Td5 D2 engine - MAF Sensor (1)

Recently become aware that the performance of the engine has declined somewhat more then expected.

Daily usage of Hx has drastically declined over the past few years and it never gets a 'good run' anymore. It used to do about 40 mile each day commuting on a good mix of single and dual carriageway. Now it can sit stationary for nearly a week and then only do a short 10 mile trip. This usage cannot be good for its general well being and it's caused me to suspect the fall in performance.

I recently connected the Nanocom to Hx and read off the engine inputs. As always there is a lot of info to digest, but I reckon a methodical approach to interrogating the various engine management sensors can only be beneficial both to me and Hx.

So, where to start? 

  

After a good read of Rave and a trawl through the online blogs attention turned to the 'Mass Air Flow' sensor commonly termed  ' MAF '

This sensor sits downstream of the air filter and upstream of the turbocharger. It provides a measurement of the airflow into the turbocharger that the ECU can interpret.

Accurate measurement of the airflow is very important to the engine running profile.

Should this sensor fail or go 'out of range' the ECU has a fail safe setting it will use to maintain the engine operation. This fail safe setting is perfectly driveable but will not deliver optimum performance.

The sensor is connected to the wiring loom via a three-pin connector ( CO149 ).

PIN 3  -  12v input from engine fuse box.

PIN 2  -  Variable output (0-5v) to ECU.

PIN 1  -  Earth path via ECU

Back probing the connector can easily confirm this. Rave gives the loom connections to test for continuity in the cabling..

Online blogs would suggest that a reading of 5v from Pin 2 equates to 60 mg/hr. which is the lower end of the range with the engine at idle.

There is another basic resistance test that can be done with the sensor unplugged...

MAF Sensor Resistances.

(Got these figures from the Web)

GOOD MAF

Pins 1-2  -  16.8 K/ohm

Pins 1-3  -  34 M/ohm

Pins 2-3  -  34 M/ohm

BAD MAF

Pins 1-2  -  16.8 K/ohm 

Pins 1-3  -  6 - 20 M/ohm

Pins 2-3  -  6 - 20  M/ohm

These 'Bad' figures are still functional and within range. The figures can get further away from the ideal before the ECU will 'ignore' the MAF sensor and select its default values.

By far the best way of testing the sensor performance is to do it 'live' via the OBD socket. I have a Nanocom diagnostic reader and have already noted the values with the engine running and it was this that convinced me the sensor was 'out of range'.

The Nanocom is able to read the airflow figures direct as Kg/hr values. More on this later.....

Markings on the removed MAF body identify it as a SIEMENS unit. The LR part number is MHK100620. I believe this to be the original factory-fitted unit now with nearly 15 years of service.

Replacements are readily available. MAF sensors are used by almost every fuel injected vehicle and this tubular housed design is very common across many manufacturers.

There is a great debate about the varying sources and prices of replacements. Siemens or VDO get top billing and can cost upward of £100 plus. Much cheaper unbranded units can be sourced for £20 - 40. Reliability is a big concern, and the full range of readings is crucial. I guess you pay the money and take the choice...

After years of service there is the inevitable build up of dirt and deposits on the wires which can effect the readings. Often with some VERY careful cleaning, the performance of the sensor can be restored again, but I reckon the best way is to replace the unit.

So just what is the money buying and how does it work...?

Inside the unit can be seen two thin fragile supported wires that are heated and then cooled by the airflow. The changes in ambient temp are quantified by electrical resistance and outputted as a variable voltage..

Or, as Rave  puts it...

".....The MAF sensor works on the hot film principal. The MAF sensor has two sensing elements contained within a film. One element is controlled at ambient temperature e.g. 25°C while the other is heated to 200°C above this temperature e.g 225°C.  As air passes through the MAF sensor the hot film will be cooled. The current required to keep the constant 200°C difference provides a precise although non-linear signal of the air drawn into the engine. The MAF sensor sends a voltage between 0 and 5 volts to the ECU proportional to the mass of the incoming air. This calculation allows the ECU to set the ratio for varying operating conditions...."

And dirty it is. This is a wet wipe that was just pressed into the air filter side of the sensor body. The dirt appeared to be more oily then dusty..... The heated wires inside would probably bake any dust particles onto their surfaces.

The trailing or turbo side of the body was a lot cleaner. It is possible to remove the circular guard to get better access to the inside of the sensor body.

Looking through towards the front of the sensor. If enlarged its possible to see small particles caught in the fine wire mesh grill that sits behind the front plastic grill.

Three photos attempting to show the actual sensor wires in close up. They looked pretty clean to me. In the last photo the curious mirrored surface of the sensor 'arm' base-plate can be seen. I have no idea of its function in there !

Must mention there was a small yellow painted dot on the base of the sensor body inside the unit. It came off with the meths I used to clean up in there. Also, there are small circular indentations on the side surfaces below the wire elements, but I can't be sure if they are drillings or not. 

I got frustrated that the sensor unit itself seems bonded to the casing and is not de-mountable. Since I have purchased a new MAF sensor, I might just break the original down to satisfy my curiosity....

TEST READINGS FROM FITTED MAF.

Target values are the following

55 - 65 kg/hr at idle

500  -  650 kg/hr at load  (3k rpm)

NANOCOM LIVE READINGS 

46 kg/hr at idle  (745rpm)

200 kg/hr no load ( 2k rpm)

250 kg/hr loaded  (2.5k rpm)

note: It was not easy to get the revs up beyond 2.5K which was a concern in itself.

Clearly something is amiss and getting the above results was reason enough to remove the MAF from Hx.

MULTIMETER RESISTANCE TEST.

Target readings accross the 3 pins.

PINS 1-2  = 16.8 K/OHM

PINS 1-3  =  34 M/OHM

PINS 2-3  =  34 M/OHM

THE HX ROGUE MAF

PINS 1-2  =  16.6 K/OHM

PINS 1-3  =  NO reading (infinite)

PINS 2-3  =  NO reading (infinite)

Like I said, there's a new MAF on its way !!!