Dictys

My car has had an issue that it runs rich through out the rev range and has hot start issues when the 02 sensor kicks in. The car shows 12-13 A/F ratio on hot idle with the 02 sensor disconnected.

Air temperatures are around 38-44c at the moment

Time and time again we have checked the usual suspects and to little avail.

Well, we found the problem we think, the air filter was the wrong way ( the curved section down), due to the bloody arrows.

This restricted air flow to MAF particularly when hot and made the car run rich through out the range. This didn't seem to effect the performance of the car outside idle however.

Cleaned then flipped the air filter over and the A/F ratio is back to just over 14.7 on hot idle with the O2 sensor disconnected!

Have ordered new air filter and will ensure it is put in correct way up and then monitor the A/F ratio's and hot starts.

FredR

Agree that the curved section should be up but what you report does not make technical sense on the face of it -however not everything in life is what it seems. The MAF is measuring air flow be it restricted or not.

If you are running with an AFR of 12 to 13 it will probably run better on full load, especially mid range. I have no answer for what you have experienced- weird!

I am also running well rich on stock maps and cannot work out why. I have St'd it out but cannot fathom what I am seeing.

Let me know if you work it out

Regards

Fred

Dictys

I st'd it out originally, but decided that was not the right way and wanted to find out why it is running rich over the whole range. The idea is get the 02 sensor adjustment to as little as possible so there is hardly any difference between 02 and non 02 maps around idle.

Everything checked out good except the air filter.

FredR

Maybe some of our bright boys have an answer for this. That you have apparently cured the symptom is interesting but what was the cause? The only thing I can think of owuld be if the upsdie down filter element is corrupting air flow over the hot wire thus somehow causing an over read of the air flow to thus throw in more fuel than the actual air flow demands.

I would be interested to know what happens to your MAF signal in ST2 at full load now that things are "normal". It would be interesting to see if your MAF numbers are lower than they were previously at similar temps giventhey seem a little higher than mine at these temps.

Regards

Fred

Dictys

As a test we put an upside down air filter in two other S4's and both started having hot idle issues after a while. Correct side up, no issues.

Something is happening, just not sure what.

Does the flow decrease one way more than the other? or does the smaller gap between air filter element and bottom of air filter housing decrease further when engine hot?

FredR

Some of us believe the inlet plenum is less than optimal in design and thus why Roger's EIS system can help extract a little more than the stock system. Thus on a system that is less than perfect, installing the element upside down may change the air flow pattern in some way that corrupts the measurement thus inducing the MAF to read higher than is actually flowing and thus cause ove fuelling.

Indeed I am wondering if something like that has happened when I fitted the EIS system thus influencing my original sharktune efforts. The ST2 can deal with what I am experiencing- just a bit puzzled as to why it has happened.

Maybe these systems are a bit more fickle than we realise/expect.

Regards

Fred

John Speake

The MAF is sensitive to the imperfect airflow of the stock intake, however Porsche mapped the car taking into account those issues. The screens on the MAF are also an attempt to smooth out the airflow into the MAF.

So if you fit an EIS you can expect to do a retune to get the best from it.

I have seen one car with a foam type engine air filter which was mounted inverted, when more than 1/2 throttle was applied it completely blocked the mouth of the MAF !

Erling G-P

The owner of a Porsche specialist workshop (with 928 knowledge) once told me that inverting the airfilter could 'burn out' the MAF, as it gets too hot.

Supposedly a very common mechanic error - mine did it last time it was serviced, despite me discussing it with him at the time. As I got the impression he had put it in correctly, I only checked after a week had passed and then discovered it was upside down. Didn't notice anything while driving with it that way, and my MAF seems to have survived, so don't know for how long - or under which conditions you have to drive to damage the MAF.

Regards,
Erling

linderpat

I wouldn't have done that - cleaning it isn't a guarantee that everything was removed, and when it goes back in the other way, any paticulate matter that might not have come out from a standard shop vac would get sucked into the intake and ultimately the cylinders. Not a chance I would take.

GlenL

Its a good point to remember:

The ECU is mapped for the _measured_ air flow which is not the _actual_ air flow.

This produces odd effects when people change the air filter and intake tube sections. I recall debates over big improvements from up-stream changes that were really about making the MAF read a higher flow and then the MAF would enrichen the mixture. Pow! More power.

If only there was a tool to change the map...

FredR

Glen,

I would suggest it is the other way round- thus why corrections are required to the theoretical stoich in the mapping and even that is not bullet proof as those with dyno experience will invariably attest to.

Regards

Fred

Ed Scherer

A photo you all have probably seen (I've already posted it at least a couple of times, e.g., the original post), but it's so entertaining, I'll do it again:

(Caption: "Ignore the arrows!")

GlenL

Sometime they point to the top and sometimes they point in the direction of the air flow. Couldn't be simpler!

Speedtoys

That explains a few things that I THINK ive been seeing.

Thanks!

WyattsRide

I'm not at home to check. But, from my latest picture taken, it looks like my air filter may be in wrong. Correct?