The MAF (Mass Air Flow) sensor is a sensor on the engine which simply measures the mass of the air being delivered into the engine. The mass (and therefore density) of the air can be affected by temperature, altitude, and rate of induction, and the ECU needs a MAF reading to balance and deliver the correct amount of fuel to the engine.
The problem
The stock MAF fitted to the TD4 is a Bosch model, and performs perfectly adequately. That is, until it malfunctions. The problem with sensors of this nature is that they don’t suddenly break; rather, their readings tend to become increasingly inaccurate over time and with use. The effect is that the ECU may start delivering slightly too much or too little fuel, affecting fuel economy, engine power, and smoothness. Once the MAF develops a fault (or becomes ‘out of spec’) the accuracy of the reading will gradually deteriorate.
The ECU has a pre-programmed table of expected MAF values, and if the MAF reading falls outside of this range the ECU will ignore the MAF and use a default setting. However, if the MAF is out of spec but still within range, the ECU will still use the MAF value because it has no reference reading with which to compare the MAF value. Furthermore, the effects of a faulty MAF are noticeable well before the MAF becomes out of range.
The MAF that is factory-fitted to the TD4 (and the model that will be refitted if replaced by Land Rover) is a Bosch model, which functions perfectly while it is in spec. However, the Bosch MAF has a reputation for being very unreliable – indeed, there are forum anecdotes of a new Bosch MAF being fitted to an engine, only to have it fail days later. In addition, the Bosch MAF is expensive; you could be looking at upwards of £200 including VAT and delivery.
This makes the MAF sensor difficult to rule out (unless you have a known-good spare lying around) but arguably the most likely component to be at fault with regards to high fuel consumption, rough idling, smoke in the exhaust gases, and poor acceleration/development of power.
The solution
Before rushing out and replacing your MAF, you should first try forcing your ECU to use its default MAF value. This is achieved by disconnecting the MAF sensor and having a drive around. This should have a very noticeable effect on the engine. If it feels better or no different, your MAF is probably faulty or on the way out. If it feels worse, your MAF is probably still ok. Of course, it’s not quite as simple as this – disconnecting your MAF may result in your engine becoming a bit more responsive to your right foot, but might also result in lower fuel economy.
Anyway, if you decide to replace your MAF, you can find it here, at the rear of the engine and just to the right of the air filter:
The MAF sensor is located at the rear of the engine, near the oil filler cap and air filter.
For access to the MAF, the air filter cover must be removed as described here, and is secured with two Torx screws which are easily removed with the correct bit.
MAF sensor partially visible behind oil filler cap
After removing the connector, the MAF sensor simply slides out (it has an air-tight seal so may resist). You have several options at this point.
Clean the MAF sensor
If forum anecdotes are to be believed, the Bosch MAF sensor rarely fails due to being clogged. In addition, they’re very delicate – so unless you expect to replace the MAF anyway I would recommend leaving it alone.
Replace the MAF sensor with a Bosch MAF
Simply pop in a brand new Bosch MAF, and hope it lasts long enough to warrant the cost.
Replace the MAF sensor with a Pierburg MAF
First of all, I must point out that there are two Pierburg MAFs that are compatible with the TD4. The Pierburg MAFs are much cheaper and reputedly much more reliable than the Bosch model.
One Pierburg model (7.22684.09.0) is allegedly a straight swap for the Bosch model, and is discussed here on LandyZone. The downside is that there seems to be little corroboration that the MAF is 100% compatible, though I couldn’t find any reports that it’s not. Feel free to be a guinea pig!
The other Pierburg model (7.22684.07.0) lacks an IAT (Intake Air Temperature) sensor and therefore does not deliver a signal which is directly compatible with the TD4 ECU. In order to use this MAF you will need something which transforms the Pierburg signal into one that emulates the Bosch MAF – such as Rover Ron’s marvellous Synergy 2. This is the option I chose, simply because the other Pierburg MAF wasn’t available, and I wanted a tuning unit anyway. The Synergy 2 has an separate selectable section which is designed to convert the Pierburg signal into a Bosch-compatible one.
Both Pierburg MAFs are available here and elsewhere.
