krahmlow

I finally tracked down and fixed the last large hidden vacuum leak on my '87 S. (Or so I think.) I tried setting the AFM again. Before, the readings from the 02 sensor jumped around quite a bit at idle. Now they hold pretty steady at .72 volts. But nothing happens when I turn the screw on the AFM. All the way closed, all the way open, the 02 sensor still reads .72 volts. 02 sensor was replaced last year.

So is the AFM shot or am I missing something?

Idle is a constant shudder and acceleration is initially weak. Not really a fun car to drive.

quinnfiske

I have been adjusting my AFM in an attempt to pass the emissions test here (I have a 951). I have found as little as 1/4 turn shows up in the O2 sensor voltage. I understand this screw is an air bypass for idle conditions. As I lean it out, it changes idle speed so I corrrect that with the bypass screw in the thottle body. (You have to jumper past the idle stabilizer valve to do this). Maybe and this is a guess from afar, your throttle body is passing so much air that it is actually moving the barn door inside the AFM. Your idle is really a part throttle condition. A CO meter would make life easier, but they seem to be expensive.

krahmlow

In that case, the question would be why I'm moving so much air at idle. Maybe the barn door is damaged/bent?

Electronically, it looked like it tested okay on the bench.

BTW, there's no bypass on the S throttle body. The ECU sets the idle.

quinnfiske

My thinking is that your throttle is open slightly to maintain an idle. I think that air should be supplied via the bypass in the AFM. That might explain why it does not respond to adjustment.

Another possibility that the bypass is clogged with something and the throttle was adjusted to get it to idle.

Clark's Garage (http://www.clarks-garage.com/) website has a proceedure for testing AFM's.

krahmlow

Oh, I think I'm understanding you. With the AFM bypass clogged and the barn door partially open, the screw would have zero effect. Very clever. I wonder if I can see/get to the bypass if I pull the unit? I can't remember.

I tested the unit on the bench, per Clark's, last summer, and it looked okay, electronically, though I had some doubts. It's hard to tell, really. Sticking a dowel in there, at that angle, I couldn't manage to get an absolutely smooth transition from full closed to full open, so it was tough to determine whether there were jumps in voltage. This was after re-tracking the wiper arm, by the way.

Legoland951

Guys, the screw is ONLY FOR IDLE air mixture. You have to adjust the AFM main by prying the rectangular plastic cap off the top. And, you have to adjust it AFTER DISCONNECTING the O2 sensor.

nickg

correct...and if you don't have a gas analyzwer, you are messing with something that you should not be messing with...end result is the car is not gonna run right.

quinnfiske

There was a post from Danno about these things and he said that the resistance on the wiper path is non-linear, so you may not see a steady change in voltage. I checked mine and only checked voltage at closed and fully open. It was in the car and you need three hands etc. I have never seen one all apart, but I am sure some here has.

Legoland951

You are right Nick about the exhaust gas analyzer. I have one in my garage but I done so many of these I can adjust by ear and almost tell if a car can pass smog by smelling it. To tell you how much difference it makes, I had a "gross polluter" 944 at the smog station" and after adjustment, it passed the stringent California smog on the dyno. People talk about replacing the O2 sensor but I had passed smog in the past with the O2 DISCONNECTED WITHOUT A CATALYTIC CONVERTER before they started to test for NOx with the dyno. This is a testament to how clean our cars run when running properly. All This I am talking about is real life experience not some theory in a book even though it helps if you understand basic theories in a Bosch motronics injection system.

People wonder why AFM can go out of calibration. One reason is people who don't know what they are doing are "adjusting" it, another reason is the contacts on the arm, and also another reason is that its coiled spring operated. As we know, spring rates change slightly over time and can even break causing failure.

951Tom

Legoland, have you ever measured what the O2 voltage reads at idle on a car with correctly adjusted AFM mixture screw? I can get my car to idle down at 840rpm, but as soon as I disconnect the B-C jumper wire and reconnect the O2 sensor, it jumps up to a steady 1100rpm. ISV is new and no vac leaks. 19 in. hg on my vac gauge at idle....22 on decel. Is it impossible to set idle mixture without a gas analyzer? Thanks.

Tom
'87 951

krahmlow

Well, my car already runs like crap, and the idle mixture is clearly off. So that's why I'm trying to adjust it at the screw.

krahmlow

Anyway, isn't this sort of beside the point? Let's assume that I know what I'm doing, and I turn the screw on the AFM. Shouldn't the voltage reading on the O2 sensor reflect the change? And if I get no change, what is the likely problem?

As an aside, I would be curious to know how adjusting the idle mixture screw would put the internals of the AFM out of calibration. And I'm not being sarcastic, by the way. I really would like to know.

quinnfiske

Just a shot in the dark here, but is your TPS switch working? It should click as you move the throttle off it's stop. I am not sure why, but the system wants a signal to indicate the throttle is closed.

krahmlow

Yeah, the TPS is adjusted. I just checked it when I had the manifold off.

quinnfiske

In this post Charlotte944 says the O2 sensor voltage should be .60 to .70 volts. at idle Sounds like you are pretty close. I am in that range also (closer to .60), but I still fail the emissions test around here. I wonder what the real number is?


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