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  #1  
Old 15th-December-2012, 15:15
NikTheGeek's Avatar
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Default MAP & MAF Sensor question

I was just reading what the MAP sensor did here: MAP sensor - Wikipedia, the free encyclopedia

It says engines typically use a MAP or a MAF but seldom both. Do our P38's use both then. I know I have a Pierburg MAF and I think I have a MAP on the fuel filter body.

Anyway being bored today and playing with the car, I decided to unplug both and go for a run. I couldn't tell a blind bit of difference. No warning lights, no difference in performance, nothing.

I know a dodgy MAF will cause over-fueling, which may account for my average 16mpg, but since I've had the car on an Autologic diagnostics, wouldn't they pick up a failure in either MAF or MAP? Or not?

Mind you, they said my EAS was ok to, but EAS Unlock detects loads of faults when I click on "read faults".

Just curious.

Nik
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Old 15th-December-2012, 15:17
Saint.V8's Avatar
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Default Re: MAP & MAF Sensor question

The later model Diesels use a MAF to give information to the ECU on the use of the EGR valve....

All models of the Diesel use a MAP to tell the ECU the boost air pressure and the ECU uses this to measure out fuel injection quantities....

On the P38 Diesel, the MAF is just for EGR operation only and not for fueling information...unlike other Diesels which may use a MAF for fueling in the same way the P38 uses the MAP

The petrol P38 uses the MAF for fueling information....

With the MAP disconnected you should have noticed an appreciable drop in power....
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Old 15th-December-2012, 15:23
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Default Re: MAP & MAF Sensor question

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Originally Posted by Saint.V8 View Post
With the MAP disconnected you should have noticed an appreciable drop in power....
Thanks for the explanation. Very useful and interesting.

Anyway, because I didn't want a "check engine" light to come on that would require clearing, I ran it at tickover, then when no light came on, I took it round the block. So it was cold and did about half a mile. It wasn't really a fair test, but I couldn't tell any difference from cold.

Will I do any harm (other than over-fueling) if I leave it off and take if for a 5-10 mile run to see if I can tell a drop in power? And then, if not, I guess I need a new one!

Nik
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Old 15th-December-2012, 15:28
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Default Re: MAP & MAF Sensor question

Other than mis fuelling, I am fairly sure it will do no mechanical harm....

You may not get a Warning Light as the ECU is only responding to the information it receives from the MAP, which will be wrong, but the ECU doesn't know this so will just fuel the engine based on the wrong information...

When the engine is cold, the ECU is using a Cold Fuel map and be injecting more fuel than normal (Like using a choke on a petrol engine) until it warms up and then starts to measure the fuel quantity from the various engine sensors.

Running with the MAP disconnected should do no mechanical harm, nor issue a fault or warning light...but the poor thing will not like going up hills or be a spritely as normal....
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Old 15th-December-2012, 15:31
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Default Re: MAP & MAF Sensor question

As attached from RAVE....
Attached Thumbnails
map-maf-sensor-question-p38_dse_map_maf.jpg  
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Old 15th-December-2012, 17:13
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Default Re: MAP & MAF Sensor question

Quote:
Originally Posted by Saint.V8 View Post
The later model Diesels use a MAF to give information to the ECU on the use of the EGR valve....

All models of the Diesel use a MAP to tell the ECU the boost air pressure and the ECU uses this to measure out fuel injection quantities....

On the P38 Diesel, the MAF is just for EGR operation only and not for fueling information...unlike other Diesels which may use a MAF for fueling in the same way the P38 uses the MAP

The petrol P38 uses the MAF for fueling information....

With the MAP disconnected you should have noticed an appreciable drop in power....
I'm sorry that is just not so. BMW introduced the MAF to meet stricter emissions regulations. The ECU uses the MAF to measure airflow in the normal way and adjusts the fueling to suit. It allows compensation for atmospheric pressure, air temperature etc which did not exist before.
This has been flogged to death before.
As above, check the pipe is not split and the spigot on the manifold is not blocked.
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Old 15th-December-2012, 17:19
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Default Re: MAP & MAF Sensor question

Quote:
Originally Posted by Saint.V8 View Post
The later model Diesels use a MAF to give information to the ECU on the use of the EGR valve....

All models of the Diesel use a MAP to tell the ECU the boost air pressure and the ECU uses this to measure out fuel injection quantities....

On the P38 Diesel, the MAF is just for EGR operation only and not for fueling information...unlike other Diesels which may use a MAF for fueling in the same way the P38 uses the MAP

The petrol P38 uses the MAF for fueling information....

With the MAP disconnected you should have noticed an appreciable drop in power....
Oh my you is getting gooderer and gooderer MAF is for EGR control only some will dipute that but it's the case never the less.
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Old 15th-December-2012, 17:21
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Default Re: MAP & MAF Sensor question

Quote:
Originally Posted by Saint.V8 View Post
As attached from RAVE....
Note it say's that the MAF is used to measure air volume and temperature. Yes it's also used to control EGR but air temperature and volume are what matter and RAVE confirms these parameters from the MAF are used to modify the fixed fueling data in the ECU.
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Old 15th-December-2012, 17:26
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Default Re: MAP & MAF Sensor question

The MAF measures the incoming air temp
The MAP measures the charge air pressure

Using these two pieces of information the ECU works out the mass and density of the air inducted and controls the EGR to ensure the burn will create emissions below a certain standard.

Read the RAVE extract above it explains the operation of the MAF....in one respect it replaces the IAT sensor on pre-EGR engines....and that is the only thing within the MAF that has any bearing on fueling information....BUT the MASS of the AIR which is what the MAF primarily measures is for EGR operation only....

On other diesel engines the MAF primarily is used for fueling....on the P38 it isn't...the Temperature is the only thing it supplies to the ECU to vary the fueling quantity nothing more.
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Old 15th-December-2012, 17:28
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Default Re: MAP & MAF Sensor question

Quote:
Originally Posted by Datatek View Post
Note it say's that the MAF is used to measure air volume and temperature. Yes it's also used to control EGR but air temperature and volume are what matter and RAVE confirms these parameters from the MAF are used to modify the fixed fueling data in the ECU.
But it is only the temp it uses for Fuel quantity....as they didn't fit an IAT sensor on the EGR engines...if they did...the MAF would only be used for EGR operation....that is its primary function, it just so happens the model of MAF they used has a built in Temp Sensor, so they used that and saved a fe wquid not having to fit an IAT aswell.....
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