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Old 12-20-2004, 03:00 PM   #1
bobporsche996
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Default MAS.. Airflow Sensor? ok to drive?

I've been busy and haven't had a chance to bring the car into the shop yet to have the mas replaced... my question is, is there any danger in driving the vehicle when the mas airflow sensor needs replacement? It's nice that my extended warranty just kicked in to pay for it.. regardless.. just curious as if it would cause damage to my new engine when driving the vehicle to the dealer with this part bad... with my luck and engines, i'm not taking any chances.. <g>
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Old 12-20-2004, 03:06 PM   #2
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No problem driving the car with a bad MAF.
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Old 12-20-2004, 03:21 PM   #3
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what exactly does the mass airflow sensor do on the porsche engine? does it simply protect the engine from future damage from a more endangering incident?
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Old 12-20-2004, 03:38 PM   #4
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It simply reports to the computer the amount of air that is entering the intake. Based on the fuel map of the computer, the injectors are iinstructed as to how much fuel to use based on the voltage from the MAF. If the O2 sensors see that the amount of fuel was too great or too little, they will instruct the computer to adjust the fuel injection accordingly.
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Old 12-20-2004, 06:05 PM   #5
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I think there is a problem driving with a bad MAF. The system goes rich and I belive it will cause fuel dilution of the oil. That is what occured when mine when bad. I think that ecessive fuel dilution could cause engine failure! YMMV
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Old 12-20-2004, 07:47 PM   #6
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If your MAF is not working properly, your O2 sensors are deigned to correct the error. If you are worried about it, disconnect the cable from the MAF and you will be fine until you get a new one.
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Old 12-21-2004, 09:37 AM   #7
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"No problem driving the car with a bad MAF."

Not True!

If the MAFS is bad causing either a too rich or a too lean condition, engine damage
may occur, e.g. burnt valves, damaged CAT or O2 sensors. The DME unit can only
correct so much for out-of-spec DME sensors, e.g. MAFS. Get the problem fixed
and avoid driving the car.
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Old 12-21-2004, 01:47 PM   #8
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hahah disconnect it! that's funny but not. Get some electrical contact cleaner that is plastic friendly and doesn't leave a residue, you can find it at fry's radio shack doesn't have the right one, although I was surprised to find out that they knew that it would eat plastic which was a nice info for waiver. It's easy to take out just a security torx. I got a set from fry's for 24 bucks if that, it has the driver and all security bits, I actually end up using it often-
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Old 12-21-2004, 01:57 PM   #9
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I will repeat again that disconnecting a bad MAF will NOT effect the operation of the car. If it does, then the MAF is not bad. The O2 sensors will manage the air/fuel ratio enough until you get the new sensor. There is a default level in the DME when the MAF is disconnected which wil keep the O2 sensors from getting out a range. It is not something to do long term. But if in doubt, just keep in connected and drive until you get the new one.

If you want to prove it, connect your scanner and take a reading of the secondary O2 sensors before and after the MAF is disconnected.
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Old 12-21-2004, 01:59 PM   #10
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Yeah if you want to change the loop mode i.e. closed v. open. Terrible
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Old 12-21-2004, 08:42 PM   #11
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Hard to imagine that a missing (disconnected) air flow sensor would even allow the car to operate. Of all the sensors used in a Motronic system, Air Flow would have to be the most important variable to determine the fuel flow. The ECU maps typically use MAF and engine speed as the x and y axis variables. While the ECU could hold the last good sensor value after failing, this would not be sufficient for a missing/bad MAF. It may be that the engine control software substitues the throttle position and manifold pressure measurements as a backup to estimate the air flow, (The Motronic book I have stops a bit short on these proprietary details). It would seem that operating the engine with a "bad" MAF must degrade performance/emmissions/operations.

I am currently suspecting my MAF due to various O2 sensor related codes I have seen from both banks. In my case I don't think the ECU has marked the MAF as a bad sensor as no related MAF codes (OBD) have appeared. Never-the-less I am concerned that the MAF is borderline bad. The cold weather seems to have cured my CEL codes for the moment, but I will try the dielectric grease for the connector and may try to clean the flowmeter sensing wires. I don't really know the likely impact of driving with the MAF disconnected, but my instincts tell me it has the potential to do some harm. I would rather someone else collect the test data on this one. Without more OBD/PST data, and assuming the engine behavior is normal, I would keep the MAF connected but have it checked out ASAP.
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Old 12-21-2004, 08:57 PM   #12
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Well, obviously if your car is not displaying any problems, why would you disconnect the MAF? The point I was making is that no harm will come to the engine by driving with a bad MAF or driving without the MAF connected for a limited period of time. I drove to NC from Chicago without a MAF last year. Other than some difficulty on cold starts, no problems. If you look at the response of the O2 sensors, they immediately correct for the missing MAF function as the Motronic DME has a fail safe setting when no info is being fed back from the MAF. In turn, the LTFT is adjsuted and before you know it, the Fuel/Air micture is right where it should be. Much like the computer's mapping specs do at WOT. This preset assumes standard airflow into the engine. It comes pretty close to matching what is needed in real life. The O2 sensors make up the difference. This is not an assumption, but a fact. The MAF reading fine tunes the fuel/air mixture only.

Some people make the MAF out to be some mysterious componant that no one can understand when it is such a simple device. So I guess for those people, if their MAF goes bad out in the middle of nowhere, they would just sit in their cars and wait for someone to come by and tow the car?

If you want to test your MAF, disconnect the cable to it while the engine is running. If the engine struggles and then adjusts, (which means that the fail safe setting takes over) the problem is probably not the MAF. If the performance of the engine does not change, then I would suspect the MAF.

What O2 sensor error codes are you getting, anyway?

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Old 12-21-2004, 11:27 PM   #13
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I found your post quite interesting as I struggle to understand the details of Motronic Fuel Injection without the right documentation. What I would like to know is what are the algorithms involved or at least what are the thresholds for each code that sets the CEL. Not sure if there is any such published manuals. I have some manuals for Bosch Motronic and for Bosch sensors. They are both good for the concepts but short on specific details.

My understanding of the Motronic software is: it takes Air Flow and computes the fuel flow proportionally. Other measurements are important but none more so than the MAF. The O2 sensors allow a slower but very precise feedback trim. There must be some clever fallback way the ECU computes the fuel amount for the car to function so well as you describe with the MAF disconnected, as even small errors in Air/Fuel ratio affect driveability.

The codes I have seen are:
P1118 O2 sensor related (3 times)
P1126 cyl 4-6 rich adapt threshold
P0159 aging O2 sensor(2 times)
P1128 cyl 1-3 rich adapt threshold
P1130 cyl 4-6 rich adapt threshold

At first I suspected the O2 sensor, so I checked for vacuum leaks and checked the O2 sensor connectors, but otherwize did nothing. I read that common problems might be air/oil separator bellows or bad MAF. When I saw the P1128, I concluded that my O2 sensors were OK as now both banks are seeing the problem and my source issue is unmetered air, as I am always adapting on the rich side. My OBD setup showed the O2 sensors oscillating normally (from too rich to too lean in a square wave etc).The CEL has been out now for some 500-600miles in the cold weather. My plan is to clean the MAF sensor wires, and the connectors. I have heard the sensor wires are sensitive to dirt in some MAF designs. If I see the CEL again for a related code, then I will buy the MAF.

I assume disconnecting the MAF would quickly give you a code for low MAF or maybe implausible MAF reading. Maybe you have some experience with how the Motronic/MAF interacts with the pressurized air from the supercharger on your 1999 C2.
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Old 12-22-2004, 12:15 AM   #14
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You are correct to assume the problem is not with the O2 sensors since you appear to have a problem on both banks. You may also never see the CEL come back on again. (fingers crossed) Many times a fault may occur because of legit reasons but correct itself.

The codes indicate that the car is running extremely LEAN. Could be caused by many things. Vacuum leaks would cause the engine to run RICH. Check that there is no air leaking into the intake AFTER the MAF. Check the MAF boot for tightness as well as the intake tubes connection to the throttle body. Make sure that there are no exhaust leaks.



You need to assume, since it is both banks, that there is a common fault condition that is making the computer think it is getting less air than it actually is. Short of a bad MAF, the possibilities inclue an air intake leak or weak fuel pressure. fouled injectors or a leak in the exhaust system. It is possible, but unlikely, that an injector or injectors on both banks are dirty causing too little fuel to be injected. However, you may want to add some injection cleaner to the gas tank as well as clean the throttle body with carb cleaner.
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Old 12-22-2004, 12:47 PM   #15
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"What I would like to know is what are the algorithms involved or at least what are the thresholds for each code that sets the CEL."

To determine fuel metering, fuel injection systems need to know the engine load
value. This is determined by values provided by the MAFS and the TPS sensor
on later engines. Simple fuel injection systems (alpha-N) use only a TPS, e.g. motorcycles,
to provide a load value. On fuel injection systems with both a TPS & MAFS, the system
can run with either malfunctioning/disconnected. Also, most fuel injection systems
store MAFS values based on RPM to use if the MAFS fails, or is disconnected.

To analyze your 996 problem, you need to monitor the TRA & FRA values which are
used to correct for AFR out of range, e.g. TRA - intake air leaks, TRA/FRA - high/low
fuel pressure. These variables provide good insights into many fuel injection problems.
Most good OBDII scanners provides these Porsche DME values. These variables (TRA/FRA)
provide the AFR adaption which relate to your fault codes.

The 996 fuel injection system has few problems with the exception of the MAFS which has
a fairly high failure rate. Obviously, the O2 sensors are problematic areas as they age,
the result of OBDII monitoring.

It's possible your MAFS may be intermittent. So you need log some MAFS values
with the engine running O.K. without a CEL at various RPMs. This can be used later
if the MAFS has a problem.

Last edited by Lorenfb; 12-22-2004 at 01:10 PM.
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Old 12-22-2004, 12:47 PM
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