Beware: Rebuilt Air Flow Meters

[Steve W]

As the air flow meter output voltage is used to compute both the part throttle AND full throttle loads, it's calibration is important in determining the correct ignition timing and fuel at part throttle, and the correct amount of fuel at full throttle up to about 5000 rpm.

Short version of the story is, customer builds a new 3.4 motor and the dyno AFR chart is extremely rich around 9.5:1 at full throttle. I suggest to him it either a bad head temp sensor, bad fuel pressure regulator, or bad air flow meter. Customer insists all is perfect including a new temp sensor, and rebuilt AFM. "Rebuilt AFM??? What was wrong with the old one?" Customer says: nothing in particular, was just replacing parts to trace a previous drivability issue. I tell him then I am 90% sure your AFM is defective, put your old one back on and get back to me.

A few days later, he calls me back and tells me after reinstalling his original meter, he is astounded by the throttle response, torque and power change - obviously from a correct air fuel ratio. The crap rebuilt air flow meter he purchased came from Fuel Injection Corp. I don't know how these guys calibrate the internal spring tension of the flap, or if there even is an attempt but obviously this one was way, way off - so buyer beware.

[Brent Burns]

steve I have a low miles 87 911 which has a slight miss at 2400 rpms. someone on the forum suggested I may have a worn space on the afm contact patch and there was a way to adjust the afm to avoid this spot. can you or anyone else help me with this. thanks brent

[imcarthur]

Brent

http://forums.pelicanparts.com/porsche-911-technical-forum/288603-fr-wilkes-afm-mod-success-easy-post2648904.html#post2648904

84 Carrera Throttle Flat Spot @2900

Ian

[chris57]

My shop has tried fuel injection corp parts in the past with really bad results. We have tried fuel distributors and afm's for various euro cars. I would not recommend them.

[mjshira]

Thanks for the heads up Steve.

[Por_sha911]

They have a website. They claim to calibrate the unit.
For the sake of peace on earth, I hope the owner is not named Loren.

[ZOA NOM]

I remember the thread that was referenced above. I recall that my "refurbished" AFM required checking with a CO meter and readjusting the mixture. It was set at full rich to protect the refurbisher from liability. If they try to set it without it hooked to your car, it can be lean, which will damage your engine if you aren't aware. Setting it full rich keeps that from happening, but it forces you to have it adjusted before using it.

[ZOA NOM]

double post

[mackskibum]

Steve, if you are still subscribed to this thread, what should mixture be at WOT below 5K?

Thanks

[Steve W]

The idle mixture adjustment screw is different from the way the unit is calibrated internally. If the rebuilder changes the internal spring tension setting (typically looser as what it seems), then the effect on the overall curve output is significant and different from the minor adjustment of the idle mixture can do - kind of like changing the main jet in a carburetor and expecting the idle screw to correct the overall mixture.

I'd target for lambda 0.88-0.89 by 3500 rpm up. At peak torque, you can go slightly richer if you don't have a way of dialing back the ignition advance at that point as you would with programmable EFI. Below this it would depend on your engine and modifications that's best determined on a dyno. For example, high overlap cams can bleed excess oxygen into the exhaust during cam overlap that will fool the wideband sensor into thinking the car is running leaner than it actually is. If you give it the fuel based sole on what looks like a lean AFR, you will actually give it too much fuel and end up reducing torque.

[mackskibum]

Thanks Steve. I have a FI corp rebuilt unit on my 3.2 that I have been chasing a rough idle problem with- you may remember my other long thread.

Last week I took a run with my LM1 to see what mixture was at steady throttle, accel and decel to make sure my issue was only at idle- I didn't want to cook anything (valves) by driving.

My AFR was 14.5-15.5 at steady cruise, 11.5-12.5 at WOT. I was wondering if my AFM was misadjusted as well.

Thanks

Dave