends on the fault).
A faulty AMM will feel like the turbocharger has stopped working.
Most modern AMM use a thin film design. This is a thin plate with resistors printed onto it.
Flat film AMM does not get contaminated but can get dirty.
Flat film AMM shouldn't need cleaning other than a wipe with a cloth.
Flat film AMM can measure reverse air flow.
How to spot a faulty AMM
The first clue to a faulty AMM is poor response from the engine, similar to limp mode. It may feel like the turbocharger is not working.
This is because the ECU has detected a problem with the AMM signal.
If the ECU has detected a faulty signal it will flag a fault code.
This will be;
Voltage too high,
Voltage too low or
Short to plus.
The ECU contains an inbuilt map for vehicle speed and estimated Air Mass flow so the car reverts to this when an AMM fault is spotted. This map doesn’t respond well to acceleration or detect when you are going up hill so the car will feel under powered.
If you switch the engine off and then on and things improve…..your fault is NOT likely to be the AMM
If the AMM is faulty, but giving the wrong reading, the ECU can’t tell and no fault code will be set.
The car will just run according to the Air Mass information and feel under powered.
What to do.
If a fault code says AMM faulty, it probably is so replace it.
If you don’t have a fault code, but suspect the AMM. Drive a couple of miles, return, unplug the AMM and drive the same route again.
If the two journeys seem the same, the AMM is probably faulty.
*Note: After unplugging the AMM you will have a fault code.
The best way to check for a faulty AMM is to use VCDS (vag-com).
Connect VCDS start engine.
Go to ENGINE controller.
Click measuring blocks.
Go to 003
You will see something like this;
YOU CANNOT DIAGNOSE A FAULTY AMM (MAF) FROM A SINGLE RESULT LIKE BELOW. YOU MUST DO A LOG OF 003.
The four 003 information boxes will have Numbers in them.
Note: EGR figures are the same thing as MAF figures so your screen may say MAF or EGR.
Typical values for the EGR (MAF) are:
The EGR actual figures are often higher than the specified figures. This is normal.
1. The EGR (specified) and EGR (actual) readings should be similar at any given engine speed.
2. At idle most diesels give a figure in the range of 200-400 mg/s so a figure much less than 200 or much more than 400 suggests a fault.
* Don't leave to idle too long. If you do the egr valve will shut and the value will be higher.
3. At speed (3200 rpm plus) most diesels give a figure in the range 700 -1200 mg/s so a figure much less than 700 is not good.
A constant " too hig" value suggests the EGR valve is stuck shut.
4. You can rev the engine and watch the two MAF figures go up and down. As long as they go up and down and stay similar, the AMM is probably OK.
5. A more accurate test is to go for a drive and graph the data
Make sure the engine is hot and make a log as you accelerate from idle to about 4000 rpm.
A graph should look something like this.
You can see that the EGR specified is just below 400 mg/stroke at idle and rises to 840 mg/stroke above 1300 rpm.
The actual EGR (MAF) spikes above EGR specified and then settles nearer to that for EGR specified.
A constantly over reading EGR (MAF) value may be due to a faulty AMM (MAF) or the EGR valve is stuck shut.
A constantly under reading EGR (MAF) value may be due to a faulty AMM (MAF) or the EGR valve is sticking open..
6. A failed AMM will give a fairly constant reading in the region of 470 mg/stroke regardless of engine revs because the engine ecu is using an inbuilt map.
A faulty AMM should be replaced with a genuine one from a VW Dealer to avoid the dubious copy ones that are on sale. (you also get a 2 year guarantee)