Mass Air Flow Sensor adjustment?

I was reading the shop manual and it says that there is a CO adjustment screw on the MAFS. I recently took my car to get smogged and it just barely failed. It passed everything but the CO at 30mph. The max CO alowed @30mph is 6.0 and my was at 6.9. So it is just a hair off. According to the manual the CO can be adjusted by that screw. My question is which way do you turn the screw to lean out the CO? Also how to you balance the MAFS. I don't want to have one leaning things out too much and the other one making things too rich. I want these thing to be balanced. I looked in the archives and found how to balance the throttle bodies, but couldn't find anything on adjusting the MAFS. So I figure I would ask you guys for some guidance

 

Hi Ernie. Glad to see you up an running again! How's it going bro?

That screw is attached to a potentiometer on the MAFS which adjusts output voltage which indirectly affects mixture. I don't have the reference voltage offhand but basically I would adjust and "balance" the two by adjusting (with a multimeter ) to make the output voltage match the opposite MAFS at idle. Having said that - I doubt this needs adjustment at all as the ECU does a pretty good job of picking up small mixture variations and adjusting accordingly using the Lambda sensors. In my limited experience MAFS usually work or don't work and having to adjust one is pretty rare as there are no moving parts to wear out. Are you sure the problem couldn't be something else, like a vacuum leak or a bad 02 sensor?

Shan

 

Hey Shan

For me things are good, for the car not so good. Just when you think you have this thing figured out it throws something else at you. But I will not be defeated by this car. At this point I'm not ruling anything out. I have checked the vacume hoses and they are all hooked up. Since this thing is only off by .9 it has to be something really small. Right now I wanna make sure that the MAFS aren't the problem. Then I'll hit the O2 sensors. Thanks for the advice. I was reading the shop manual and the voltage is to keep the heating wire at a constant temperature. Then as the resistance to this wire changes according to air flow it adjusts the fuel, in combination with the O2, crank position, and cam sensors. Other than that it doesn't tell you much. The manual is very vague. They didn't do a very good job writing them. I don't remember if it even said the amount of voltage that it used to get the wire to the needed temperature. So if you know what it is that would be of great help. Thanks again bro.

 

6% sounds like a lot to me, like you might have some other problem. But if you want to check the adjustment on that air flow meter, see my attached picture. Take a resistance reading between the 2 indicated pins. It should read 383 ohms. Readings are usually within 3 ohms.
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Hey oldman,

Thanks alot that is the info I needed, the picture is of great help too. Should I check it with the car running or off?

 

Ernie,

Try this website.

It lists all Bosch sensors and gives specifications.

A good learning site when trying to understand what these things actually do and what signals to look for.

I used it to close down my crank sensor problems

Cheers

John

http://www.boschautoparts.co.uk/mcSens1.asp?c=2&d=4

 

Thanks John

More great info. I will check the site out.

 

Ernie you have to check it with the car off. Just disconnect the plug and take your reading.

 

Hi Ferrarioldman,

Re: your ohm readings for the maf that number is correct as a starting point right? Or from your experience with 348's that has always been the number or FNA is lazy and always sets them to this number? Or?????All kinds of other engine parameters make up the final tailpipe CO and you remove that cap on the left of your picture and adjust from there while the car is running with a CO meter up the butt or better yet in the header CO sample ports. I'm not telling I'm asking. Is that right?

 

During a 348 school at FNA they told me this was the setting, period. If there was evidence of tampering we would know where to reset the adjustment back to. Those units cost too much to be fiddled with. A properly set up car drives very well and in my opinion these "extra" adjustments are not necessary. Your CO adjustment is automatic anyway so if you try to make any changes, the ECU is always going to try to compensate so that the reading at the O2 sensor is what it is suppose to be. If you try to make big adjustments, you may end up with an O2 sensor that is out of range and then you will end up with a check engine light.

 

FOM,

Thanks for the info. Ernie and I learned something today.


Ernie,

Are you O.K. with how to confirm the proper readings per FOM? I have the multimeter if you need one.

FBB

 

Hey ernie,
I PM'D you. I am having the same problem with the engine missing around 5000 rpm's. I am also not passing emissions. Any info. would be great.

 

Billybob:

I'll give you a call next week sometime to set up a date for us to get together.

Viperguy:

I got your pm and sent you a reply.