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[CaptainZok]

One of the lads at work is having trouble with an astra, stealer says fault code is pointing to a maf sensor faulty. He's changed the maf which cured the fault for about an hour but then it returned.
I seem to remember TB saying MAF faults are rare as its usually caused by something else, but what I don't remember. Can anybody give me a clue where to tell him to look?

[Marks DTM Calib]

I can count on one hand the number of REAL MAF sensor failures I have seen.....but I have seen bloody hundreds of MAF sensor fault codes.....

What you have to remember is that the ECU uses the MAF sensor to measure the mass of the air entering the engine, it can then add a proportionate amount of fuel to the air to get good conbustion and then apply a correction based on Lambda feedback etc.

As a check, the ECU also looks at engine RPM and throttle positon (plus others depending on the system) to get a second opinion on if the amount of air going into the engine is being measured correctly.....if it thinks its not it flags a fault.

All well and good I hear you ask but, what does this have to do with MAF codes being raised if its working fine......well, if you have some other route for 'unmetered' air (i.e. air that has not been measured by the MAF) to get into the engine, the ECU will flag a code as the MAF would read low.....reality is there is an air leak.....

So, how many wrongly diagnosed faulty MAF sensors do you think have been changed over the years... !!

My advise to him would be to check very carefuly all the breathers and inlet ducting etc.....look for splits in the corrugated sections and any signs of chafing.

Plus, some do daft things like, leaving a breather hose off or having a loose fitting oil filler cap.....this results in an effective air leak via the breather system.......even leaving the dipstick out can have some effects (again, it depends on the management system!)

Even worse are oiled air filters.....the oil can get drawn through the filter medium and contaminate the MAF sensor element!

If you have a tech2 or similar good diag kit you would carry out various tests like, actuating the EGR and watching the MAF value.....or popping a breather off and watching the MAF value again....

[CaptainZok]

Cheers Mark, I'll pass on your advice in the morning.

[Chopsdad]

That's the reason I joined the forum in the first place.

Started off as Lambda fault and progressed to MAF.  I cleaned breathers, throttle body & replaced gasket and checked all the pipework that was fine.

3m of trying this and that and I eventually had to change the MAF, which did cure the fault but then it was probably only dirty.  Mark has suggested cleaning the element with computer foam clean and I'll do that one day and put the old one back just out of curiosity.

But in answer to your original question - the phrase you were looking for is - air leak  

[TheBoy]

If you have a tech2 or similar good diag kit you would carry out various tests like, actuating the EGR and watching the MAF value.....or popping a breather off and watching the MAF value again....

believe it or not, i don't think tech2 can actuate components and do simultaneous live data....

[Andy B]

Very interesting! The EML on the Astra diesel comes on at around 3200 rpm which I assume is about when the turbo starts to kick in. My cheapo Tech II says something along the lines of EGR fault, which a couple of people have said could be MAF faults. I'll check it out for air leaks on Thursday when I'm off.

[Marks DTM Calib]

Turbo should b on boost at much lower rpm's than that....it is more likely to be an EGR fault as it should be closed at that sort of rpm on a diseasel....

[Matchless]

Has someone blanked off the EGR on the Astra? or is it stuck open?

Egr sticking faults should come up as a EGR feedback error but worth a check.
ECU expects reduced MAF when the EGR valve is open, if its been blanked MAF wont change so should report MAF error but I seem to remember it reporting MAF instead (ie it thinks the MAF is too high rather than no gas flow through EGR)