[BER]

I read with interest a post on the Pelican BBS a month or so ago concerning idle and low speed (i.e. low RPM's) drivability problems on the Motronic-equipped 911's, 924's and 944's.

Basically, the symptoms are a rough idle and the car just running rough up to about 3000 RPM. This asumes all other systems are properly tuned and functioning properly and you have no vaccum leaks which is a whole different "kettle of fish".

In my case, I tune my '84 Carrera every year. I install new plugs, rotor, and cap; I change the oil and replace all filters (air, fuel and oil), and finally I adjust the valves. Oh, I also adjust the idle speed if needed. My O2 sensor and reference sensors are functioning properly.

After doing the above, the car should run smooth and purr like a tiger. Not so, because lately the car still idles rough and surges occasionally at low speed when driving around town.

The Pelican post intreged me because it described problems identical to mine. The cure is to open up your AFM and do a little cleaning and make a critical adjustment to the air flow wiper assembly. Let me stop and reference the following website which provides some excellent Motronic information and describes the adjustment I made to my AFM:

http://frwilk.com/944dme/

Do not be concerned that this website only refers to 944's. The 3.2 liter Carrera's used the same late-DME (ML3.1), so the basics and Motronic theory apply to both 944's and 911's.

Once you access the website, click on the "Air-Flow Meter (AFM)" link on the left side of the page. This section of the website tells you most everything you want to know about the AFM.

The problem many Motronic Porsches have is the wiper arm will wear into the wiper track and cause continuity problems with the electronic pulses sent to the Motronic ECU "brain". Since the wiper arm sweeps across the wiper track hundreds of thousands of times, it stands to reason a "track" will be worn into the ceramic wiper track material and cause continuity problems.

In my case, that was the problem. There were two visiable tracks where the two wiper arm "tits" contacted the wiper track. In the idle region of the wiper track, it appeared the wiper arm track had completely worn through the ceramic material.

Let me stop and discuss for a minute the removal of the AFM. I have the stock air cleaner assembly, so I first had to remove the air cleaner assembly and AFM as one unit. First, remove the air cleaner cover and air filter. Then there are two nuts and a vacuum line that need to come off the air cleaner assembly. The AFM has one electrical connector that needs to be pulled off. There are two nuts that support the front of the AFM and finally loosen the large hose clamp around the rubber air intake boot. Once the AFM and air cleaner are out, I seperated them by removing the 4 nuts that hold them together.

What you need to do now is gain access to the electrical innards of the AFM which are located under the plastic cover that is epoxed in place by the factory. This is done VERY CAREFULLY using a razor knife. I suggest you wear thick leather gloves just in case the knife slips; spending the afternoon at INOVA Fairfax Hospital is not my idea of a fun.

Basically, you run your razor knife around the seam between the plastic cover and the metal case of the AFM. Just go "around and around" and pry the cover a bit at a time; be careful because breaking the cover is not a good idea. It will eventually come loose. Once I got the cover off, I went around and carefully removed all remaining epoxy material taking care that none fell into the AFM.

This is what you will see:



At this point, you will clearly see if you have "tracks" worn into the wiper track which is "1" in the above photo. What you want to do is reposition the wiper arm onto fresh ceramic material on the wiper track. You do this by loosening the 7mm nut on the wiper assembly retainer (item "7" in the photo). In my case, I looseded the nut and mover the wiper assembly down the shaft that it is connected to. This moved the wiper arm out on the wiper track so the wiper is now riding on fresh ceramic material. "Never touch the Phillips screw holding the wiper in place as this will move your angular set point."

While I was in there, I also cleaned all connections and grounds. You can use any electrical contact cleaner or just plain rubbing alcohol along with a Q-tip. I cleaned all electrical/ground contacts with alcohol and a fresh $1 bill because the raised ink on the $1 bill provides just enough abrasive cleaning action without removing any metal material.

Finally, I replaced the plastic cover using black RTV sealant and let the AFM assembly sit for an hour to let the RTV cure. I then re-assemblied the AFM and air cleaner housing and re-installed them on the engine.

Now for the road test...we like road tests!

Let's just say the car drove like new! Idle is rock solid and no stumbling or other signs of rough running. I wish I had known about this simple fix five years ago.

In summary, the total time invested was about 2 hours and the cost was about $10 for a tube of black RTV. I highly recommend this easy repair if you are experiencing any of the drivability issues I described in my opening paragraphs.

Stay tuned for my next post about refurbishing your Motronic flux capacitor.

[Jazzbass]

Nice work, good write up.

[Jase007]

Thanks Bruce.

I've used / referred others to that site (even when it was down) from the BMW crowd as well (E30 and other AFM users) as it applies to the same Bosch units there (as well as other makes).

Did the arm / wiper mod on one AFM in one of my E30s and will eventually do it to the other as well. The thing I have to research / source is a replacement for the factory installed intake air temp sensor in the AFM that also often goes bad / supplies no / bad signal.

Cheers.

[smdubovsky]

Originally Posted by Jase007

replacement for the factory installed intake air temp sensor in the AFM that also often goes bad

Hmm, I've graphed that once. I threw a spare E30 AFM in the temp chamber to get points to calibrate the datalogger. I'll look for it...

[BER]

Jase,

My air temp sensor tested within spec, so I did not mess with it other than to clean all electrical connections. I guess I was lucky.