A MAF (Mass Air Flow) sensor measures the air intake passing into the engine, and along with a number of other sensors and a base map, it can work as close as possible how much fuel to inject into the engine for a perfect burn depending on what you are doing.
The MAF sensor has a number of designs, but in the K11 Micra, it’s a small wire, not to dissimilar to that of an old style bulb which has a specific current pass through it to heat it. As the air passes over it, it’s cooled. The resistance of the wire (which changes with temperature) is used to calculate the amount of air entering into the engine.
The MAF sensor can sit in different places depending upon the car, but in the K11 Micra, it sits in the throttle body housing – with a torch and a wide open throttle you may be able to see it.
If this fails completely then the ECU will go into a limp mode and will go to a safe known values in the base map and read info from other sensors. It’s a limited map so the car will run like a bag of spanners, over fuel like no tomorrow but will allow you to either limp home, or to a garage. In my experience in having an iffy maf (no reading) which results in limp mode, the engine may not idle and stall.
Even though the air is filtered, a small amount of dirt always gets past no matter what and can contaminate the wire. This can affect the reading and may under-read the air entering the engine thus giving you a lean mix.
You can attempt to clean the wire, and there is many a topic on here about it and various ways of doing the job. If you are not that experienced with tinkering with such sensors and don’t want to blow £150 (well, that’s how much they were last time I looked years ago!) then it would be best to leave it in place, as it can be a pain to remove and they are also quite fragile. With the engine running with some revs spay in some electrical cleaner (you can buy it from Halfords) in short sharp bursts directly into the intake, but from around 20-30cm. This allows the engine to drag the cleaner into the engine rather than it being forced. The engine will bog down but should recover too much spray/not enough revs and the engine could stall. Don’t worry, just give it a minute and restart it.
I would say at the moment that the fuel tank pressure is a bit of a red herring at the moment, especially as your mechanic has suggested that your MAF maybe misreading and the symptoms do match that of a dirty maf (although, it could also match a whole host of other faults….one at a time though!).
A vacuum leak in a hose somewhere can also give similar fault readings as it allows unmetered air to enter the engine, so the engine does not put in enough fuel for the air entering, thus can give you the symptoms you are having so worth having a good look over for split hoses.
And lengthy reply’s – not a problem, just doing some work on servers at work but there is a lot of waiting for stuff to happen so thought I might as well make myself useful!
