What does the MAF do?

[DrNick]

Can anyone explain the significance of the air flow sensor in the general workings?

For example, the output from the sensor goes up with air flow. This is then fed to the computer, which presumably adjusts the fuelling dependent on the pedal sensor as well.

So if the air sensor starts to read low, then there is more air going into the engine than the computer knows about, so presumably it reduces the fuelling thus resulting in a loss of power.
But if you tread harder on the accelerator, presumably the computer injects a bit more fuel to cause more air to be pulled in, but the sensor always says that the air flow is not increasing, so does the computer then add more fuel to compensate? This would result in overfuelling?

So does the car run rich or lean, or does it depend?

(I suspect my air sensor is failing as I get a loss of power, but I'm wondering if this would also result in poor fuel economy as well)

Not a well stated question, but I hope you get my drift!

Nick

PS Mines a diesel

 

[DriftworksJames]

as far as I understand MAF's usually read low when they go wrong.
This will mean the ECU sees less air, so puts in less fuel. So the car runs lean.

 

[R2D2]

What does MAF do!

Answer : Drive up and down the M4 grinning from ear to ear!

 

[silverarrow]

faulty maf will cause slow accel at low speed ie pulling out at a junction common fault easy to test ie unplug if same then duff main problem is they fail slowly and this does not cause a code to show on the obc best to change at 60 - 80 k thats when they fail in my experience e :bannana: very time the ign is on the element heats up and like all coils they wear out and cannot burn any dust oil debris etc so duff readings

WHEN CHANGING THE AIR FILTER VAC UP ANY DUST TO STOP IT HITTING THE MAF SENSOR :bannana:

 

[grasmere]

Price for a new MAS anyone

Has anyone got a photo of their MAS / MAF.
From what I've read the item seems to be fairly common across cars, is that right? ie same actual item on MB VW etc - the differnece is how much is paid for it - any comments . . .

Also, can anyone on the forum do a price for a new MAS better than a $tealer?

 

[GrahamC230K]

I was expecting someone like Dieselman to step in with the answer to what a MAS does. Maybe he will soon.

The only reference I have sever seen regarding Bosch MAS from a source other than the dealership is here:

http://www.mbclub.co.uk/forums/showthread.php?t=10429


I did a post with my old MAS taken apart to some degree:

http://www.mbclub.co.uk/forums/showthread.php?t=3006

 

[glojo]

What does MAF do!

Answer : Drive up and down the M4 grinning from ear to ear!

I noticed it,

And yes, I am also waiting for Dieselmans contribution.

John

 

[silverarrow]

if bosch part go to a bosch agent not with mbz part but with bosch part num £125 all in from memory less than half mbz price. ONLY DIFFERENCE NOT IN A MBZ BOX

 

[Brian WH]

Although the link is for a Ford the principle is the same and describes the MAF sensor workings and function.

MAF Sensor

 

[grasmere]

I did a post with my old MAS taken apart to some degree:

Thanks Graham, thats the one I remembered - similar but different to what I think is my MAF - I'll get a piccy and post it shortly

 

[GrahamC230K]

Thanks Graham, thats the one I remembered - similar but different to what I think is my MAF - I'll get a piccy and post it shortly

I don't do engines, but from what I recall talking/looking at Jimmy's C25D, you won't have a MAS full stop.

There is a different type of sensor in a similar location, but it's not a MAS.

 

[grasmere]

There is a different type of sensor in a similar location, but it's not a MAS.

Your correct Graham Boo Hoo ! I feel so deprived now

I checked my Haynes manual, and MAF is for eg multipoint petrol injection, see Haynes manual section 4A.10 and on the diesel its simply an intake air temperature sensor, see section 4B.2 with piccys

Well it looked like a MAS for a while, it was very grubby and the bulb at the end of the 2 wires is cracked with a tiny piece missing so I've cleaned it up and will replace it this week anyway.

Thanks for the replies

 

[Dieselman]

Dr Nick

Sorry for the delay I can't sit for log so working the PC is a pain..Literally.

The MAF on your car, yes diesels have them, just not Jimmy's 250TD, works on the principle you have explained. The only caveat being that the accelerator potentiometer acts as the torque limiter and the MAF measures the amount of air ingested into the engine.
Depending on how much torque is required, and how much air is being injested, the ECU calls for more fuel to be injected.

As has been said the MAF normally causes underfuelling due to the dirt on the sensor plate creating insulation, thus the plate stays warmer.
The sensor can fail for other reasons as well, but dirt is the most common failure reason.

Try cleaning the sensor with some carburettor cleaner before condemning it.

As the engine revs rise a faulty MAF will cause the mixture to become progressivly leaner resulting in less torque so using more fuel and revs to perform the same amount of work.

Have you measured the output yet?

 

[league67]

How difficult is to take maf of w208, pictures would be appreciated

 

[GrahamC230K]

How difficult is to take maf of w208, pictures would be appreciated

No pics of a 320, but typically its a pair of quick release clips one end and a circle pipe (jubilee?) clip the other end. The wire just unplugs.

 

[league67]

Graham, thx, but is it close to air filter square box, (complete newbie when comes to engines).

regards

 

[DrNick]

OK I've measured it now.

I don't know how to post graphs so I'll just give the numbers

idle 1.27V
1000rpm 1.52V
1500 1.67
2000 1.95
2500 2.07
3000 2.36
3500 2.4

Didn't do under load as these reulsts seem pretty conclusive
Compared with dieselmans curve, these reading look low but are pretty linear. Maybe a clean up will do the trick?

A contributing cause was a very loose inlet manifold (which was also the cause of a vibration at idle that has recently come on!) I imagine tightening all the bolts to stop the manifold moving about is likely to have a significant impact on performance!)

Nick

 

[Dieselman]

A contributing cause was a very loose inlet manifold (which was also the cause of a vibration at idle that has recently come on!) I imagine tightening all the bolts to stop the manifold moving about is likely to have a significant impact on performance!)

Nick

Now let me think!!

 

[Dieselman]

OK I've measured it now.

I don't know how to post graphs so I'll just give the numbers

idle 1.27V
1000rpm 1.52V
1500 1.67
2000 1.95
2500 2.07
3000 2.36
3500 2.4

Didn't do under load as these reulsts seem pretty conclusive
Compared with dieselmans curve, these reading look low but are pretty linear. Maybe a clean up will do the trick?

A contributing cause was a very loose inlet manifold (which was also the cause of a vibration at idle that has recently come on!) I imagine tightening all the bolts to stop the manifold moving about is likely to have a significant impact on performance!)

Nick

Can you repost your results again after actually putting the engine together this time.

 

[glojo]

Can you repost your results again after actually putting the engine together this time.

John