Maf

[Guest Eric Simpson]

This may sound like a daft one ... but why not ... does a petrol engine have a MAF? It seems to keep coming up with the TDI on the list. I saw one chap had to replace his cat conv to solve his power loss. Oh nooo. That is my problem ... and looking for ideas. It seems the MAF is also a culprit (at least for diesels) B)

[Guest chrisH]

I dont know about the Gal/Alam/Shaz petrol versions but my wifes Fiesta has got one and so has my mates scooby.

 

Maybe its an injection thing!

 

 

Anybody throw any more light on the subject...........

[seatkid]

Mafs are increasingly more common on all types of engines. They are used to aid compliance with ever stricter Euro emission regulations. They are one of many sensors used in the calculation of the "correct" fuelling quantity and to avoid over fuelling in particular.

 

Because the technology is so unreliabe (maybe even deliberately so?) I personally think in their present form they are a mistake. It is possible to calculate air mass using other (plus additional) sensors, engine speed etc. but the in thing is for designers to choose the MAF approach. Remember people like Bosch design the fuelling and engine management, not VW or Ford.....

 

One reason why MAFs are more unreliable on diesels than petrols is that a diesel has no throttle and umpteen times more air (and exhaust gas recirc) is sucked into the engine via the MAF.

[Guest neil_wiles]

Further to Seatkid comments, whilst the inaccuracy of current MAFs are an issue with many manufacturers the main reason that MAFs are not utilised in Petrol engined vehicle is that in a Diesel engine the amount of fuel is metered and the air flow is free and unrestricted but must be measured to adjust the fuel injection period. On a petrol vehicle the fuel is also metered but the rate of flow of the air is controlled by a "throttle" valve, so the airflow is not measured but controlled. The throttle position has its own sensor which in effect acts as a MAF but through control and not passive measurement.

 

New designs of MAF are more accurate and the new Bosch "Hot Film" MAF is so accurate it can measure air pulses caused by flow restrictions in the manifold and engine. This also means the emissions can be controlled far more accurately.

 

The new Hot Film Sensors are now begining to emmerge into the petrol engine market and will further improve the reliability and life of the Cat by reducing further the levels of unburnt fuel.

 

Hope this adds to the discussion point and help to understand the technology.

If you search for the EOBD item that I posted about 10months ago you should see more about the MAF and its control.

[seatkid]

in a Diesel engine the amount of fuel is metered and the air flow is free and unrestricted but must be measured to adjust the fuel injection period.

I believe in a diesel the MAF is used to determine the upper limit of fuelling (before smoking/pollution occurs) for the amount of air being drawn at the current engine revs.

 

When the air filter becomes blocked or air pressure falls or air temperature rises, the mass of air measured reduces and so the ecu reduces max power at that engine revs to stop emission infringement.

 

Unfortunately the shit etc that builds up in MAF (including hot film types) has the same effect - it thinks the air flow has dropped - hence the "no power" syndrome.

Likewise erosion of the hot element.

 

IIRC the ecu has the ability to recalibrate the MAF at intervals so it compensates automatically until a certain level of deterioration. After that, it's time to post the same old question "no power - please help!"

 

 

In a petrol engine the MAF is used to control the fuelling almost directly and gives a much more accurate metering than throttle plate position (plus several other inputs) - I believe many upmarket petrol must have MAFs - my 1978 Saab 99GLE had one - in those days the Bosch fuel injection system used a sprung flap to measure airflow.

 

New designs of MAF are more accurate and the new Bosch "Hot Film" MAF is so accurate it can measure air pulses caused by flow restrictions in the manifold and engine.

Until they get some shit in them....but Bosch ain't complaining, its nice bread and butter work, especially if you're the only supplier.

 

What I want to know is why VAG wont use Pierburg MAFs anymore - the general consesus is they last forever.. :unsure:

[Guest neil_wiles]

Just a little more on the finer points of the MAF, if like mine your Gal does lots of small journeys and you switch the engine off regularly the MAF last much longer as the ECU is programmed to apply a very high current to the wire raising the temp to 2k degrees for a very short period after the engine has stopped to burn off the crap that accumulates on it.

 

If nothing alse this prolongs the life and nothing shows that better than the life of the MAF on a Galaxy that is used as a taxi. It also demonstrates that the manufacturers know the problem of contamination exists and can only attempt to "burn" it off after each drive cycle.

[Richmond]

It might make the MAF work better, but I would expect frequent cycling up to 2k C to shorten the life of the wire.

[Ivor_E_Tower]

I would expect the MAF to melt long before it reaches 2k deg C!!!

:unsure:

[Richmond]

I would expect the MAF to melt long before it reaches 2k deg C!!!

:ph34r:

I think it's only the wire, not the whole MAF, that gets to 2k C!

[Ivor_E_Tower]

Even so, I don't think that there are that many materials that will stand 2000 deg C for very long. We have recently struggled at work to find an insulated cable that will withstand 450 deg C for more than a few minutes (one hour is required and we have ended up stripping off the insulation and using ceramic beads protected by glassfibre sleeving, supported by metalwork, plus caking most of it in ceramic adhesive).

[Guest neil_wiles]

Sorry Gents got a digit wrong its only 1000C when the burn off occurs.

 

Extract from the Bosch text regarding MAF sensors.

 

"Contamination of the hotwire will quickly cause inaccurate readings that can result in loss of performance or foul running. To ensure it is kept as clean as possible, the hotwire is heated to a temperature of 1000 degrees C for a one second period each time the engine is shutdown. This should burn off any potential contaminants. This burn-off signal is triggered by the ECU so it is recommended that you wait at least twenty seconds after shutdown before attempting to remove the connector plug from the ECU."

 

The site also has a mini video of the MAF wire rapidly glowing white and then cooling.