C5 MAF sensor. No difference when unplugged

bencowell · Post by **bencowell** » 09 Oct 2010, 19:27

Dear All,

2006 48k mile C5 2.2 HDI 173 reports P0104 with a clone Lexia. Symptoms are an occasional misfire and hesitation below 2000rpm.

I cannot recreate them on demand.

Unplugging the MAF made virtually no difference to performance or smoothness of the engine, but did bring up a fault code saying the sensor was unplugged.

Do you think it is a good gamble to replace the sensor?

Thanks
Ben

wheeler · Post by **wheeler** » 09 Oct 2010, 19:53

The Airflow meters main function on most diesel injection systems is to monitor if the EGR system is working properly hence the reason it doesent make any notable difference to the running. This also means that an EGR fault can also bring up airflowmeter faults. So this may not neccisarily be a airflow meter fault.

ataxidriver64 · Post by **ataxidriver64** » 24 Oct 2010, 20:51

I am intrigued as to how a MAF is linked to the EGR system.

bencowell · Post by **bencowell** » 24 Oct 2010, 20:59

Shouldn't the MAF notice less air coming in when the EGR is open?

Citroenmad · Post by **Citroenmad** » 24 Oct 2010, 21:06

I dont think thats quite right, the MAF is there to sense air which effects the mixture.

ataxidriver64 · Post by **ataxidriver64** » 24 Oct 2010, 21:22

A MAF, mass air flow sensor, measures the amount of air passing through the air filter for use in combustion,(if it isnt measuring accuratly, your engine wont perform properly) were as an EGR exhaust gas recirculation valve, opens on over run to bybass exhaust gasses back into the combustion chamber to burn it a bit more!!!!
EGR systems do nothing to improve performance or fuel economy, unless its stuck open were it reduces both.
Like DPF systems EGRs should be removed from the car ASAP.

cachaciero · Post by **cachaciero** » 25 Oct 2010, 21:29

O.K lets get logical

A diesel does not require a MAF to measure the air flowing into the engine, unlike a petrol engine.
There is no throttle butterfly in the inlet so for any given engine the air flowing into the engine will be directly proportional to engine revs and air pressure.

So given that the ECU has engine revs as an input and inlet pressure it doesn't need a MAF to calculate the amount of air going into the engine. However an EGR valve complicates things in that it adds gas into the inlet system so now airflow is no longer proportional to revs and pressure. My understanding is that the MAF is used to apply correction to the calculated mass airflow during EGR operation and also monitor correct operation of the EGR system.

Caveat the first bit is true the second bit is my understanding culled from various sources.

cachaciero

KevMayer · Post by **KevMayer** » 25 Oct 2010, 22:31

Why do you get lots of black smoke out of the exhaust if there's a turbo boost leak?

The rpm and pressure will be input to the ECU but, you get lots of overfuelling.

I always believed that this was because the air flow through the MAF (sucked in by the turbo) is giving data to the ECU which enables it to work out how much fuel to inject. But, with a boost leak, the air isn't going into the cylinders but the amount of fuel to match the air flow is injected anyway.

I've had a quick search around the net on the subject and a few sites describe that a diesel engine will run without accurate MAF input using a default ECU map whereas a petrol engine needs accurate MAF input to run efficiently. However, A diesel will run better with accurate 'live' MAF input.

wheeler · Post by **wheeler** » 25 Oct 2010, 23:09

bencowell wrote:Shouldn't the MAF notice less air coming in when the EGR is open?

Spot on

If the EGR valve is open & directing exhaust gas into the cylinders then there is less room for 'fresh' air to fill the cylinders therefore less will come in past the airflowmeter.

ataxidriver64 · Post by **ataxidriver64** » 08 Dec 2010, 21:47

I am afraid you are getting confused between a MAP sensor and a MAF sensor. 2 different sensors doing 2 different jobs, the MAP sensor can tell if the egr is working not the MAF.

Chlorate · Post by **Chlorate** » 08 Dec 2010, 21:59

Somewhat surprised that the ECU didn't throw a hissy when it noticed the sensor was disconnected.
But I'm not really surprised that the engine ran without it, diesels will run perfectly fine on quite a large band of AFRs, as opposed to petrols that usually don't get much below 9 or above 15.

RE the EGR debate:
A MAF sensor will definitely be able to tell when the EGR valve is open; since the engine is aspirating on it's, own exhaust as well as fresh air. So the MAF will see a lower mass of air than it would normally. A MAP sensor, however, probably wouldn't care either way.

RE the original question:
Since it didn't seem to do much, replacing it may be an idea. Would be a better idea to get hold of a car that runs fine and swap them over for a wee bit before making a purchase.

-Alex

wheeler · Post by **wheeler** » 08 Dec 2010, 22:11

ataxidriver64 wrote:I am afraid you are getting confused between a MAP sensor and a MAF sensor. 2 different sensors doing 2 different jobs, the MAP sensor can tell if the egr is working not the MAF.

I'm definitely not getting confused & i'm well aware of what the difference between a MAP & MAF sensor is & how they work and on the 2.0HDi with Bosch EDC15 injection system the MAF monitors the EGR operation. The more exhaust gas that is drawn into each cylinder the less room there is for 'fresh' air hence less comes in past the MAF & thats how the ECU works it out.
Also the 2.0 HDi doesent even have a MAP sensor, the 110bhp version however does have a boost pressure sensor that has a similar kind of roll as a MAP sensor.

JamesQB · Post by **JamesQB** » 09 Dec 2010, 17:38

The MAF measures air density as well, so the ECU can calculate how much oxygen is likely to be present for the given volume of air. Since cold air is denser, the same volume contains more oxygen than warm air. Fuel quantities can then be adjusted for the oxygen present to achieve most efficient burn.

Also, the MAF will be able to detect if the turbo is working or not by the volume of air being sucked in, and adjust fuelling as required to match airflow. It would also detect the changes wrought by an EGR opening, through less demand on fresh air intake.

That's my understanding of the MAF, anyway.