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MAFM is the Gremlin!
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tikibus
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Location: Rochester, NY
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PostPosted: Fri Aug 17, 2007 8:20 pm    Post subject: MAFM is the Gremlin! Reply with quote

Hi all!

Been grappling with a ongoing issue with the Van that some may have experienced.

Here's the deets:

Starting cold, engine hesitates, misfires, overall not happy. Fumes from tailpipe "gassy" smelling.

O2, Cat, new.

In certain conditions feels like the whole/engine tranny is governed.

Well, Well, Well. Dig three holes in the ground! Rolling Eyes

It is the MAFM that gives the troubles.

Poor start.
Gas eater.
Retarder in gears. Makes the V fight and making you crap trying to figure out WTF is wrong.

Simple fix, heavy on the $, unless a good "pick in' pull" can make the day.

Put in a Pick in' pull- $25 MAFM swapping out the PO MAFM... WOW
Very Happy

Just drove 30 miles to verify. All systems go. The difference is astounding.

Thought I whould share and any thoughts are welcome.

Cheers!
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Mark
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Perales
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PostPosted: Sat Aug 18, 2007 7:26 am    Post subject: Reply with quote

Great story
... what is as MAFM ?
I am perhaps too old to know all the acronyms in use today, sorry.
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?Waldo?
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PostPosted: Sat Aug 18, 2007 8:39 am    Post subject: Reply with quote

MAFM is the Mass Air Flow Meter. Often the carbon track gets worn in spots (especially idle). Often pulling the lid and bending the contacts slightly to rub a different path will make it all good again.

Andrew
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mightyart
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PostPosted: Sat Aug 18, 2007 8:54 am    Post subject: Reply with quote

The early AFM (don't know about the late ones) contain a temp sender,
or temp sender 1.
this measures the temp of the incoming air.
cleaning the tracks or messing with the spring tension won't do anything if this is bad.
It can be checked for resistance using a multi-meter and the bentley manual.
(this is for aircooleds)
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Alan Brase
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PostPosted: Sat Aug 18, 2007 10:11 am    Post subject: Reply with quote

Perhaps the reason some do not recognize that acronym is because that is not the correct name of this part in a Vanagon AFC or Digifant injection. It is an AFM . It does not measure mass, it measures volume. There is a critical, if subtle difference here. Mass and volume vary at different pressures and temperatures and because internal combustion engines are very picky about how their air should be mixed with their gasoline, we really need to measure mass.
The similar part in later injections(VW as well as other brands) DOES measure mass and is called a MAF, usually. Late ones usually use what is called "hot wire" technology. In this, a wire is stretched across the air flow and a current applied. The resistance will vary, and hence the current flow, according to how many molecules of air hit it. Colder air is denser, thus more more molecules per volume. (i.e. liter). Similarly, sea level air has more molecules per liter than air at high mountain passes. Volvo used these at least as far back as 1987.
Our AFM's are throwbacks to 1974. The MAF's have no moving parts to wear out or get sticky.
I sure wish I could upgrade to a more modern part. I sorta have found one to retrofit. It is fastened to a 2.2 Subaru engine. I'm gonna transplant the whole lump.
Al
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Al Brase
Projects: 67 sunroof bug, 67 Porsche 912 Targa, 70 Westy
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Dogpilot
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PostPosted: Sat Aug 18, 2007 10:34 am    Post subject: Reply with quote

GoWesty has made a hotwire MAF to replace the troublesome AFM. It is essentially put into a gutted AFM. This then goes to a small computer to measure the flow and send the appropriate signals to the ECU mimicking the AFM's signals of position and temp. It is a lot more accurate and responds quicker. It is supposed to make the Van have a peppier feel. There is only so much you can do with this system, the computer only has 52,000 lookup points for ignition and fuel injector pulsing based upon sensor readings.

The Digifant is not a very clever computer. Modern computer compute values and compare them to to what it needs to be achieving, then adjusts the motors output until it gets what it is supposed to be. This is why it has a throttle position switch. The TPS is a command sent to the engine for a certain power output. The Throttle Switch on the van only commands three regimes; idle, normal running and full throttle.

The AFM is like a carburetor, it uses air flow and temp to to give a lookup point on its matrix. The AFM does not have a variable resistor. The fuel is metered in zones, so the resistance jumps from zone to zone. The Digifant is only an 8 bit processor, so it only works with numbers up to 32,768. The A to D converters are more than likely only 4 bit so it only measures 4,096 points.

The upstart of all this is the processor of the computer GoWesty puts in the hotwire is more powerful than the Digifant. That and they want like $700 for it.
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tikibus
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PostPosted: Sat Aug 18, 2007 6:45 pm    Post subject: Reply with quote

Dogpilot is there with deets!

Cheers Dude!

For a quite a time hit the old Tik'ers with parts. PO put in a Junkyard special AFM. Digijet mode. There all F'ed up, 24 years makes 'im worse.

Swapped out this Gremlin... 15 mins. tops... Drove with a complete grin ear to ear. WOW. Will it do the job? Ahh, Makes the Van run happy now. Who knows.

The point is that the AFM- Mass Air Flow Meter_ has a lifespan. What ever it is that makes it work and you happy it is FINITE.

Once it goes, the battle goes to other parts that make the Van run for a time, but is a Band-Aid to the fix. My eyes have been opened to this issue, and I hope many folks out there grasp the MAFM or AFM Mystery.

Cheers! Safe Travels to you.
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Happy Trails!
Mark
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Founding member of the Empire State VW Camping Club.
http://www.empirevwcamping.org/
1984 Westy - Tiki
1997 Honda Civic -The Green Hornet
1971 Volvo P-1800E- needs TLC
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