What is an out-of-calibration Mass Air Flow Sensor (MAF)

Why is MAF service needed?
Rich Andrade. www.electronkrepair.com EMAIL: electronikrepair@earthlink.net

Electronik Repair provides no-charge MAF testing to 928 OC members, you just pay the shipping!  Just mention your membership when you contact me. All 1985 - 1995 Porsche 928s have a hot-wire MAF.  Euro/ROW 1984 models also have a hot-wire MAF.

I know many Porsche 928 owners like to understand the technology in their great cars (I own three Porsche 928s), so I’ll provide you the details of the problem.  

I have a customer who owns two Porsches, a 928 and 968.  His 928 MAF was faulty, the car wouldn’t run so I recalibrated it and he was very pleased.  I later told him likely his Porsche 968 MAF was also out of calibration.  He had one MAF where the engine was running so badly he replaced it with a used one and he sent me the bad one for recalibration.  I found this MAF to be quite bad, -130 mV [over 4x the acceptable limit] out of calibration.  The negative in the voltage means it is running lean and the highest value where the engine will run “OK” is about -30 mV.   When the rebuilt MAF was installed, he was very happy with how the engine was more responsive and smoother running.  I recently recalibrated his “back-up” unit and found it was -40 mV out, not quite as bad but still past the acceptable limit.  Under inspection I also found an issue with the hot wire that could have resulted in a much larger change in the future.  He was quite surprised, as he thought the engine was running OK with this MAF.  

A friend owns a 1987 928 S4.  His Porsche had the original MAF and he said it was running OK.  He accepted my invitation to check the calibration, I found it was -66 mV out of calibration at idle and -90 mV at high throttle.  This is proof of the of the amount of drift over 39 years!  

I’ll explain how the engine can run “OK” when the MAF is “slightly” out of calibration.  These Porsches (and other European cars of this generation) have a “Lambda” circuit (often represented by the Greek letter λ) to fine tune the air/fuel ratio.   This circuit uses an O2 sensor to measure if the exhaust air/fuel mixture is lean or rich of the desired 14.7 : 1 stoichiometric air to fuel ratio.  If the O2 sensor reports a lean condition the fuel injection module incrementally enrichens the mixture by incrementally lengthening the “open time” of the fuel injector until the O2 sensor reports a rich condition then it incrementally decreases the “open time” of fuel injector.  The λ fuel injection strategy is for the air fuel ratio to constantly be “hunting” back and forth to achieve an “average” of 14.7 : 1.   

The engine λ system only operates under two conditions, when the engine has warmed up and when the throttle position is ~ 60% or less.  This is called “closed loop” because the O2 sensor is providing the necessary lean-rich reporting of air/fuel ratio.

Why doesn’t the λ system operate all the time?  When the engine is cold, it cannot run at 14.7 : 1 as combustion is incomplete at colder engine block temperatures, more fuel is needed.  So, aided by the coolant temperature sensor, more fuel is injected and slowly decreased as the engine warms up.  [In a carbureted engine, a “choke” is used to manually enrichen the fuel ratio].  When the throttle is more than ~ 60% more fuel is needed for smooth and higher RPM acceleration.  The fuel injection system uses a throttle position sensor to inform the module to go into this “acceleration mode.”  The digital computer in the fuel injection system uses a 3-D “MAP” to inject the proper amount of fuel.  This is called “open loop” operation.

When the MAF is out of calibration, the incorrect amount of air is reported to be entering the engine so the improper amount of fuel is injected.  Customers often report hesitation or jerky operation in open loop operation with an out-of-calibration MAF.  At some point as the out of calibration status gets worse, the engine just won’t run and goes into “limp-home” operation where the engine will barely run.

The λ system can only compensate so much for an out-of-calibration MAF during closed loop operation.  MAFs normally drift to producing a lean air/fuel ratio which means the engine produces less power.  It is a slow drift, so the change isn’t noticed. However, when a recalibrated MAF is installed, many of my customers report “wow, it feels like a new car, it is so responsive” because the proper amount of fuel is being injected without extreme management by the λ system. 

For this reason, I recommend all owners of hot wire MAF equipped cars to send us the MAFs that we service for recalibration.  EMAIL your Bosch part number and we’ll advise if we can service it. It isn’t an “if” your MAF is out of calibration, it is a “how much” is it out of calibration.  

So why are hot wire MAFs drifting out of calibration?  It is in the design of the Bosch hot wire system.  If you were able to purchase a new Bosch MAF, it still would drift out of calibration.  We’ve found MAFs that were previously recalibrated have also drifted past the acceptable limit.  We’ve determined that this issue affects all the hot wire MAFs of this product generation.  We’ve discovered the root cause of this issue and it is corrected as part of our recalibration process.  Our units will likely last longer than the remaining life of your Porsche once repaired!

Some MAFs have not only calibration failures; there is a bridge circuit and a power transistor inside the module that can fail.  If we find problems with these items, they are also replaced so your serviced MAF is as good (well, better) than new!  Our MAF service is a rebuild service.  Contact us to schedule testing the MAF from your Porsche 928 or Porsche 968.

Caution: NEVER USE MAF CLEANER spray on a hot-wire MAF.  It isn’t needed as a burn-off cycle at the end of each drive cleans the wire.  Secondly, the spray stream can break the wire and the spray damages the plastic cap to the point where it cannot be reused after a MAF service.