Maf sensor problems.

19 Mar 2015 | Author: | Comments Off on Maf sensor problems.



The info below relates to BMW engines but also applies to many diesels including Mondeo/X-Type, Vauxhall, Saab, Fiat, Renault etc.

If your BMW engined vehicle suffers from poor performance below 1800-2200rpm it is most likely due to an out of spec maf sensor.

This can happen from as low as 30k miles.

The MAF sensor is fitted in the inlet duct on the outlet of the air filter and measures the mass of air flowing into the engine. In theory, therefore, the ecu can inject the correct amount of fuel for the amount air going in and thereby meet the latest emission regulations. Its located on the duct just behind and to the right of the oil filler cap. the connector is obscured by the overlapping top of the air filter housing lid. (more maf sensor info here )

In practice, the Bosch sensor is notoriously unreliable and drifts out of spec as it ages. This is understandable, since the sensor element operates at 200 o C and semiconductors deteriorate at these temperatures, even though Bosch seem to have overlooked this fact!

The result is that the ecu most commonly receives a reduced signal, and because it cannot know if the signal is reduced from say a clogged air filter, it must assume it is correct.

MAF sensor degradation NEVER shows up on standard diagnostic tests this means your dealer will NOT SPOT IT! (dealers rely too heavily on fault code readers)

(Complete Maf failure – where it is short or open circuit will show up and will also light the engine management light on the dash.)

The maf sensor has most impact below 2500rpm. In the 1000-1700rpm range, where the turbo is not generating much boost pressure so there isn’t a surplus of air, it is difficult to meet emission regulations without accurately metering the air flow so the maf signal can have a very pronounced effect on the performance.

Above about 18000rpm, the turbo starts to become effective and above 2500rpm there is often a surplus of air and so the ecu does not need to rely upon the maf signal quite as much. It can simply be programmed to limit the maximum amount of fuel and hence control the torque.

So if the maf signal has fallen due to a degraded sensor, the first and most obvious sign is very sluggish acceleration when pulling away from rest. This heart stopping delay pulling onto roundabouts and into heavy traffic is downright dangerous.

It also makes the car tedious and unpleasant to drive at speeds below 2000rpm because the fairly sudden fall off of torque requires more gear changing and even the automatic models are just as bad, some even stalling when you pull away.

You will also often find that maximum torque occurs at much higher rpm than intended. Possibly not until 2500rpm, whereas on the 75 ZT it should be around 2000rpm, and on the Freelander, around 1750rpm (thanks to its variable nozzle turbo)

In about 25% of cases, the signal becomes elevated above normal and this will overfuel the engine at low rpm. Because this doesn’t result in poor performance, it is all too easy to assume the maf is working okay. Very poor mpg, exhaust smoke, exhaust reeking of diesel when idling are tell tales of an overfuelling maf.

So to sum up . the maf signal must be correct for optimum performance AND economy. You can’t just stick in any old maf, a cheap Ebay clone or ones sold for BMW cars and expect to get a good result. Also, no tuning product – box, chip or remap will work properly if your maf sensor is out of spec, so if you are disappointed, after having spent Ј100s on such a product, look to your maf sensor.

Many owners have sold their pride joy because they were sick of the poor performance when a RonBox 2 will cure the problem and boost performance to uptodate levels.

FREE TIP. Save yourself a fortune and sort a maf problem out yourself. I hear regularly from owners who have been charged upto Ј2000 by incompetent garages who could not diagnose a weak maf and replaced just about everything else.

NB: DO NOT FIT PIERBURGH MAF 7.22684 09.0 with or without a RonBox 2 as this ONLY for BMW vehicles .


Which of the following scenarios best describes your car?

1) UNDERFUELLING MAF . Zero maf signal from a dead maf (but not open circuit) will under fuel the engine to such an extent that it will NOT START. If you don’t believe me, remove the maf from its housing, reconnect it and try to start the engine. No airflow = no maf signal = no fuel.

Very poor throttle response with very slow initial rise in engine revs

Performance below 2000rpm is poor / has recently got worse and the turbo can’t be felt until about 2200rpm if at all.

I have to rev up and slip the clutch to pull onto busy roundabouts or there is a heart-stopping delay before it picks-up.

When I open the throttle below about 2200rpm, initially there is a moderate response, but most of the remaining travel is not felt to have any effect.

As the revs pass 2000-2300 the power starts to come in suddenly and quite strongly in comparison to lower rpm. (all or nothing effect)

The engine is very reluctant to rev over 3000rpm even in first gear.

If I unplug the maf connector, the low down torque is much better, though at higher revs it may be worse.

My 50-70 time in top gear is well over what it should be.

I am disappointed with the results of having a remap/chip/tuning box fitted, especially below 2000rpm.

Fuel consumption has fallen somewhat.

My auto stalls easily when pulling away.

RECOMMENDATION: Fit A RonBox 2a or e but be prepared to fit a Pierburgh maf as well – sooner or later.

2) OVERFUELLING MAF. A very high maf signal from a faulty maf or short circuit to 5v will overfuel the engine to such an extent that it will stall as soon as you touch the accelerator.*

Performance below 2000rpm very good – the car is very flexible and responsive

Fuel consumption is poor or appalling

When I open the throttle, there is a delay, then a puff of smoke and then the car accelerates.

The exhaust smells of diesel when idling

Idling is lumpy or uneven and fluctuates up and down and it ‘chugs’.

If I unplug the maf connector, the performance is about the same or a bit worse.

*In extreme cases, as soon as I touch the accelerator the engine chokes and stalls, with a cloud of black/grey smoke smelling of diesel


RECOMMENDATION: Fit a PIERBURGH MAF RonBox 2a or e (You must change the maf sensor as well – a RonBox 2 will make the symptoms caused by an overfuelling maf worse)

3) MAF OKAY-ish

Performance below 2000rpm is the same as ever

Fuel consumption is normal

I can just about feel the turbo from around 1800-2000rpm

My 50-70 in top time is about 9.5-10.5 seconds, (75/ZT)

However, the engine doesn’t really want to rev past 3500rpm except in 1st gear. (normal on a 75/ZT cdt)

I want much better overall performance from below 1500rpm to over 4000rpm

RECOMMENDATION: FIT A RonBox 2a or e. Expect to have to fit a Pierburgh maf in due course if the performance and economy drops off.

NB: DO NOT FIT PIERBURGH MAF 7.22684 09.0 with or without a RonBox 2 as this ONLY for BMW vehicles .

Fi tted a new Bosch maf and /or RonBox 2 + Pierburgh maf and the performance is still not right?

Here are some other possibilities. (more info here )

Lack of performance improvement. The RonBox has been proven 1000s of time to work as claimed so if you do not get the same result as all our other customers this will almost certainly be due to some issue with the vehicle:

· Sticking / leaking or clogged egr valve – remove and clean, refit but leave vacuum pipe off (the 10mm braided rubber tube)

· Leaking turbo or intercooler hoses or seals, also internal delamination can occur often causing a brickwall rev limit.

· Sticking wastegate or faulty actuator (75/ZT) The linkage has been known to come apart, leaving the wastegate open.

· FREELANDER ONLY: Sticking turbo vnt mechanism, faulty actuator or solenoid, leaking vacuum pipes. If the vnt mechanism has jammed, the performance will be poor at low rpm (but no smoke as boost pressure is low) if its stuck in the high rpm setting, or vice versa.

· Clogged injectors or worn nozzles. If the injectors cannot flow the extra fuel you will not see any increase in performance. In jectors should be considered a service item once they’ve done over 100k miles.

· Clogged catalyst. These have been known to collapse or disintegrate and therefore cause back pressure that ruins the performance. Hard to diagnose however.

· Faulty fuel rail sensor. TD4 more prone due to water ingress in the loom – see LR Tech bulletin 00052. Very common problem on the Td4, less so on the 75/ZT.

· A clogged crankcase ventilation valve (pcv valve) can also affect the performance, causing smoke due to too much oily crankcase fumes being sucked it. A filterless cyclone type is available from BMW, part no. B11.12.7.799.367 and costs Ј27.50 inc VAT


I did some comparisons of a dead maf kindly donated by an owner, my good maf and a Pierburgh Maf.


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