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Discussion Starter · #1 ·
Hi

Trying to get to the bottom of my issue with this P00BC code (no other codes reported by car). I have replaced the MAF sensor but still get the same issue which suggests an air intake leak as the code refers to air flow being low. Where would I start to see if there is a leak? Where could this leak be if airflow is too low? I have replaced the air filter. The air filter box has a broken corner that slightly exposes the air filter but not much. Surely it's not leaking too much here? Or maybe it's not an air leak? After clearing the code, I can drive anywhere between 10-30 miles before it shows up again and the car goes into limp mode, losing power.

Any help in getting to the bottom of the issue would be much appreciated.
 

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2013 Mondeo Titanium X Sport Ecoboost
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I first came across OBD systems 19 years ago when I bought my American 'toy' - a 1996 Pontiac Firebird, and ever since, on various forums and facebook groups I often see exactly the same response to trouble codes - and it would appear that you have fallen into the same trap - and that is to suspect a faulty sensor.
What you have to understand about the On Board diagnostics is that the sensors such as MAF, MAP, O2, Cam, Crank etc are there to provide data to the PCM so that it can manage the running of the engine correctly. In the event that there is a failure of a component, such as a fuel injector, pump etc, the data received by the PCM from certain sensors will differ - and so, it throws a code. The codes are there to aid with diagnosing the fault and what is crucially important to understand is that the code is alerting you to a condition that the sensor is reporting on.

Your particular fault P00BC is pointing you to the air intake, or Induction system and can be as simple as a clogged air filter, or an air leak

More detailed information on this fault code can be found here - P00BC Mass Air Flow (MAF) A Circuit Range/Perf Flow Too Low

It's always worth doing a little bit of reading when getting a fault code rather than suspecting a sensor fault as some of the sensors can be pretty expensive to replace - especially frustrating when you discover that there was nothing wrong with the sensor in the first place.
 

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As above the OBDII codes shouldnot be taken as that part is faulty. Its a starting poing for the real diagnostics to
start in that area.

The problem that some generic OBDII readers give different codes than a reader thats dedicated to a particular brand.

As you can drive for a little while before it throws a code you should check the airflow live data before and after.
Also check the turboboost pressures.

What engine do you have what year is the car? What code reader are you using?
 

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Discussion Starter · #4 ·
Thanks for the responses. As I have already tried to replace the MAF I don't think that's the problem.

Car is Ford Mondeo 2010 MK4, 2.0 Duratorq TDCI 140

With this code, would the low airflow problem be occurring before or after the MAF? Or either? I'm gonna do a smoke test with a cigar to see if I can find any leaks. If it's not an air leak, what would be the other suspicion? I just put a new air filter in the car so I don't think that's the problem. Could it have anything to do with the EGR?
 

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Think your going to need a very large cigar to fill the inlet with enough smoke. :)

What is the airflow measurement before and after the fault code? My first test would be air
entering the system after the MAF but before the turbo.
 

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Discussion Starter · #6 ·
Think your going to need a very large cigar to fill the inlet with enough smoke. :)

What is the airflow measurement before and after the fault code? My first test would be air
entering the system after the MAF but before the turbo.
I've cleared the fault codes not long ago so now whenever I drive I'll try and monitor the airflow to see if I can catch when the error code comes up to see what it reads.

As for testing for leaks after the MAF, that's where I'm planning to fill it with cigar smoke. The pipes appear to go behind the engine so not much else I can take apart there easily. Why would it develop a leak there? A seal maybe? Could it be not enough air entering before the MAF?

Either way I'll try to log the airflow
 

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Discussion Starter · #8 ·
Try the obvious and take the air filter out and see if that cures the fault, if so then replace it with new.
I have replaced the air filter with a new one and the problem still appeared. I'm thinking of trying to run it without the air filter. Could it be that I have a blockage before the MAF sensor?
 

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Discussion Starter · #9 ·
I've been monitoring the MAF sensors reported g/s and that did not change before and after the fault happened. Maf reads g/s and Air Intake Temp correctly I think. My base value on idle is around 12 g/s. When accelerating, it peaks around 110 g/s. It feels normal really. One interesting I have noticed, which I'm not sure is normal, is my revs will stay at a certain value if I keep the clutch pressed as if it's expecting me to change gears before it drops to idle speeds. This creates a curve on the MAF sensor that's descending then drops again. Is this a feature or is my EGR getting stuck or something?
 

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The revs will hang slightly for the reason you mentioned. It will make gear changes smoother.
 

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Discussion Starter · #11 ·
The revs will hang slightly for the reason you mentioned. It will make gear changes smoother.
Cool, thought that might be the case, cheers! Now just the strange P00BC issue. Still don't know what could be triggering it. I'm yet to do a smoke test but could air leaks after the MAF sensor be triggering it? I guess so, right? MAF might be reading normal readings but code might be triggering because engine is not getting enough air?
 

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Going back to your question about where the problem might be, let's step it back a little and look at it from a logical point of view.
Think about the direction of the air flow. The MAF sensor monitors the air flowing across it - that's how it works.
Now think about what might cause it to show an error. If there is a leak AFTER the sensor, air would still flow across it as normal, wouldn't it?
If the air flow across the sensor is lower than the PCM is expecting, it would indicate a blockage or a leak in the intake between the air filter and the MAF sensor
 

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Discussion Starter · #13 ·
Going back to your question about where the problem might be, let's step it back a little and look at it from a logical point of view.
Think about the direction of the air flow. The MAF sensor monitors the air flowing across it - that's how it works.
Now think about what might cause it to show an error. If there is a leak AFTER the sensor, air would still flow across it as normal, wouldn't it?
If the air flow across the sensor is lower than the PCM is expecting, it would indicate a blockage or a leak in the intake between the air filter and the MAF sensor
Thank you for clarifying. This was also my thinking but a friend of mine made me doubt myself. In this case, I have identified a split on a plastic tube going into the air filter which I have superglued and taped over (not sure how good this seals but the cut was around 2cm long. The error still came up. I also have a new air filter and the previous filter wasn't too bad either, both of which still threw the error. The strange thing is that I'm still getting values that I'd think are reasonable. On idle, I get around 12 g/s. Either way, my next attempt will be to remove the air filter and maybe replace it with some women's tights so it catches larger debris but lets a lot more air through. Do you think it would be safe to run 100 miles like this? Once I clear the code, it tends to take a while for it to come back.
 

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I would rather suffer the code and have to drive slower to avoid it coming on than run without an air filter.
 

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Discussion Starter · #15 ·
Right then. So I've been keeping an eye on my MAF sensor readings and noticed on a longer trip today that it was reading 7 g/s on idle and going up and down like normal (as far as I can tell). Then, on the final leg of the trip, the engine malfunction popped up again on the dash and my MAF readings when coasting/idle went up to 15 g/s and would not drop further. Interesting. It might just still be the sensor? I should replace it with a quality part probably, the previous sensor I bought was cheap.
 
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