Unplugging Air Fow Meter at idle

[stlrj]

With my engine warm and at idle, I can unplug my AFM and my engine keeps on running like nothing has happened.

Is this typical?


Cheers,

Joe
86 3.2

[DRACO A5OG]

Whoa Really? Can you post a vid?

[stlrj]

Wish I could.

[KTL]

I would say NOT typical. A friend had an off track excursion that knocked his MAF away from the rubber elbow and the car would not run. In this case the meter was still plugged in, but not secured to the rubber elbow.

[stlrj]

With the bellows knocked off, of course, would give a false signal but how about no signal at all?

Anybody try that?

I know that if I have my base mixture set on the rich side, the engine dies right away but when the mixture is set just right, it will keep on running without even the slightest change in rpm. I may have stumbled on a simple test to see if your mixture is set just right.

Oh yeah, if I set it too lean, the engine speeds up slightly when I unplug it, but keeps on running too.

Jose' aka "Joe"
86 3.2, stock chip etc.

[scarceller]

On my 3.2L it stalls if I pull the AFM plug. But it does sort of seem to want to run for a few seconds before it stalls.

The signal from the AFM at idle is about 1.1 to 1.2 volts at idle of 880RPMs. And if you pull the plug that signal just goes to about 0Volts, I measured it.

Wonder if some chips/DMEs have specail code for this condition? What year is your DME from?

Mine is a 86 DME.

I'm surprised any car runs with a 0v signal from the AFM, but of course some do as we can see from this thread.

[scarceller]

Jose,

Very interesting theory. I have a WBo2 monitor in my car and my mixture is set to AFR=14.0 (at idle) which is rich. I can easily try setting this to 14.7 which is spot on lambda setting (not rich not lean). Then I can also try setting it to 15.5 (lean) as well. I will try this as soon as I can.

Keep in mind mixture is set via the set screw on the AFM and NOT the throttle body idle screw.

WOW, if you are correct on this theory you may have just stumbled upon a great test for mixture setting!

Originally Posted by stlrj

With the bellows knocked off, of course, would give a false signal but how about no signal at all?

Anybody try that?

I know that if I have my base mixture set on the rich side, the engine dies right away but when the mixture is set just right, it will keep on running without even the slightest change in rpm. I may have stumbled on a simple test to see if your mixture is set just right.

Oh yeah, if I set it too lean, the engine speeds up slightly when I unplug it, but keeps on running too.

Jose' aka "Joe"
86 3.2, stock chip etc.

[DonMo]

I can electrically disconnect my afm on my 3.2 and it will idle normally. Any movement of the throttle kills it.

[scarceller]

Great feedback. Thanks.

Now we need to answer why some cars can continue to idle while others do not.

I will do some testing in mine with several diffrent idle mixtures, I'm going to try the following Air Fuel Ratios 14.0 (Rich) 14.7 (Lambda) and 15.5 (Lean).

Will post results once I test.

Originally Posted by DonMo

I can electrically disconnect my afm on my 3.2 and it will idle normally. Any movement of the throttle kills it.

[scarceller]

OK,

Tried various AFR mixtures between 13.8 and 15.2 and no luck on my car, it stalls. But it does seem to have a sweet spot of about 14.3 AFR, at this mixture it does not stall right away, it tries to run for a few seconds and then stalls.

Here's another question: For those cars that do not stall are you running a O2 sensor? would be nice to know.

My car does not run on O2 sensor but I can if I wish.

Thanks.

[scarceller]

One more thought on this. For anyone with a car that runs with the AFM harness disconnected could you see what voltage you have on the signal line of the harness (green/red stripe) wire. Maybe some DMEs have a voltage divider made up of 2 resitors that produces about 1.1 to 1.2 volts wich is the approx. AFM signal at idle.

This is an easy test. Just take your digital volt meter and connect the ground lead to a good known ground then poke the red lead into the back of the harness (pull back the rubber boot on the harnes) at the green/red wire and see what voltage you have when you disconnect the harness from the AFM with the car running.

By the way I have a word doc on the Theory of operation for our AFM devices see here:
http://members.cox.net/widebody/AFM/AFM3.2LCalibration.doc

Many hours of research and toying with AFMs is sumurized in that doc.