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The MAF or Mass Airflow Sensor or air mass meter relies upon the principle of the cooling effect of moving air (think ' windchill'). It's a fairly unreliable and temperamental sensor. If it just worked or didn't work, it would be easier to diagnose a faulty one. But it doesn't - it can become less sensitive over time or can change sensitivity suddenly for no apparent reason. 

IMPORTANT  -  I CANNOT EMPHASISE THIS ENOUGH:-

Out of spec mafs DO NOT show up on diagnostic tests. So DO NOT assume that because you are told that no fault was found, that nothing can be done to cure lacklustre performance other than an expensive remap or tuning box. Ignore anyone who says "they're all like that"...!

Having read this page, and you conclude your maf might be suspect, then a Mafam will most likely effect a dramatic improvement. 

L SERIES MAFAM PAGE

75/ZT/FREELANDER MAFAM PAGE.

Here is my theory of how the Maf sensor affects the fuel quantity and hence torque: How the Maf and Ecu interact.

A: All ROVER 25, 45, 75, MG ZR, ZS ZT,  & Freelander TD4 DIESELS.  (Not to mention most turbo diesels from 1999/2000)

But NOT the 200/400/600 Sdi/Gsdi or SD. On these the maf is ONLY used to control the exhaust gas recirculation system. It is not used to modify the fuelling.

A: Many are poor below 2000rpm and as dealers cannot usually detect an out of spec Maf sensor even with a Testbook connected, they could have a point BUT they should eliminate the MAF sensor by trying a new one. Or even by just disconnecting it.

a)  it is nothing to do with the engine design, valve timing, spec of turbo fitted, or injection pump.

b) It is due to lack of fuel as a result of an out of spec maf sensor and/or poorly developed firmware (fuel maps mainly).

c) It is completely fixable by means of a Mafam or new maf. PLEASE NOTE: A remap or a rechip will mask a faulty maf and not produce the expected increase.

A1: Loss of power throughout the rev range, can indicate a dead maf (possibly even worse at low rpm).

A2: Loss of or absence of low down power (below about 2100rpm - i.e. before the turbo comes in) tends to indicate an out of spec maf.

A3 : Not too bad low down, but a reluctance to rev past 3800rpm indicates dead maf OR disconnected maf OR bad connection.

TIP  Unplug the MAF plug and go for a drive - you'll soon notice a difference if your maf is faulty or out of spec!  You will find that low down driveability is vastly improved, but max torque and turbo boost is often reduced somewhat and there is an unwillingness to rev past 3800-4000rpm. If you get gains throughout the rev range then your maf is seriously ill, but still fixable with a Mafam unless completely dead. (rare)

A: Because it is a not a serious enough fault. In fact the ecu is probably not even aware of the 'out of spec' maf defect. It just takes the signal as being an accurate measure of the air flow and adjusts the fuelling accordingly. In the case of a faulty maf - it reduces it, ergo less torque. The ecu can really only detect short or open circuit mafs, this is why they never show up as being faulty / out of spec.

A: The sensor element is heated to a high temperature and 'ages' over time, which reduces the sensitivity. It can also be affected by contaminants. Oil impregnated air filters could be one source, though the manufacturers of these deny this (naturally, they would!) Also relocating the air intake so it can suck in moisture is asking for maf problems.

I prefer BMC &  ITG filters - they are pre-oiled with a very sticky oil and never need re-oiling. Other makes can be over oiled.

A: If you examine the attached Bosch chart, you can see that at low flows, corresponding to low rpm, the curves are steepest so a small change in air flow produces a large change in signal.

Conversely, you may not think your maf is defective because performance over 2500pm may seem the same. This is because at higher airflows, the curves level off so only a small signal change occurs for quite large air flow changes. Thus even with an out of spec maf, the loss of signal hardly effects the performance at higher rpm unless the maf is very faulty.

The MAFAM is very effective because at low flow rates, even a small change in maf signal can have a dramatic effect on performance.

A: It is worth trying. and also clean the electrical contacts. BE VERY GENTLE. Here's the details. Its usually only a temporary cure, if one at all.

A: A Pierburgh maf works well but MUST be used with a Mafam.  I now stock these for £53.50 delivered or £50 if bought with a mafam.  More here

A: Fit a MAFAM!

A: Unplug the maf connector and go for a drive. If you notice an immediate gain in low down power but often some loss of performance when the turbo comes in, a MAFAM will give you at least the same low down power AND NO loss of power anywhere else. Possibly even a tad more upto 3500rpm. If your maf was completely dead, then you may gain power right across the rev range.    

Q11B I've tried this and my car goes much better,  so why can't I leave it disconnected?

The maf sensor also incorporates the intake air temp sensor (IAT) so without this, the ecu defaults to -5C, over fuelling the engine at all temperatures above this, significantly so in warm weather.

The maf signal also is used to control the egr valve, overrun fuel cut off  (& hence engine braking)  and injection timing.  So you will find your mpg drops without a maf connected.

A: Certainly. These work even better with a Mafam fitted as well. The MAF signal plays an important role in modifying the fuel values in the fuel map before they are sent to the pump ecu. No matter whether its the standard map, a remap or a rechip, unless the ecu is receiving a 'good' maf signal, the fuelling may not reach 100% of the fuel map values. This explains why some tuning companies have stated that they can't guarantee results with Rovers. They haven't realised how important the maf sensor is!!

A: NO. The Mafam does not affect max rpm to any significant extent so it has negligible effect on smoke measurements.

Even with a Superchip remap, boost turned up, no cat and a Mafam, my previous 45 passed the MOT first time.

1. The maximum fuel quantity that can be injected at any given revs is contained in the fuel maps. These values are determined by the vehicle manufacturer to tailor the torque curve taking into account engine and transmission power ratings, emission requirements, the rev limit, poor servicing, poor fuel and minimising warranty claims.

2. This maximum amount is modified by the ecu based on various sensor signals before being sent to the pump ecu.

3. The crucial one is obviously the throttle position signal - clearly the ecu uses this to adjust the fuel quantity from almost zero to the maximum it is programmed to supply at any given rpm.

4. The signal from the Maf sensor also is used to adjust the fuel depending upon the mass of air going into the engine.

5. On a turbo engine, the airflow depends upon the boost pressure as well as rpm, and there is a surplus of air once the turbo starts to work. The fuelling can then be determined by the fuel maps, and the maf sensor signal is somewhat less crucial.

6. Below 1800-2000rpm, however,  when the turbo is barley working, there is not a surplus of air and to avoid smoke and comply with ever tighter emissions regulations, the air flow must be measured. 

7. Consequently, if the maf signal is below par, the fuel quantity NEVER reaches the maximum allowed value (whether this value is from the original maps or a remapped ecu or a chipped one) and thus torque is reduced.

8. The Mafam maximises the signal from the maf sensor, thus enabling as close to 100% of the fuel values as possible to be sent to the pump ecu.

9. When the actual fuel values in the map are replaced with larger ones, as is the case with a remap or a rechipped ecu, more fuel can be injected giving the increased power. BUT, crucially, the maf still plays its part. So if its signal is below par, the ecu will still reduce the fuel and you will not be getting the most from your modified ecu. So a Mafam can make a big difference after a rechip or remap in just the same way.  I HAVE HAD MANY EMAILS CONFIRMING THIS  see Mafam feedback

1. Fit a Mafam first. If this restores lost performance then this is all you need. 
2. If top end power is still down from what it was, then the maf may be completely dead so replace it, either with a Pierbugh one, or a Bosch one AS WELL.
3. If you are thinking of having a remap or rechip, fit a Mafam first. This might give you the performance you've been expecting, especially in conjunction with an ITG air filter and 3-4psi boost increase, and it will still ensure you get the most from any subsequent remap or rechip. You can use the on/off switch to compare.