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Accidentally messed with adjustment screw on MAF and now running rich and failing smo

S

s1kboy

New member
Joined
Mar 13, 2013
I have a 1985 244 and while I was cleaning the MAF I messed with the adjustment screw and must have tweaked it way out of proportion. It is running rich and failed smog. The tech said I need a mechanic to look at it. Anyway I can just adjust it back to where it needs to be?

Everything else on the car is fine, I just had to clean the MAF and install a new air filter. It almost passed smog before this. But now that I messed with the adjustment screw it failed smog as it's running rich. Anyway I can find out how to correct this? The screw seems to be able to go infinitely in either direction. It doesn't affect the idle and it runs fine so I really have no way to test this.
 
Connect an LED between the mixture test point and battery positive. Put the positive probe of your DMM set on voltage into the back of the AMM plug, alongside the yellow wire (pin 6) if you want to track where you are while turning the mixture screw. Negative probe to battery negative.
 
cleanflametrap said:
Connect an LED between the mixture test point and battery positive. Put the positive probe of your DMM set on voltage into the back of the AMM plug, alongside the yellow wire (pin 6) if you want to track where you are while turning the mixture screw. Negative probe to battery negative.
Click to expand...


I don't know what you're talking about when you say yellow wire (pin 6).

So negative on negative, but what do I do with the positive? And where do I want to set the mixture screw?
 
s1kboy said:
I don't know what you're talking about when you say yellow wire (pin 6).

So negative on negative, but what do I do with the positive? And where do I want to set the mixture screw?
Click to expand...

Sorry.

I was trying to show you how you could determine where the mixture screw was actually adjusted by watching the electrical result of the adjustment. That appears as a varying voltage on the AMM plug, pin 6 on LH2.2.

The LED watches the mixture test point, so the setting you choose is that which causes the LED to flash on and off with equal time. Kinda tricky. Stop on by and I'll show you and bring a friend with a camera, we'll make a youtube.
 
cleanflametrap said:
Sorry.

I was trying to show you how you could determine where the mixture screw was actually adjusted by watching the electrical result of the adjustment. That appears as a varying voltage on the AMM plug, pin 6 on LH2.2.

The LED watches the mixture test point, so the setting you choose is that which causes the LED to flash on and off with equal time. Kinda tricky. Stop on by and I'll show you and bring a friend with a camera, we'll make a youtube.
Click to expand...

Okay, still not sure what you mean by the pin 6.

You also lost me on the second paragraph. I am very close to passing smog. I just need some help determining which way and how much to turn that adjustment crew. Thanks
 
s1kboy said:
Okay, still not sure what you mean by the pin 6.

You also lost me on the second paragraph. I am very close to passing smog. I just need some help determining which way and how much to turn that adjustment crew. Thanks
Click to expand...
There is not set amount of turns to screw it in. Each on is different. We also don't know what you failed emissions for. You could be lean and failing for NOx or rich and failing for HC.

You have to use the LED trick. Pin 6 is the yellow wire on the AMM connector. You want to adjust it until the LED blinks on and off for the same amount of time each.
 
Lando said:
There is not set amount of turns to screw it in. Each on is different. We also don't know what you failed emissions for. You could be lean and failing for NOx or rich and failing for HC.

You have to use the LED trick. Pin 6 is the yellow wire on the AMM connector. You want to adjust it until the LED blinks on and off for the same amount of time each.
Click to expand...

Let me get some pictures of what I'm talking about. It is a screw which appears to go either direction infinitely. Trust me, I have done everything and this like my 5th retest. I just messed with that screw when cleaning the MAF and installing a new air filter. I know that getting that screw to where it needs to be will score a pass.

Okay, can you please give some more info on the LED trick? Now I know where to plug the positive probe of the DMM. But where does the LED fit into the whole equation? If the negative probe is on the negative terminal and the positive probe is in the AMM connector, where does the LED fit in?
 
I need to spend some time on my 1986 740 GLE that failed smog for high CO.

Yeah, what are your smog numbers? I'm close to becoming a smog repair tech.

I recently learned what the test point does. It's a single wire pigtail hanging from the driver side fender. It shows how many times the o2 sensor crosses 0.45v. I can't poke out how an o2 sensor works right now. Look it up.

Basically when the LED is blinking steadily it means the o2 sensor is no longer reading a rich (.9v) bias or a lean (0.1v) bias.

Problem is, this only works if everything goes else is in good working order.

Getting a car thru CA smog that is this old can be tough. I'm sure you are out close to $400+ failing 5 times! 2001 and up don't even have to go on the rollers and get a tailpipe sniff. It's just an OBD2 plug in test.
 
cleanflametrap said:
Connect an LED between the mixture test point and battery positive. Put the positive probe of your DMM set on voltage into the back of the AMM plug, alongside the yellow wire (pin 6) if you want to track where you are while turning the mixture screw. Negative probe to battery negative.
Click to expand...

Not exactly sure on all those points, but would like to learn. Most have told me the only way to test the mixture setting is with a tailpipe device. Feel free to to elaborate on any details, again thanks !! :nod:
 
Otto Mattik said:
Not exactly sure on all those points, but would like to learn. Most have told me the only way to test the mixture setting is with a tailpipe device. Feel free to to elaborate on any details, again thanks !! :nod:
Click to expand...

I see where cleanflametrap lost you guys. He is saying to use a voltmeter to read the MAF output voltage.


One could graph the 02 sensor voltage but I am still refining my techniques and learning how to interpret the data. It's not simple. You can't just look at it and say it's rich or lean very easily. Modern cars do calculations over a prolonged periods of time and look for a rich or lean bias on the forward o2 sensor in order to trigger a CEL.

On a 1985 there is no CEL. YOU are the computer that gets to do the diag.

I'm still trying to Pass the ASE advanced engine specialist test and this kinda stuff is on that test; the were more than a few questions on interpreting 5 gas analysis.

I wish I had access to an exhaust gas analyzer. I watched my last shop neighbor use his old one and it was pretty impressive.

Anyways, back to your car. Post up the 5 gas chart. Snap a pic would be best. I'm curious and take every chance I can to train.

My experience is getting a 2.2 car to pass is much harder than a 2.4 car. This is because 2.4 adjusts the mixture. It doesn't take much for 2.2 to get thrown off. An air leak, fuel pressure being off, may even an injector leaking; or yes a tampered MAF screw. But just because you THINK it's the problem doesn't mean it is.

I just started playing with our shops Pico labscope. I hope to practice graphing o2 sensor voltages and get better at interpreting it. I haven't learned the software yet. I was trying to use an amp clamp to do a parasitic battery drain test on a Prius today but didn't have the time to devote to getting it dialed in.


On these old cars with no OBD you can't hook up a scan tool and read data. On modern cars I could graph the o2 sensor data, look at short and long term fuel trim, fuel injector pulse width, etc.


OK so sorry if that was too many tangents! High CO is usually too rich. But we also want to look at HC and NOX. It's not a simple relationship between the 5 gasses. They are interconnected but it's not cut and dry. I'm trying to figure it all out myself!
 
ZVOLV said:
I need to spend some time on my 1986 740 GLE that failed smog for high CO.

Yeah, what are your smog numbers? I'm close to becoming a smog repair tech.

I recently learned what the test point does. It's a single wire pigtail hanging from the driver side fender. It shows how many times the o2 sensor crosses 0.45v. I can't poke out how an o2 sensor works right now. Look it up.

Basically when the LED is blinking steadily it means the o2 sensor is no longer reading a rich (.9v) bias or a lean (0.1v) bias.

Problem is, this only works if everything goes else is in good working order.

Getting a car thru CA smog that is this old can be tough. I'm sure you are out close to $400+ failing 5 times! 2001 and up don't even have to go on the rollers and get a tailpipe sniff. It's just an OBD2 plug in test.
Click to expand...

You nailed it here. OP "knowing" it could be fixed by a turn of the screw appears to be wishful thinking. Might have been true on a new 87 240, but not now.

Allow me to repeat:

...this only works if everything goes else is in good working order.

...this only works if everything goes else is in good working order.
Click to expand...

That yellow lead I was asking to be monitored is not the measured air output of the AMM, but the output of the mixture pot -- a steady voltage normally given to the fuel ECU to adjust the ECU's response to the AMM's measured air output which appears on the white/red wire.

The yellow wire voltage varies inversely with the resistance setting of the screw, so if you watch that voltage while messing with the screw, you know where in the range the pot is set. Most of y'all unplug the AMM and check the resistance with an ohmmeter, and come up with some value like 315 ohms, for instance, then pass on this value as a "magic" setting to make things work. Looking at voltage gives you the same info, but you can get it without disconnecting things, watching it while you monitor the CO.

The actual voltage (resistance) doesn't matter. It just corresponds to the limits of the screw's turning, which is, as OP suggested, rather vague in its start and finish. If he would only just try it, he'd smile at the usefulness. This isn't in the books, but there's this "Volvo Problem Solver" book out there which might suggest it if any would.

And yes, that mixture test point follows the ECU's interpretation of the oxygen sensor's output. Back in the day, our high impedance input digital multimeters were still only found in labs mostly, so today you'd monitor the O2 sensor instead of Bosch's test point. But the LED is a better tool for that, because it gives a visually distinct lean/rich verdict instead of you trying to follow the flashing numbers on a DMM.
 
Otto Mattik said:
Not exactly sure on all those points, but would like to learn. Most have told me the only way to test the mixture setting is with a tailpipe device. Feel free to to elaborate on any details, again thanks !! :nod:
Click to expand...

You may be thinking of LH2.0, which has the test point, but can't drive the LED test tool.
 
cleanflametrap said:
Wow, blast from the past. I must have written that 15 years ago. But nothing's changed. :nod:
Click to expand...

Thanks Art. I have used it for years.

Volvo 2.2s will pass emissions.
My 1988 244 NA last week

25/25 test Allowed
HC ppm 45 123
CO% 0.03 0.69
NOx ppm 300 878
 
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I have basic knowledge of how to use a voltmeter. I don't quite understand where the LED fits into all of this though. Or where to put the positive probe. Besides that I think I'm almost there: adjust mixture screw either direction until the LED blinks steadily and evenly. So can someone please elaborate or be a little more clear on what exactly and how I need to do it? I am very close to passing.

I you guys want the full scoop, here goes. After the first fail I did spark plugs and some vacuum lines. Then it failed again so I did the o2 sensor. Then it failed again so I did the cat. It was very close to passing. I did the air filter and cleaned the MAF. I am VERY confident everything else is good to go, I just have to tweak the screw to where it needs to be. I just need some help figuring out how to do it. Thanks.
 
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