Bad MAF without CEL ?

[mat911]

Just wondering, is it possible to have a bad MAF without triggering CEL?

My previous car was an AUDI TT and although the MAF wasn't working properly there was not any CEL triggered. The MAF wasnt totally busted but on the other hand it miscalculated the air mass going in the engine, the only way you could tell something was wrong was only by comparison to how the car used to fell before (stronger accelaration etc). As soon as a new MAF was fitted the car felt totally different again.

The problem is that a MAF gradually deteriorates and there is any objective way to know for sure if it doing the job properly or not.

Can I rest assure that in my Carrera if the MAF isn't 100% ok a cel light shall notify me and will save me from engine failure ? I think not since both Audi's and Prosche MAFs are made by Bosch...The CEL shall only warn you when the MAF has gone really bad.

In simple words how can anyone verify that the MAF is 100% ?

Mathew

CARRERA 40th Anniversary

[1999Porsche911]

Just wondering, is it possible to have a bad MAF without triggering CEL?

My previous car was an AUDI TT and although the MAF wasn't working properly there was not any CEL triggered. The MAF wasnt totally busted but on the other hand it miscalculated the air mass going in the engine, the only way you could tell something was wrong was only by comparison to how the car used to fell before (stronger accelaration etc). As soon as a new MAF was fitted the car felt totally different again.

The problem is that a MAF gradually deteriorates and there is any objective way to know for sure if it doing the job properly or not.

Can I rest assure that in my Carrera if the MAF isn't 100% ok a cel light shall notify me and will save me from engine failure ? I think not since both Audi's and Prosche MAFs are made by Bosch...The CEL shall only warn you when the MAF has gone really bad.

In simple words how can anyone verify that the MAF is 100% ?

Mathew

CARRERA 40th Anniversary

The MAF and related componants are designed to operate in a specific range, If the MAF operates outside this range, it WILL throw a CEL. If you do not have a CEL, either everything is working right, or the bulb is burnt out. When the MAF gets close to it's limit, but not enogh to throw a CEL, you may experience slight idle roughness and hesitation from stop which many times can be corrected by cleaning the MAF.

Edited March 25, 2006 by 1999Porsche911

[c928jon]

Hi

The MAF will deteriorate as it ages, it can remain within parameters but still not be quite right.

The signs are the rough idle and a feeling of low power at higher revs.

In these cases cleaning works well, but not always!

My experience was to clean the MAF and it felt like I had a new car. I then fitted a new one just to be on the safe side.

John

[Richard Hamilton]

You will find that with RoW cars (like mine) the CEL is NOT triggered for certain emissions-related faults. P1123 and 1125, for example, do not show the CEL on UK cars, but they do on US cars. (Ask me how I know).

[1999Porsche911]

You will find that with RoW cars (like mine) the CEL is NOT triggered for certain emissions-related faults. P1123 and 1125, for example, do not show the CEL on UK cars, but they do on US cars. (Ask me how I know).

Yeah, the European flash ignores many of the sensors (as far as emission faults are concerned), where here in the US, we have all the bells and whistles. However, the US chips can be reflashed with the European program if you don't mind not being able to pass emission testing and can find someone willing to break federal law.

[wwest]

You will find that with RoW cars (like mine) the CEL is NOT triggered for certain emissions-related faults. P1123 and 1125, for example, do not show the CEL on UK cars, but they do on US cars. (Ask me how I know).

Yeah, the European flash ignores many of the sensors (as far as emission faults are concerned), where here in the US, we have all the bells and whistles. However, the US chips can be reflashed with the European program if you don't mind not being able to pass emission testing and can find someone willing to break federal law.

When the engine is at idle the primary A/F mixture control is the oxygen sensors between the engine and the catalyst. The MAF can be, is, checked for validity at this time. But during acceleration or high torque developement by the engine the A/F mixture is enriched beyond the "detection" capability of the oxygen sensor.

There are numerous simple HP improvement devices on Ebay that simply add a small value resistor in series with the IAT section of the MAF/IAT module. These work by making the engine ECU "think" the incoming airflow is colder than actual, say 45F intead of actual 72F, and that results in a richer mixture.

Insofar as I know the engine ECU has no method of detecting a "lying" (failed??) MAF/IAT module other than at idle.

A 20,000 ohm 1/4 watt resistor in parallel with the IAT sensor changes the ECU's intake airflow measurement from 72F actual to 92F according to AutoTap's OBD-II parameter readout screen.

Hope that answers your question.

[mat911]

Thanks for the info guys, so in order to recapitulate, at least for Europian Porsches, CEL shall no be triggered if the MAF is "partly" deteriorated and not doing its job properly.... I think that's bad news, and is exactly what had happend with my TT...

I can assure you I never had such problems with my Scooby....

Anyway how can I determine the health status of the MAF by using a diagnostic OBD software ? Can you tell by reading the g/s value of the MAF at idle at normal operating temp ?

What should it read in order to ascertain the MAF is 100% OK ?

Thanks

[1999Porsche911]

The MAF reading is used by the ECU at ALL times the engine is running, whether at idle or at WOT. The MAF measures the amount and temperature of the air passing by it and in turn, the ECU (based on it's mapping) decides how much fuel to inject into the engine. During times where you are not at or near WOT (closed loop), the O2 sensors then sense how rich or lean the burnt mixture was and will adjust the amount of fuel injected to obtain the proper Air/Fuel balance. As long as the O2 sensors remain within range, there should be no CEL.

At or near WOT, the car goes open loop at which time the readings of the O2 sensors are ignored and the amount of fuel injected is controlled only by the MAF readings alongs with the fuel maps of the ECU. The O2 sensors continue working, but do not make any adjustment to fuel.

[wwest]

The MAF reading is used by the ECU at ALL times the engine is running, whether at idle or at WOT. The MAF measures the amount and temperature of the air passing by it and in turn, the ECU (based on it's mapping) decides how much fuel to inject into the engine. During times where you are not at or near WOT (closed loop), the O2 sensors then sense how rich or lean the burnt mixture was and will adjust the amount of fuel injected to obtain the proper Air/Fuel balance. As long as the O2 sensors remain within range, there should be no CEL.

At or near WOT, the car goes open loop at which time the readings of the O2 sensors are ignored and the amount of fuel injected is controlled only by the MAF readings alongs with the fuel maps of the ECU. The O2 sensors continue working, but do not make any adjustment to fuel.

The sensitivity and range of the oxygen sensors is such that they are used only in an OFF/ON sequence. At idle the engine ECU continuously modulates the A/F mixture slightly above to slightly below the target, the ideal A/F mixture from the standpoint of minimizing emissions deemed harmful to the environment. So at idle the exhaust flow alternates between absolutely NO oxygen content and the base minimum. In short the output of the oxygen sensor, ideally, will vary up and down continuously as a function, and in sync with, the modulation of the intake A/F mixture.

There are actually two parts, components, to the MAF "module", the mass airflow sensor and the IAT, the intake air temperature sensor. Colder air is denser air and at low intake flow volumes, near idle, the IAT plays a bigger part in the computation. Not to say, by any means, that it isn't always a part of the computation.

As you open the throttle initally above idle, and then more and more, provided the engine is under load, the A/F mixture will begin to move away from the ideal A/F mixture insofar as emissions are concerned, and into the region of higher, richer, A/F mixture ratios wherein the oxygen sensor becomes totally and completely useless. The engine is now operating "off-the-curve" of the oxygen sensors sensing capability.

It is in these conditions that the MAF/IAT module signals come into play for controlling the optimum level of fuel to mix with the intake airflow volume. As a trial I have inserted resistors in the IAT circuit, series and parallel (at different times) to modify the intake sensed temperature above and below the actual intake temperature, ~72 actual to 92F "sensed", and 72F actual down to 45F. False sensed voltages were verified via AutoTap's OBD-II scanner/reader.

There was never an indication that the engine ECU "knew" or detected the flawed intake temperature signals apparently accepting them as "true" readings.

The test runs, typcally over 400 miles of highway driving at a reasonably constant ~70MPH, of the vehicle in question, a 2001 AWD RX300, resulted in no meaureable difference in average MPG with the intake temperature falsified above, below or with actual sensing.

But I fully believe that had I tested on a dyno the available engine HP output would have changed in the range of +/- 10% or more. So the IAT false "cold" modification that resulted in an even richer mixture than the factory default target would have undoubtedly shown itself at track time.

[1999Porsche911]

The MAF reading is used by the ECU at ALL times the engine is running, whether at idle or at WOT. The MAF measures the amount and temperature of the air passing by it and in turn, the ECU (based on it's mapping) decides how much fuel to inject into the engine. During times where you are not at or near WOT (closed loop), the O2 sensors then sense how rich or lean the burnt mixture was and will adjust the amount of fuel injected to obtain the proper Air/Fuel balance. As long as the O2 sensors remain within range, there should be no CEL.

At or near WOT, the car goes open loop at which time the readings of the O2 sensors are ignored and the amount of fuel injected is controlled only by the MAF readings alongs with the fuel maps of the ECU. The O2 sensors continue working, but do not make any adjustment to fuel.

The sensitivity and range of the oxygen sensors is such that they are used only in an OFF/ON sequence. At idle the engine ECU continuously modulates the A/F mixture slightly above to slightly below the target, the ideal A/F mixture from the standpoint of minimizing emissions deemed harmful to the environment. So at idle the exhaust flow alternates between absolutely NO oxygen content and the base minimum. In short the output of the oxygen sensor, ideally, will vary up and down continuously as a function, and in sync with, the modulation of the intake A/F mixture.

There are actually two parts, components, to the MAF "module", the mass airflow sensor and the IAT, the intake air temperature sensor. Colder air is denser air and at low intake flow volumes, near idle, the IAT plays a bigger part in the computation. Not to say, by any means, that it isn't always a part of the computation.

As you open the throttle initally above idle, and then more and more, provided the engine is under load, the A/F mixture will begin to move away from the ideal A/F mixture insofar as emissions are concerned, and into the region of higher, richer, A/F mixture ratios wherein the oxygen sensor becomes totally and completely useless. The engine is now operating "off-the-curve" of the oxygen sensors sensing capability.

It is in these conditions that the MAF/IAT module signals come into play for controlling the optimum level of fuel to mix with the intake airflow volume. As a trial I have inserted resistors in the IAT circuit, series and parallel (at different times) to modify the intake sensed temperature above and below the actual intake temperature, ~72 actual to 92F "sensed", and 72F actual down to 45F. False sensed voltages were verified via AutoTap's OBD-II scanner/reader.

There was never an indication that the engine ECU "knew" or detected the flawed intake temperature signals apparently accepting them as "true" readings.

The test runs, typcally over 400 miles of highway driving at a reasonably constant ~70MPH, of the vehicle in question, a 2001 AWD RX300, resulted in no meaureable difference in average MPG with the intake temperature falsified above, below or with actual sensing.

But I fully believe that had I tested on a dyno the available engine HP output would have changed in the range of +/- 10% or more. So the IAT false "cold" modification that resulted in an even richer mixture than the factory default target would have undoubtedly shown itself at track time.

The on/off your referred to regarding the O2 sensors if determined by whether you are in closed or open loop at the ECU. If your are in closed loop, the O2 sensors are never off but are increasing and decreasing in current in response to the mixture it reads. These adjustments are fed into the fuel LTFT. At idle, closed loop, the amount of fuel injected is a combination MAF, O2 sensors and LTFT. The MAF has no way of controlling the PROPER air/fuel ratio in the engine other than what has been mapped into the ECU. At WOT, if the MAF for some reason is reporting the incorrect amount of air and temperature (in cases where you have an intregrated MAF/IAT) then the incorrect reading will NOT be adjusted for by any other engine sensor.

Modifying the air temperature reported to the ECU will also effect the engine's timing and this has been a method of fooling an ECU for years. Remember, the O2 sesors are always controlling the fuel/air mixture except in open loop (near or at WOT) whether under load or not. It's a fairly efficient method of contolling the mixture.

[Richard Hamilton]

You will find that with RoW cars (like mine) the CEL is NOT triggered for certain emissions-related faults. P1123 and 1125, for example, do not show the CEL on UK cars, but they do on US cars. (Ask me how I know).

Yeah, the European flash ignores many of the sensors (as far as emission faults are concerned), where here in the US, we have all the bells and whistles. However, the US chips can be reflashed with the European program if you don't mind not being able to pass emission testing and can find someone willing to break federal law.

I had P1123 and P1125 for quite a while, and since I switched my MAF it hasn't re-occurred. The fault was stored in the fault memory of the DME, and it showed the number of occurrences. You could read the faults with a PST2, but they did not show up on a scan tool. Because the "fault" was intermittent (it showed as something like 10 events per month) it wouldn't fail an annual UK emissions test. I suspect that Porsche consider that such faults would show up on a service visit, and could be rectified at that time.

Mathew - where are you located? - you didn't say if it was a US or RoW car.

[mat911]

You will find that with RoW cars (like mine) the CEL is NOT triggered for certain emissions-related faults. P1123 and 1125, for example, do not show the CEL on UK cars, but they do on US cars. (Ask me how I know).

Yeah, the European flash ignores many of the sensors (as far as emission faults are concerned), where here in the US, we have all the bells and whistles. However, the US chips can be reflashed with the European program if you don't mind not being able to pass emission testing and can find someone willing to break federal law.

I had P1123 and P1125 for quite a while, and since I switched my MAF it hasn't re-occurred. The fault was stored in the fault memory of the DME, and it showed the number of occurrences. You could read the faults with a PST2, but they did not show up on a scan tool. Because the "fault" was intermittent (it showed as something like 10 events per month) it wouldn't fail an annual UK emissions test. I suspect that Porsche consider that such faults would show up on a service visit, and could be rectified at that time.

Mathew - where are you located? - you didn't say if it was a US or RoW car.

I am located in Greece, that means my car is in the RoW category.

Are the attachments you posted taken from PST2 ? Can I buy a scan tool to read the error codes P1123 & P1125 to do the job myself ?

[Richard Hamilton]

The screen shots were taken from a PST2.

I used to have an EZ-Scan 5000 which didn't display the faults, but they were there when you read the fault memory with the PST2. I'm not sure whether other scan tools or code readers would show them.

I wish I could be more help, sorry. :(