C5 2.0HDi antipollution fault + loss of power

[cazshie]

Got a C5 2003 150k miles that has been pretty reliable but has just started giving a problem.

Yesterday I thought it felt like it didn't have much acceleration on a motorway journey home (in heavy wind and rain) but wasn't sure if it was just down to conditions.

Today there was a definite loss of acceleration and it started showing an "antipollution fault". Seems to be fine until I have it in a higher gear and try to accelerate whilst below 2300 revs. However, if I drop a gear and accelerate it will do so very slowly until it hits about 2200 revs and then it kicks in and accelerates as normal. If I go up a gear though and the revs drop back down to 2000 revs I lose acceleration again (although it will maintain speed).

Anyone had this problem or have an idea as to what the problem might be?

[myglaren]

I had very similar symptoms that turned out to be the MAF sensor.

[cazshie]

What does an MAF Sensor do, and why would a faulty one cause power problems (and an antipollution warning) at >2200 revs but be fine >2200 revs?

[Gibbo2286]

It measures the air inflow and sends signals to the computer, I can confirm that the diagnosis given may be correct, it was on my C5 when I first acquired it. Easy to replace and available on Ebay for the mid thirty quids.

[CitroJim]

What does an MAF Sensor do, and why would a faulty one cause power problems

MAF - Mass Air Flow. It measures the mass of air being consumed by the engine and this helps the engine ECU determine the amount of diesel that should be injected for the conditions the engine is operating under. Measuring airflow in this way gives the ECU a reasonable estimate of how much load is on the engine given the prevailing engine speed and accelerator position.

A 2.0 HDi will run with the MAF sensor faulty and even disconnected. In fact I believe that it will run with it totally disconnected although I've never tried this. I believe further that if the engine runs better with is disconnected then that's a sure sign it's faulty. Don't do this unless someone else can corroborate this and don't do it unless you have access to a Lexia to reset the fault codes it will generate through being disconnected.

So, if the MAF is duff, the engine will run but fuelling will be wrong and it will not develop optimum power.

[cazshie]

Thanks for all the replies posted so far.

Slight change of symptoms, similar to above but now car won't rev above 2500.

Does this still point to an MAF Sensor fault?

Also, where is the MAF Sensor and how easy is it to replace?

[CitroJim]

Also, where is the MAF Sensor and how easy is it to replace?

Just after the airbox. It's a round thing with a medium sized electrical connector going to it. It's held by a jubilee clip each end and just pulls out after loosening the clips and disconnecting (with ignition off) the electrical connector.

If the engine won't rev above 2500rpm then it's gone into limp home mode. You really need to get the car on a Lexia diagnostic to see the cause and confirm it is the MAF sensor causing it.

[centurus2400]

The symptoms are very similar to those which my car had, and which proved to be the MAF.

Try running the car with the MAF unplugged. It will show a MIL light, but chances are the performance will be greatly improved.
If so, it will definitely be the MAF.
On the C5, or at least on my 2001 model, it's held by a jubilee clip at one and and a couple of torx screws at the other.

Don't buy one of the cheap 30 quid jobbies off ebay, their lasting qualities are dubious to say the least.

If you shop around you should be able to pick up a genuine Siemens or Bosch replacement for £80-90

As a matter of interest when I put mine on a Lexia, the MAF should have shown a maximum of 150 (whatever units its calibrated in) at idle.
The actual value was 700, so no wonder it wasn't running very well!

[Gibbo2286]

Sorry to disagree centurus2400 but the 30 quid jobby from Ebay is likely to be just as good as the £90 one from the dealer. they're probably all made in the same factory in China anyway. Mine has worked fine now for nearly two years, it's also fairly easy to see the most usual damage, if you look down inside the tube you will see what looks like a couple of light bulb filaments, they tend to corrode and drop out.

[centurus2400]

The important thing is not to stroll along to the main dealer and buy one, I was quoted £138

[CitroJim]

Sorry to disagree centurus2400 but the 30 quid jobby from Ebay is likely to be just as good as the £90 one from the dealer.

I think this may have arisen from a truly bad batch of MAF Sensors. I don't believe they were for PSA cars. I seem to recall they were for Dagenham Dustbins.

I agree, all likely made in the same factory, just different labelling... It's the case with lots of stuff these days.

[myglaren]

Mine looked like this inside:


click for larger image

It was soaking wet when extracted too
This was the second one, ruined by sucking up water/salt/grit last winter, caused by the wheel arch liner being missing.

Dealer part, as is the current one @£135 as I was advised against the use of pattern parts.

An electronics wizard fried was appalled at the cost as the parts involved are quite inexpensive (he designed and built them for power stationsand oil/gas rigs. His were £1,000 a go though.

The thermistor also fails on them (the little green bead) that measures the incoming air temperature that helps calculate the charge.

They can sometimes be cleaned (carburrettor cleaner) but on no account actually touch the fine Platinum wires with anything.
They acumulatea bit of muck over time that can affect their performance but by your description I reckon yours is beyond that.

You could try unplugging it and treating the plug and socket contacts with an electronic cleaning spray (Not WD40!)

If you fit a new one, retrieve the large 'O' rig seal as they do not come with the replacement unit.

Incidently, a simple resistance test is innacurate as they generate a pulsed signal, the frequence depending on the airflow.

YouTube Video

[Citroenmad]

Dealer part, as is the current one @£135 as I was advised against the use of pattern parts.

The old C5 estate 2.0HDi we had was fitted with a pattern MAF by the local Citroen specialist, it didnt work at all very well and was replaced with a genuine Citroen item, which transformed the car.