The mass airflow meter (often abbreviated to MAF) uses a platinum wire to measure the amount of air flow that enters the engine. At the heart of the MAF is a very fine (0.07mm diameter) platinum wire that’s heated to 100 degrees Celsius above ambient air temperature. As incoming air flows past it the wire cools, and its resistance alters. Circuits within the MAF increase the current through the wire to maintain the constant 100-degree differential. The output voltage from the MAF varies with this current, and according to the flow of air. This in turn provides information to the ECU that is used to ensure that the correct amount of petrol is fed to the injectors to give the correct air/fuel mixture. It is a very sensitive instrument, especially in the lower volume range. The instrument not only senses the amount of cubic meters entering the engine, but it also has a temperature sensor which registers the air temperature and this density which relates to the amount of oxygen available. The LH senses the output of the MAF and adjusts fuel based on this airflow value, rpm, and engine temperature.
The MAF is used in the 928 since the LH jetronic was introduced, which is basically the 1984 model. The MAF did not change during the years up to 1994 but in the 1987 and later models the MAF CO adjustment screw was not used anymore (not even wired). The MAF remained the same. Part number is 928 606 141 00, Bosch number 0 280 214 001
What can go wrong with then MAF?
The thin platinum wire may get contaminated with oil vapor from the engine, dirt that passed the filter or oil that was used in a KN filter. This would cause erratic output and the LH will respond by adjusting fuel in a wrong way. Even though a lambda sensor will cover a part of the obviously wrong mixture, it can only do so much and the fuel will be either way too lean or to rich. To correct this contamination, Bosch designed a burn off feature. This heats up the wire to glowing hot for one seconds. It happens a few seconds after the engine was over 60C and then (later) turned off.
The platinum wire does however deteriorate over time and the burn-off does not do it any good. The material changes characteristics, causing the MAF to change output related to flow. An error of about 5% makes the car hard to drive, cause fuel economy problems, unable to pick up fuel quickly, stalling, bad idle, black smoke etc etc.
A MAF failure is often a gradual degrading of performance, often first noticeable with "flat spots" causing hesitation during acceleration. Real serious trouble with the MAF can also get logged in the LH error memory which a diagnostic tool can read and evaluate.
If a MAF is really bad or even completely defective (which does happen) you can unplug the cable, and use the limp-mode feature that is inside the LH. It sees a not working MAF when rpm exists and a flow value is not existing. That can't be good, and it enters the limp-mode program which makes it fuel 3,5ms for rpm under 2000, and 6,3ms for over 2000 rpm. This helps to start the engine, and while gently using the throttle you can drive home. It works, although putting the automatics in gear and trying to move off can be pretty hard (the engine tends to stall due to the sudden load).
To check the MAF you could measure the output while power is switched on but the engine not started. You should see about 1,7volt on pin #5 of the MAF and ground. Measure this at the LH connector. Anything like 5v or 0v is obviously wrong. But this does not say that the MAF is ok, a 5% off will cause problems as well.
You can also look for the operation of the burn-off : the car must be at normal operating
temperature. Take off the upper air cleaner casing and remove the air filter.
Start the engine, and take the revs above 2000rpm for a few seconds. Then switch
off the ignition and observe the hot wire through the air filter housing. 4
seconds after ignition switch off, the hot wire should glow an red/orange color
for about 1 second.
I test the MAF on a test facility that I built. It tests all functions of the MAF and also simulates a 5.4L V8 engine at 6000 rpm. Imagine the air flow :) When the MAF is diagnosed, we can replace the internal electronics which can't be properly repaired (old design hybrid pcb) with our own design which can be adjusted to match the platinum sensor wire characteristics and thus tune it back into proper output. We also replace a sensor wire if needed, the internal thermo sensor and a measuring resistor. After a repair the MAF performs like new. To verify a repaired MAF is tested in my GTS so that I know it feels right also under harsh engine conditions.
regards
Theo
1992 928GTS midnight blue