What does the MAF do?

[DrNick]

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

Cheeky sod!

Those are the results with the engine back together.

Since doing up the bolts (cheap garage - you get what you pay for) the car has been as it always was ie no sudden losses of power. Having measured this sensor I think that the car must be down on power anyway over a good one, but I've never driven a good one to compare. What I'd really want is to improve the mpg a bit as I've always felt it a bit low, but as the MAF in its current state is resulting in underfuelling I've a worry that changing it may actually make the mpg worse. Unless there is some peverse law of physics that says that if you underfuel a Merc you get less mpg. Could be the case I suppose as it can't be very efficient.

MB want £180 + VAT, Eurocar parts £162 +VAT and del.

Anybody know anywhere else to try?

Nick

 

[Dieselman]

Cheeky sod!

Those are the results with the engine back together.

Since doing up the bolts (cheap garage - you get what you pay for) the car has been as it always was ie no sudden losses of power. Having measured this sensor I think that the car must be down on power anyway over a good one, but I've never driven a good one to compare. What I'd really want is to improve the mpg a bit as I've always felt it a bit low, but as the MAF in its current state is resulting in underfuelling I've a worry that changing it may actually make the mpg worse. Unless there is some peverse law of physics that says that if you underfuel a Merc you get less mpg. Could be the case I suppose as it can't be very efficient.

MB want £180 + VAT, Eurocar parts £162 +VAT and del.

Anybody know anywhere else to try?

Nick

LOL

It deffo looks like your MAF is K..nackered.

You could try your local diesel injection specialists for a price. Ring around the price may vary.

The reason your MPG goes down when the MAF is Kd is that the engine becomes less efficient because it has less torque per firing stroke. This means you then have to rev it harder to make progress.

This is why tuning boxes and chip tuning can give better MPG in a diesel.

 

[jgevers]

Hi,

The engine control unit (ECU) needs engine load information in order to calculate fuelling, ignition and boost level. The Mass Air Flow sensor is a primary load sensor. I won't go into the workings of the MAF sensor as there are various principles. The MAFs that fail often are based on the hot film principle. As the MAF starts failing either by dirt or cracking of the film, the voltage feedback to the ecu starts to reduce. The ECU sees this reduced voltage as reduced load. In turn it will reduce fuelling and on turbocharged engines it will reduce the boost command by means of reducing the dutycycle of the boost solenoid.

In my experience cleaning does not bring back a dirty MAF sensor. Rather then checking voltage on oscilloscope, it is more accurate to check air mass (g/s) on diagnostic software 'live data'. This will show both 'nominal air mass' (what the ECU expects) and 'actual air mass' (what the ECU sees).

regards,

Job

 

[glojo]

Hi,
In my experience cleaning does not bring back a dirty MAF sensor. Rather then checking voltage on oscilloscope, it is more accurate to check air mass (g/s) on diagnostic software 'live data'. This will show both 'nominal air mass' (what the ECU expects) and 'actual air mass' (what the ECU sees).

regards,

Job

Hi Job,
Thankyou very much for an excellent and very informative message.

Do you know if MAF units have now been modified?

The MAFs that fail often are based on the hot film principle. As the MAF starts failing either by dirt or cracking of the film, the voltage feedback to the ecu starts to reduce

John

 

[DrNick]

The discussion is getting interesting!

But I stil don't understand how a duff MAF increases fuel consumption, as everything is lean, surely.

The only way at cruise that fuel economy can be reduced is if the engine is doing more rpm. The engine cannot be overfuelled as there is no smoke and the MAF is screaming that there is a low amount of air anyway.

I thought the autoboxes had a mechanical lock-up in 4th and 5th so it can't increase the rpm for a given road speed when cruising can it?

Can anyone explain this, or am I not going to improve my mpg with a new sensor. I'll know next week anyway as I've got a sensor coming, but I'd like to understand whats going on a bit more!

Nick

 

[jgevers]

The MAF design for some Bosch sensors have been changed, but not in the way they operate. The airflow guidance has been changed. They're still failing every 30/40K miles or so though.

The reason why the fuel consumption increases when the MAF is faulty is that the ECU reduces the duty cycle to the turbo. This means that turbo boost is less. Turbo boost allows the engine to run much more efficiently. For instance, in order to accelerate and maintain 70 mph without turbo boost it will take more diesel then doing the same thing with boost, because the engine runs more efficiently with boost (needs less fuel for the same energy produced).

I hope this explains it somewhat.

regards,

Job

PS. In my workshop we use Pierburg part no. 7.22684.07.0 for MB and BMW diesels. Works perfect, inluding my own ML270. This doesn't cost anywhere near £100?

 

[Dieselman]

PS. In my workshop we use Pierburg part no. 7.22684.07.0 for MB and BMW diesels. Works perfect, inluding my own ML270. This doesn't cost anywhere near £100?

Do you mean as a direct replacement for Bosch senors as fitted to MB indirect engines as opposed to CDis?

Also is this the same part as fitted to CDi engines.

Having used a Pierburg as a replacement on a VW 90Tdi, I can testify to it working perfectly, with no power loss, even though the measured output is slightly lower on a bench test.

 

[jgevers]

Hi Dieselman,

Yes, it will replace the Bosch sensor providing the plug is the same. All parameters will still work the same incl. target boost etc. The petrol engine requires a different one though.

Regards,

Job

 

[guydewdney]

does the comment about the lack of stomp off the line also relate to petrols? Mine has laways been a little slow off the lights, and i have always driven accordingly - i just assumed it was the two tonnes that had somthing to do with it..

guy

 

[DrNick]

I've changed the MAF now - easy job.

Can't say I've noticed much difference in go. Its probably a bit lighter on its toes but nothing earth-shattering. Probably just the way I drive. Too early to comment on fuel economy, but its certainly no worse but probably not much better either!

Are you sure the computer doesn't need to be reset? How long does the computer need to readjust settings after learning the old ones, assuming it does modify things as it goes along?

Nick

 

[Dieselman]

I've changed the MAF now - easy job.

Can't say I've noticed much difference in go. Its probably a bit lighter on its toes but nothing earth-shattering. Probably just the way I drive. Too early to comment on fuel economy, but its certainly no worse but probably not much better either!

Are you sure the computer doesn't need to be reset? How long does the computer need to readjust settings after learning the old ones, assuming it does modify things as it goes along?

Nick

The engine ECU isn't adaptive, it works on the real time data.

No ECU reset is required.

Measure the outputs again to see if there is a change.

 

[glojo]

does the comment about the lack of stomp off the line also relate to petrols? Mine has laways been a little slow off the lights, and i have always driven accordingly - i just assumed it was the two tonnes that had somthing to do with it..
guy

Hi Guy
Your signature shows a CL420?? slow away from traffic lights must surely be relative????

I know if our car is in 'C' mode it is slower at accelerating, but surely a CL is not 'a little slow off the lights' Are you sure you were not alongside a certain CL55 AMG???

Regards,
John

 

[guydewdney]

no - its graceful, unless full kickdown is applied - and yes its in S mode not W..

its like the throttle is logrithmic - the first inch is nothing, the last inch makes a huge difference - it doesnt seem to get 'on cam' until 2.5kish - which is a known thing, but when up against a mates 735i (R reg) its v slow off the lights..

 

[glojo]

but when up against a mates 735i (R reg) its v slow off the lights..

Crikey, a 735i is no lightweight, I see your point.

Get a diesel Imagine a 400CDI V8 in a CL??

I can hear you down here in South Devon shouting out about all the virtues of this terrific engine!!!!

How does your CL compare to similar powered models?

Sadly down here they are not a popular model and I have only seen one in our Avenue (CL500 in a funny deep purple type colour)

Regards,
John