stock MAF details!

[Ehab]

Ahmet, you might be able to help me out with this ...

I looked at the haynes today, and looked up the specs for the 4 terminals on there, one has the power voltage, and the other has the signal, while the other two are the resistance of the air temperature sensor.

My power was fine (12V)
The signal at idle was around 450mV. The manual claims 150 to 250 mV when barn door is closed, so I guess it was fine.

My temp. sensor, however, had only .5 kOhm, whereas the range was from 3.3kOhm to 1.54kOhm. From your knowledge/experience, do you think that is screwing me over and making my mixture rich and my MPG so low?

BTW, I increased the tension on the spring by 3 teeth, and got an amazing improvement in gas milage. I have like 170 miles and the gas meter is still above half!!! I've never seen that before with my car.

Also, away from the black box where the MAF sensors are, to the left at the edge of the MAF, where it meets the intake rubber tube, there is a hole with a hex screw in it, and playing with that screw affects idle speed, even though it doesn't affect the electrical readings from the MAF ... what the hell is that thing?????


I would appreciate any ideas/input from all.
Thanks.

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Lock Stock '84 944 and two smokin' tires ...

[Ehab]

back on top!

[1.2gees]

Ehab, I've been busy, I'll look into it tonight, and get back to you.
Ahmet

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It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[1.2gees]

It sounds like the air temperature sensor in your AFM is shot, I would think that the wrong signal from the air temp sensor would contribute to your bad gas milage.

The screw you're talking about changes the mixture at idle (Talking about the alen bolt right? There's supposed to be a cap over that so you don't mess with it!).

If I have time, I'll see if your temp sensor reading is cold, or hot (if it reads cold, that'll make the car run richer, if it reads hot, that'll make the car run leaner at a given temp).
Ahmet

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It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[Ehab]

Yes, you are completely correct about the temp sensor. The allen bolt has also been a mystery, but I do agree that it changes the mixture at idle. That bolt was all the way out for as long as I had the car, again, it was tampered with by the previous owner.

So, I guess what he was trying to do was to increase the response by loosening the MAF spring, but then compensating for the extra rich mixture at idle by loosening that bolt to get it back to lean again.

Now that I've tightened the MAF spring back to where it should be, I am still trying to figure out where that bolt should be as well, but the complicating factor is that as the air gets hotter, my idle sometimes drops all of a sudden from 1000 all the way to 700 and starts oscillating!!!

Do you think it is time to buy a new MAF from eBay or something? There is one for sale right now for $50.

Thanks G,
E


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Lock Stock '84 944 and two smokin' tires ...

[Ehab]

Nevermind that last comment, I just remeasured the resistance, but this time, I realized I had to disconnect the connector from the back ... and it came out to about 2.16KOhm, which is within range according to the Haynes. Now the only mystery is why does my idle drop so damn low sometimes?

[1.2gees]

It does sound like somebody tampered with the AFM...

If your idle is set too high, it will 'hunt' it's been that way on many 944s I've seen.

I'd take the car to a shop with an exaust analyzer, and try to set the correct mixture, after you re-index the AFM door spring, and idle speed (which should be around 900 rpm, warm).
Ahmet

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It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[Ehab]

Yes, exactly, it sort of hunts by dropping slowly then all of a sudden jumps up to a higher rpm by a couple of 100 rpms. What I'm trying to figure out is, what determines the lower and the higher rpm limits that the idle hunts between???

From playing around, I found that the lower rpm is more affected by the throttle body idle screw, whereas the higher rpm can be modified by the MAF idle screw ...

I just want to know HOW does the MAF screw change the mixture at idle? I mean what mechanism does it use to increase or decrease the fuel ratio?

[1.2gees]

I believe the AFM door position at idle (spring ressistance?) is controlled by that screw, if you say the signal doesn't change one bit. An electronic (-re)"calibration" for the referance of position vs. signal in otherwords. This adjustment's really not supposed to be used for adjusting idle speed, rather only the right idle mixture (which'll happen to be at about the highest rpm that you can set through the adjustment). Let me try that again, the right mixture will have the rpms highest, if everything else is kept the same. So for a rough adjustment without any other measurements or tools, turn the screw so the idle is as high as it'll go. (backing off 7 full turns is about halfway from full tight, to give you an idea) Then you can set the idle to around 900 rpm, it shouldn't hunt after this.

PS: If you can check the current to your aux. air regulator, see if it opens and closes when the idle fluctuates, I might be able to give you more insight.

Is this helping at all, or am I just rambling along? Do let me know
Ahmet

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It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[Ehab]

Hey Ahmet, this is exactly the advice I'm looking for, this problem has been haunting me for quite a while now.

I guess you are right, I should set the throttle screw and leave it, then set the MAF screw until I get the highest rpm possible at idle, then readjust the idle using the throttle screw again ... Does that sound right?

And for reference, do you have any idea how far away from 'tight' is YOUR screw on your MAF???

Thanks a bunch man, I really appreciate it.
Ehab

[Ehab]

As for my AAV, it is working perfectly, as you recall, I took it out, cleaned it, recalibrated it, and checked the votage at the connector ... it is operating well, I believe.

The rpm is around 1200 when it is cold, and it goes down as the engine warms up, which is a sign that my AAV is working fine.

the only complicating factor is that my alternator is kinda weak, so that when the fans kick in, the idle drops a little bit, but 50 to 100 rpms, that is why I can't set my idle (while the fans are off) to 900, rather, I have set it at 1000, so that when the fans kick in, the idle does not go below 900, and no shaking or shuddering results from it.

[1.2gees]

Originally posted by Ehab:

I should set the throttle screw and leave it, then set the MAF screw until I get the highest rpm possible at idle, then readjust the idle using the throttle screw again ...
Ehab

Yeah, I suppose you could do that (as a rough adjustment that is). Set the idle to let's say 900 through the adjustment at the throttle body, then play with the bolt on the AFM until the rpms are as high as they'll go (then you can back off a little bit more to make it run a bit leaner for fuel economy if you wish, but just a slight bit, less than a full turn of the screw if that). After that re-set the idle speed (I'd go a little under 1000rpm), and that should be it.

Now as for the aux. air valve, see if the valve is signaled to open/close when your idle is fluctuating (Usually, you can get it to fluctuate by setting the idle to something too high, like 1200rpm).

When the weather becomes colder, you can unplug one of the fans so you don't use as much power (and keep on eye on the temp gauge). The alternator problem could very well be just the brushes, or even just the voltage regulator. Fluctuations in power supplied is bad for all the electronics, I'd probably one swap that alternator when you get a chanse (it's not hard at all to change an alternator on an NA 944).
Ahmet


------------------
It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[1.2gees]

My AFM screw is set at about 8 turns from tightest, and the screw has a range of movement of about 14 turns full. At the loose end, it's hard to judge when the screw is engaged, and when it's free, so I go from tightest. 7 Would be midway, but I'd think that a range of up to 3 turns either way from middle is acceptable considering wear, and all mods.

PS: Take my setting with a grain of salt, my AFM door is drilled, spring pressure is altered, so is my ignition timing (ambient air temp), and there's no cat, modified exaust etc.
Ahmet

[This message has been edited by 1.2gees (edited 07-06-2001).]

[Ehab]

Cool, thanks G,
I'll try that, although I'm kinda happy with the way it is running right now (at 1000, no hunting, kicking down to about 950 with the fans) ... I just changed my voltage regulator (thanks to Dave951, who sent it to me for $15 and saved me loads of money) and that has new brushes ... didn't change much except for a little flicker that went away and now I have a steady voltage.

As long as the car is asymptomatic, I'm happy.

E

[Ehab]

Ahmet, sorry to bother you with this, but I just tried playing around with that damn allen bolt, and the way you suggested didn't really 'work' ... Besides making the idle higher or lower, turning that bolt through certain ranges tends to make the engine idle up and down between 1000 and 700 continuously.

I can get rid of the oscillation if I tighten it more, or loosen it away from that 'problematic region' ...

Now, to get the right setting, should I tighten it past that point, or loosen it just to the point right before it starts happening??? What makes it oscillate like that?

Thanks.

[1.2gees]

Well, it sounds like you need to set the idle lower first (through the throttle body). The effect of the AFM adjustment to the engine rpm at idle shouldn't be too big, before you run too rich/lean.

Also, do me a favor, and check to see if the AVV is getting signal to open/close when the rpms surge.

PS: You have to give some time to the adjustment on the AFM to 'set-in' (a few 10 secs before you turn it more), while you play with the throttle openning. I'll try to type in something a little more in dept, but I just got home from a party, and am getting ready to hit the bed.
Ahmet

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It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[1.2gees]

Well, it sounds like you need to set the idle lower first (through the throttle body). The effect of the AFM adjustment to the engine rpm at idle shouldn't be too big, before you run too rich/lean.

Also, do me a favor, and check to see if the AVV is getting signal to open/close when the rpms surge.

PS: You have to give some time to the adjustment on the AFM to 'set-in' (a few 10 secs before you turn it more), while you play with the throttle openning. I'll try to type in something a little more in dept, but I just got home from a party, and am getting ready to hit the bed.
Ahmet

------------------
It's all the driver...
My page over1g@hotmail.com
Porsche owners Gallery.../My 944

[Ehab]

Hey, Right now, the MAF is behaving very well, as it is not making the idle hunt, and I have very little shudder when I let off the throttle. However, I have the bolt set to the tightest position, which seems to be producing the best behavior.

As for the AAV, the signal going to it is continuous power, all the time, and it opens and closes with temperature. The electricity is there to make it close a bit faster than the temperature change of the engine alone, by heating up the bimetallic strip with a coil. So I don't think it has anything to do with the idle speed once the engine is up to normal running temp.

[Nick L]

You keep saying "MAF", but your car does not have a Mass Air Flow sensor. FYI.

[mideastmafia]

he means the AFM. (air flow meter)


SHAUN