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>

No problem driving the car with a bad MAF.

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?

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.

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

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.

"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.

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-

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.

Yeah if you want to change the loop mode i.e. closed v. open. Terrible

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.

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? .

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.

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.

"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.

I'd guess the most common cause of running lean is a clogged fuel filter.

Maybe we are stealing Bob's original thread here, but the topic is still Mass Air Flow Sensors and their replacement. Some good ideas, but they raise more questions. My OBD setup records: FUELSYS12 (OL, OL; CL, CL) open or closed loop mode for O2 feedback LOAD_PCT % Calculated Engine Load ECT F (Engine Coolant Temperature) SHRTFT1 % Short Term Fuel Trim Bank 1 LONGFT1 % Long Term Fuel Trim Bank 1 SHRTFT2 % Short Term Fuel Trim Bank 2 LONGFT2 % Long Term Fuel Trim Bank 2 RPM rpm VSS mph (vehicle speed) SPARKADV deg IAT F (Inlet Air Temp) MAF #/min (Mass Air Flow) TP % (Throttle Position) AIR ON/OFF secondary air pump O2SLOC SEE/? O2S11 SHRTFT11 V,% O2 sensor voltage and associated short term trim O2S12 V O2 sensor voltage (downstream of the CAT) O2S21 SHRTFT21 V,% % O2 sensor voltage and associated short term trim O2S22 V O2 sensor voltage (downstream of the CAT) Most key variables are measured and recorded in a log file. My previous log files give me some background data, but most is at idle. Only a few short test drives with the laptop connected. Not sure what TRA or FRA parameters are, but I cannot see them with my set up. Aha!!, now that I have written all these parameters out I think my log files contain a ton of data that I never even looked at. I can see the Fuel trim corrections - they are described as "% decrease rich correction" and while the short term numbers vary +/- 6% or so they are on average negative. Long term corrections are virtually identical for both banks (-.78%). I read this as a negative decrease or an increase in the rich correction. It seems like the Motronic must make separate Bank 1 and Bank 2 corrections (learn something every day). While some of the short term corrections do seem larger on one bank, and the average O2 sensor voltages are a bit higher on one Bank, I still suspect the MAFS. It seems to me that Bosch has been struggling to find a cheap, fast responding, reliable, and accurate flowmeter for electronically fuel injected cars since the early days of L-Jetronic. The current generation MAFS is Hot wire or the similiar Hot film types (I think my C4 has the film type). They use a platinum wire or film that is heated via current, and cooled by flowing air. Platinum is routinely used for temperaure sensors (RTD's or Resistance Temperature Detectors), because it's resistance changes predictibly with temperature and it appears Bosch uses this single platinum wire/film for both the resistance heater and the temperature sensor. They must have calibrated each MAFS design in some type of wind tunnel with known air flows. With no moving parts it should last forever, but I think the slightest dirt film changes the way the air moves over the platinum and as such they fail by reading too low. In other words they under-report the air flow so the ECU under-delivers the fuel and the system runs too lean. The O2 sensors see the lean mixture and the ECU makes a correction. Once the correction exceeds a threshold, the CEL is lit up. I have heard that the oil on the K&N filters can lead to failures of the MAF Sensor. So just what kind of dirt or film could impact the MAFS and how could it be best cleaned? I've only used Porsche paper elements. I cannot find any vacuum leaks, my fuel filter was replaced in the middle of the codes I have, well before the maintenance schedule. The engine seems to run fine. As I am not driving in the Boston salt, I may need to wait a bit before I can collect more data and see if the CEL returns.

Your are correct that the STFT will change rapidly as the engine runs, making quick changes in the amount of fuel being delivered. The LTFT is a learned percentage of what corrections have had to be made by the STFT. If the number is positive, it means that the computer is, on average, having to add that prcentage of fuel to the engine. Negative LTFT means it has had to reduce that amount of fuel. One thing that you might look at is the secondary (after cat) O2 sensor readings. You need to look at what the reading is at WOT. It both sensors should report .85 volts or higher. If they do not, it woul be an indication that you are running lean. Your fuel trim looks fine, so it appears that the MAF is properly reading the amount of air that is passing by it. You state that your car runs properly, so are you getting a CEL? And if so,what is the error code reported? It is a good idea to keep some of the old logs from the scanner. It gives you something to compare to when you do have problems.

"Not sure what TRA or FRA parameters are," They are the long & short adaptation variables you're getting on your scanner. Bottomline: 1. both banks should indicate zero short/long term corrections 2. both front O2 sensors should vary over the voltage range and must respond within a time limit, i.e. they must have minimum frequency (Fsensed - O2 sensor > Fmin) 3. the rear sensors must be stable Note: You can determine fuel injection problems by monitoring the readiness states, i.e. incomplete states indicate potential problems.

Loren: I have to disagree with you on the LTFT. They do not need to be, nor are they rarely close to zero. My current setup runs about - 12%. Stock was a little positive on both banks. My Toyota runs about +11; BMW about -5; Honda +6 and Nissan +9. I have yet to see a car with LTFT at ZERO. I am sure they are out there, but chances of getting a perfect match of an injector size would be very difficult, stock or otherwise. Also, everyone should be reminded that the after cat O2 sensor MUST Jump to a high voltage immediately at WOT. A safe rule of thumb is a,85 volts or higher. Less than this is an indication of a lean condition. This is the primamry reading you need to take after any type of tuning of the fuel system.

Hey, out of curiosity what obdII interface and software are you using? I had a setup once, ISO type that worked but would make my laptop crash so I returned it! Thanks, btw cheaper is always better as long as it gives the same info etc.

It's very helpful to see the know-how and data collected by others, so that one might interpret the data from their own engine and learn to maintain it without replacing perfectly good parts via trial and $error$. My understanding is that any long term corrections (or short term corrections for that matter) are an indication that the MAF based calculation of the fuel amount has errors. If you look at all the other variables the ECU uses to compute the fuel amount (ECU computes the fuel amount as the duration the injectors stay open), you would suspect the MAFS to be the major source of error. i.e. a 2% error in the MAF with all other variables perfect would give a 2% long term correction in fuel amount after the O2 feedback correction. A 2% error in manifold pressure, manifold temperature, rpm etc... would be much less adjustment to the long term correction. To meet the compromise of emissions (run lean), fuel economy (run close to stoichiometric A/F ratio), and power (run rich) the ECU has a 3-D MAP, so that MAF is not exactly proportional to the fuel, but still quite close. The ECU interprets WOT (I hope so) as a request for power, thus operation on the rich side, thus 0.8 volts or higher on the O2 sensors, a good test to show the O2 sensors are responsive. Not sure where Bosch leaves off and Porsche picks up when they program the ECU for a particular engine. I want the MAP for my car. Where is it? I played back a few more log files. I did see some large short term corrections +/- 20% just after starting the engine. I also saw some long term corrections of up to -8%. But based on previous posts I now think I need to collect data on a longer drive, under normal conditions and also to get the O2 data for WOT. So why is all of this technical stuff so important? In Massachusetts we now have new vehicle emissions testing (June 1, 2004): http://www.mass.gov/dep/pao/news/imfinal6.htm To coincide with the program improvements announced today, Massachusetts will introduce pass/fail On-Board Diagnostics (OBD) emissions testing for all model year 1996 and newer cars and SUVs, and most light trucks this summer. The new test relies on data from the internal computer that continuously monitors a vehicle�s engine, transmission, and emissions control system. As soon as OBD pass/fail testing begins, tailpipe testing will no longer be needed for more than 60 percent of the cars, light trucks and SUVs in Massachusetts. Within four years, less than 10 percent of the state�s motor vehicle fleet is expected to need tailpipe testing. Now about saving money and keeping the engine running perfectly: Can I effectively clean the MAFS with "electrical dust remover"? Like radio shack tetrafluoroethane in a can? Can I wipe the platinum film with a cloth or brush? Blow it out with compressed air? Yes this is fun!! Why? Because Porsche, Mercedes, BMW... would love to have you at their mercy for service. I've heard that Volvo only allows the dealer to connect to a central Volvo computer for diagnostics. Yuk. With a $120 OBD link and the know-how to use it, the user community might come up with some clever ideas keep our engines purring or even making some improvements. I guess it's about having some control over your own destiny and the satisfaction of successfully maintaining such a complex piece of machinery. Where the heck can you get a good manual for DME 5.2.2 or maybe my C4 is actually DME 7.2?? Is there a label on the ECU? There is no DME version on the engine lid. SPR I use this OBD setup: http://www.obd-2.com/

"As soon as OBD pass/fail testing begins, tailpipe testing will no longer be needed for more than 60 percent of the cars, light trucks and SUVs in Massachusetts. Within four years, less than 10 percent of the state�s motor vehicle fleet is expected to need tailpipe testing." The key now for emissions testing is the use of OBDII data and the readiness codes which provide an indication of the state of the emission system. Here's some info from my web site: Common Generic OBDII Fault Codes (DTC) X1XX, X2XX - Fuel & Air Systems X3XX - Ignition System X4XX - Emission System X5XX - Auxilary Functions X6XX - Engine Control Module Functions X7XX - Transmission Interface Functions 1XXX - OEM Proprietary XXXZ - Same type of fault, Z - specific fault location/value 0100 - Mass Air Flow Sensor (MAFS) 0105 - Manifold Abosolute Pressure (MAP) 0110 - Intake Air Temperature (IAT) 0115 - Engine Coolant Temperature (ECT) 0120 - Throttle Position Sensor (TPS) 0130 - O2 Sensor, Bank 1 - Front Sensor 0140 - O2 Sensor, Bank 1 - Rear Sensor 0150 - O2 Sensor, Bank 2 - Front Sensor 0160 - O2 Sensor, Bank 2 - Rear Sensor 0200 - Injector Circuit 0230 - Fuel Pump Circuit 0300 - Random Cylinder Misfire 0330 - Knock Sensor (KS) 0335 - Crankshaft Position Sensor (CKP) 0340 - Camshaft Position Sensor (CMP) 0400 - Exhaust Gas Recirculation (EGR) 0410 - Secondary Air Injection (SAI) 0420 - Catalytic Converter Efficiency, Bank 1 (TWC) 0430 - Catalytic Converter Efficiency, Bank 2 (TWC) 0440 - Evaporative Emission Control System (EVAP) 0500 - Vehicle Speed Sensor (VSS) 0560 - System Battery Voltage 0605 - General Control Circuit (ECM) 0650 - Malfunction Indicator Lamp (MIL) 1. OBDII DTCs are accessed via the diagnostic link connector (DLC), located in the passenger compartment near the steering column. 2. OBDII DTCs are not cleared by ECM/battery removal. 3. Most OBDII scanners provide a description of the generic DTCs, in addition to the numeric code, but not for the proprietary DTCs. 4. Some OBDII DTCs do not set the MIL but are stored. Basic OBDII Readiness Codes Non-Continuous Tests - Three Way Catalytic Converter (TWC) Effeciency - Fuel Tank Ventilation (EVAP) - Secondary Air Injection (SAI) - Oxygen Sensors - Oxygen Sensor Heating Continuous Tests - Fuel System (TRIM) - ECM Faults (DTC) - Cylinder Misfire 1. Monitored & Set During Three Cycles; Warm-up, Driving, & Trip Warm-up Cycle - SAI, EVAP, (from cold start to operating temperature, 2 min. minimum) Driving Cycle - O2 Sensors/Heating, TWC, TRIM, (2 min. idle, 5 min. drive, 2 min. idle) Trip Cycle - Diagnosis of All ECM Systems, (2nd Driving Cycle after shut-down, 20 min. max.). 2. Completed Prior to an Emissions Test. 3. Accessed via the OBDII Connector (DLC). 4. Reset by a DTC clear or a battery disconnection. 5. Some non-continuous tests can be run ("short test") via special scanners. "I have to disagree with you on the LTFT. " You need to buy the Porsche OBDII manual (WKD 483 320) and read beginning on page F-15.

More than a bad MAF will effect the STFT and LTFT. As the air filter gets dirty, the LTFT will grow negative. Put in a new filter and it will go more positive. As the injectors get dirty, LTFT will get more negative. Change in fuel quality or fuel pressure will effect it. The temperature of the air going into the intake will effect it. This is an ever learning variable that will adjust to many driving conditions. Keep this in mind when comparing current readings to past ones.