Anyone experienced a bad 3.2 AFM?

[Mike the mechanic]

Has anyone experienced a bad 3.2 airflow meter's symptoms? I am having a problem with my car and have narrowed it down to a bad afm, but not 100% sure. The idle won't stabilize and low load/low throttle input driving makes the engine pop and stumble. Once I press a little more throttle in order to load the engine slighty, it runs alot smoother and just goes. richening or leaning out has NO affect. The a/f mixture meter reads eratically when this occurs, so I can't tell if it's running lean or rich when this is occuring.
All cylinders are behaving equally, so it's not one cylinder misfiring, it is definately a fuel delivery problem. The injectors have been balanced and blueprinted, fuel pressure in the rail checks ok, timing is dead on, etc...
I may run a voltmeter in the car while cruising to measure the afm's output signal, to make sure it is not oscilating.
Anyone have any other ideas short of swapping afm's?
Thanks!

Just wanted to add that the car cannot be driven slowly, as the hesitation and stumble is very pronounced at low throttle input. If I drive aggressively, it goes well.

[Milu]

The symptom I had (twice) was the engine cutting out everytime I lifted off the accelerator. It would idle ok, but play up when driving. On one car it needed to be replaced, on another cleaning it so the flap moved more freely was enough. Life expectancy is about 75k miles in theory, both of mine started playing up at less than 50k. My daily driver 3.2 has 120k with no problem so far.

[andyu911]

I replaced mine couple years ago at 110k. The symptom was more or less like yours. I checked all vacuum hoses before replaced the AFM. Problem solved.

[stlrj]

Before you toss the old one, send it to me and I'll pay the shipping.

Cheers,

Joe

[Rick Lee]

You can open it up by prying off the plastic cap and then adjust the spring tension on the potentiometer. Sometimes this works, sometimes not. That spring will slacken over time and it can be adjusted to make the barn door open and shut correctly. But I think it's a feel thing and may require a lot of trial and error.

[beepbeep]

Are you running flapper-type or hot film? Flapper's potentiometer can wear down at light openings (as most of the movement ocurs at light openings, thus resistance-film wears down), open it up and use some contact-spray and see if it works. Hot-film ones usually get bungled-up when people use open cotton-filter and lot's of filter oil. Use too much and oil breaks free from filter and gets itself onto the Wolfram-wire (which is kept @ 100 deg. C]. When car is shut down, Bosch Moronic heats up the wire to 1000 deg. C to burn all deposits. Consequently, burned oil gets caked on the wire and either eats trough it or just influence it in the bad way so ECU gat false readings.

[Dan Owens]

Often the contact surface is worn on the potentiometer arm where it meets the face material and loses contact in one of the segments. There is a way to move the contact arm to ride on a different portion of the face material although that is only one of several problems it can have. Have you checked the o2 sensor for proper operation?

[Mike the mechanic]

Thanks for all the replies. I have already opened the plastic cover, cleaned the AFM inside out, adjusted, etc.. but no luck. I have had a problem similar to this a while back, with another car, and even though the AFM resistance/voltage tested ok, it caused the engine to stumble. replacing it did the trick.
Today, i noticed that my alternator is overcharging at 16.6-17.2 volts! Maybe this is causing a problem with the Motronic? Injectors getting too much voltage, DME also? Can this be the problem????

[mjshira]

Mike, I have a simular problem and went through changing many of the senors before finding that it must be the AFM unit. What happens in my car is temp. sensitive in that it only happens when the car is at normal opp. temp. and has been driven. Out of nowhere the engine will want to die, giving it gas helps a little but in the worst of cases I have to pull over. In the winter it never happens but here in the east the humid temps seem to make it happen more frequently. If your issue is anything like mine save the $$$ on the other parts and change the AFM.

[marcesq]

Mike:

Did you try unplugging the O2 sensor and then driving.

My car did exactly what yours is doing and I swapped out AFMs with another Carrera and the problem continued. The next day, I unplugged the 02 sensor and it drove like a dream.

Replaced 02 sensor with a 40 buck special from www.oxygensensors.com and haven't looked back since.

[Mike the mechanic]

Marc, thanks for the reply. The symptoms I mentioned are the exact same symptoms encountered with a bad 02 sesor, but i should have mentioned, I run the car without the o2 plugged in. These cars seems to run smoother that way.
Mjshira, thanks for the reply. The problem is now narrowed down to a bad AFM or voltage regulator. I'll post the fix soon.
Thanks!

[Dan Owens]

Once you get the problem sorted out you may want to plug in and use the O2 sensor so the Motronic system works correctly and completely. Unplugging the O2 semsor to make the car run smoother quite often is just masking another problem in the system by allowing the DME to go into override mode. Just my .02 cents worth here. If you're looking to replace the AFM there are a number of companies around that sell rebuilt units.

[Wil Ferch]

Before you go any further you better fix ( replace) the voltage regulator before you fry the entire electrical system which cannot take 17 volts. Look at my write up in the Pelican Tech Section , written in particular for 84-89 Carreras but good for other years too.
--Wil Ferch

[rick-l]

Interesting stuff at http://frwilk.com/944dme/
Click on air-flow meter on the left.

[kstylianos]

Mike,

I talked to JW last week regarding very similar symptoms on Richard LeSchander's car. One of the first things he had me do was test the alt. output voltage (14.0V +_.2). He said the DME computer will act funny, causing strange engine behavior, if the the voltage is too high.

BTW: I'd check it at idle and at higher rpm's. The regulator may be working correctly on the higher RPM range explaining why the cars runs great when its run hard. Just a theory......

I agree with Wil...change the voltage regulator first.

[Mike the mechanic]

Thanks for all the informative replies! Greatly appreciated. I actually removed the voltage regulator from the alternator yesterday, and will receive a replacement tomorrow, as today is Canada Day. I freaked out when I saw the overcharge voltage, I am happy I didn't fry the DME.
Will, thanks, great article!
Rick, thanks for the link.
Charlie, thanks for the info, good stuff.
Dan, thanks for your concern but the DME doesn't really need the O2 to work correctly, on most of these types of engine control systems, The O2 sensor circuit is an "add on" and they do actually run better unplugged. The O2 is needed for a car with a catalytic converter, in order to help it do it's job.
Thanks guys, will post the results of the voltage regulator swap tomorrow night.

[Dan Owens]

Mike,

Glad you found what seems to be the problem. As for the O2 sensor I don't know that I would agree with your statement about the sensor and it's function in relation to the DME but hey it's your car.

[Mike the mechanic]

I found the problem!! It wasn't a bad AFM, and the voltage regulator swap did nothing to change the situation (needed to be done nonetheless). The problem was the crank sensor and reference sensor were reversed. You see, I am running the Patrick Motorsports clutch, flywheel setup with my new shortened G50. The flywheel design dictates the two sensors be swapped in each others positions, and I hadn't done this. Today I will go cruising and I won't come home until the break-in mileage has been achieved!!!!!!
Thanks for all the help guys.
notice my new signature!

[kstylianos]

Good stuff Mike....nice catch.

Now go run the P!SS out of it......