AFM Mixture - Am I That Lean?

[Smoove1010]

My 87 Carrera has the classic flat spot/hesitation/stumble/jerkiness in the 2,300 - 2,600 rpm range, and I've been combing all the related threads for about week.

Narrowing the causes down, I've checked thoroughly for vacuum leaks in all the notorious places - many old hoses had been replaced as part of my top end project, and she's nice and tight. I did the FRWilk AFM wiper retrace and test - there are no dead spots. I noted that the AFM has been molested by a PO - the mixture screw cover is long gone and the plastic wiper/circuit board cover was clearly pried off and re-glued back on at some point.

Reference sensors and the O2 sensor were replaced about 1,200 miles ago as part of the top end project.

I have no idle or starting issues, but for good measure today I carefully checked the base idle, following the wire-jumper procedure. I then disconnected the O2 sensor and took a voltage reading on the male pin per this recommendation: http://forums.pelicanparts.com/porsche-911-technical-forum/247905-carrera-balkiness-season-2.html#post2260059

Here's the issue: whereas I was looking for something between .2 to .7 volts off the O2 sensor, I could only get it to a maximum of .2 with the mixture screw turned all the way in. While this has improved the flat spot, it's not totally gone, and I'm concerned that I'm running too lean for the good of the engine.

I'm thinking that a PO may have mucked with the spring tension on the AFM, and I'm considering moving the tension wheel a few notches toward the rich side, and then repeating the adjustments and tests. Does this make sense? Any thoughts on how many notches I move the wheel to make a measurable difference?

In the mean time, I've got a Word document that's about 3 pages long compiled from the most authoritative threads on the bucking/stumbling/jerking symptoms that I think I'll turn into an "ultimate" troubleshooting thread.

All thoughts and suggestions welcomed!

Thanks in advance,
GK

[Justin@Athens]

Just send the whole thing off to Bavarian Restoration and for $145 they will refurbish/recalibrate the whole unit. Air Flow Meters - Bavarian Restoration

Per Steve Wong, you need to be between 1-3 turns from fully clockwise/tightened on the AFM adjustment screw. Anything more than that means you have issues outside the AFM.

You could try messing with the spring tension yourself. The thing about it is, the adjustment screw is only for the idle mixture, not the rest of the rpm band...you might get it perfect on idle, but you don't know what it'll be doing at 3500rpm without an exhaust gas analyzer. If you had known figures to bounce your readings off of as you went up the throttle....that'd be useful.

[Justin@Athens]

You might also consider putting a fuel gauge on the rail and seeing if the fuel system is delivering at spec pressure. That is just a no cost test, and those are my favorite kind. You still won't know if the injectors are delivering properly though.

[stlrj]

Bad ignition could be the problem since those are the classic symptoms, not the mixture. If it were too lean, you might notice it a bit down on power and lower oil temperatures.

Cheers,

Joe

[T77911S]

have the AFM rebuilt. the PO may have adjusted the spring tension in the AFM. sometimes people do that to pass emissions.

[Smoove1010]

Originally Posted by stlrj

Bad ignition could be the problem since those are the classic symptoms, not the mixture. If it were too lean, you might notice it a bit down on power and lower oil temperatures.

Cheers,

Joe

Thanks for the reply - the cap, rotor, wires and plugs are all new - could it be a coil or distributor wear issue?

[Smoove1010]

Originally Posted by Justin@Athens

Just send the whole thing off to Bavarian Restoration and for $145 they will refurbish/recalibrate the whole unit. Air Flow Meters - Bavarian Restoration

Per Steve Wong, you need to be between 1-3 turns from fully clockwise/tightened on the AFM adjustment screw. Anything more than that means you have issues outside the AFM.

You could try messing with the spring tension yourself. The thing about it is, the adjustment screw is only for the idle mixture, not the rest of the rpm band...you might get it perfect on idle, but you don't know what it'll be doing at 3500rpm without an exhaust gas analyzer. If you had known figures to bounce your readings off of as you went up the throttle....that'd be useful.

I left it at one turn open, which left me at about .19-.2 volts. I checked Bavarian Restoration out on line, and it looks like they'll do some of the same measurements and adjusting that I can do DIY, but the real value seems to be in the oscilloscope testing and spring tension adjustment. I'd be curious to know if anyone on the forum has dealt with them and what their experience has been. Seems like a reasonable price.

On the mixture screw - I'm trying to reconcile how that adjustment only affects idle mixture since it allows air past the barn door at all times. Is it because the barn door is open such a small amount at idle, while at high RPMs the volume of air passing is so great that the amount passing through the mixture port is a much smaller percentage?

Thanks very much for the feedback.

GK

[Justin@Athens]

I just got my AFM back from them, not yet installed, but it looks to have been well serviced. Reasonably quick turn around, and excellent email communication. Well worth it considering what unknown condition USED units are selling for.

I think you understand the idle mixture situation perfectly, or at least in the exact same manner as I interpret it. Your reasoning seems spot on.

Stlrj was smart to offer the possibility that you could have an ignition issue. Specifically what type of plugs are you using and how old are the coils? Coils don't always fail in a sudden death fashion, they can deteriorate to produce a weakened spark. I pulled a used coil off a buddies 944 for my 964 as they are a shared part. You should be able to find a spare cheap. Be a nice spare to have even if its not related to your problem.

[Smoove1010]

For spark plugs I'm using NGK BPR6ES. These seemed to be acknowledged as the go-to plug on the forum.

Checking the coil and distributor is now on my troubleshooting regimen.

Justin - please post your results once you install your refurb'd AFM. I'll probably do the same when I take her off the road for the winter and start the rear suspension refresh project.

Thanks again,
GK

[Smoove1010]

Keeping in mind the input I received here, I settled on the theory that the engine was adjusted at some point to run lean - perhaps for emissions, or perhaps someone thought it was running too rich and fouling plugs when she was actually burning oil and fouling plugs.

I recalled that when I sent my heads out to Verden, Bob found that one of the exhaust valves was burned (Bob then replaced all the valves.) At the time I suspected she was running lean due to vacuum leakage, but I'm second-guessing that now, and decided to adjust the AFM spring tension lower for a richer mixture.

Today I checked one of the plugs and found the appearance to be consistent with a lean-to-normal running engine, which validated my O2 sensor reading, and gave me the green light to go ahead with the AFM adjustment.

In doing so I believed I discovered a clue in this mystery. (Owning a car with nearly no documentation is a constant process of discovery...)

Just as I was about to put a dab of paint on the tension adjustment wheel to index my starting point, I noticed a scratch on the wheel three notches over from one of the raised pins on the AFM housing:



This indicated to me that a PO or his wrench had marked the wheel earlier before turning the wheel clockwise for a leaner mixture - more validation for the theory!

I moved the wheel counter-clockwise three notches, assuming that the mark was made to align with the pin, though it's also possible that the PO made the mark to align with the wire clip that holds the adjustment wheel in place.

I'm hoping for the rain to stop tomorrow so I can take it out for a ride, warm the engine up to operating temp, re-adjust the base idle, and take a reading off the O2. If it's between .2 and .7 I'll call it good. If not, I'll loosen the spring a few more notches and see where the reading lands.
...more to come.

GK

[Justin@Athens]

Nice forensic work!

[Smoove1010]

Today I gave her a thorough test drive after backing off the AFM tension by three notches (as described above) and while she ran noticeably better, there was still a subtle flat spot though it was at lower RPM's than previously. More concerning was that the O2 sensor output hadn't changed, and was still +/- .2V.

I decided to back the tension off further, assuming that the index mark on the tension wheel was originally aligned with the spring clip. After doing so, it became evident that this was indeed the original position of the wheel as the scratch mark is perfectly aligned with the clip:


...clearly, whomever changed this setting made the scratch alongside the clip to keep track of the starting point. I buttoned it all back up, re-set the idle and took a ride. She runs better now in every way - much better throttle response, very free-revving, no flat spot/stumble/jerkiness at all. She used to idle like a car with a hot cam - very bumpy, but no hunting. Now the idle is as smooth as silk. I can coast along in first gear at idle without any balkiness and accelerate smoothly from there. This is my first air-cooled 911, so I have no frame of reference as to what is normal for these cars - I really didn't know it could run this well!

So, it's all good but for one thing: The reading I'm now getting off the O2 sensor makes no sense - I'm getting roughly .04V when I was expecting to get .4V! On occasion it will bounce to .1. If I rev the engine it might go to .14 momentarily.

I understand that it's important to use a good quality, high-impedance voltmeter when taking this reading, so I'm going to borrow a better one than I have and re-do the test.

So, while my engine has never run better, I still have one unsolved mystery.

...still more to come.

GK

[Justin@Athens]

Nice! Keep us updated. It is always great to see these issues resolved and learn about how other approach problems that a common to many of us. Curious where you sourced your info on 02 sensor parameters for determining proper mixture?

[dicklague]

Have you considered a wideband and senor and AFR instrument that is accurate and fast?

Also it is nice to have one that has RPM, and MAP redings and data recording. Gives you a good idea of what is going on.

[Smoove1010]

Originally Posted by dicklague

Have you considered a wideband and senor and AFR instrument that is accurate and fast?

Also it is nice to have one that has RPM, and MAP redings and data recording. Gives you a good idea of what is going on.

As I've read more on this topic, I'm learning that using voltage readings off an O2 sensor is a dubious practice to begin with as they (apparently) don't output a steady voltage.

I haven't thought of a permanently installed wideband/AFR gauge. I've thought about the best way to get access to better equipment for taking these readings, but the cost of doing so for essentially just one use would lead me to think that I'd be better off taking her to a shop that already has that kind of gear and the expertise to use it.

...unless there's a Pelican on Long Island who's got one and is willing to help me out?

GK

[scarceller]

My advice is stop messing with the tension, the sensor can also be adjusted with the other adjustment.
Please PM me with your email I'll send you detailed doc I wrote about the adjustments for the AFM.

But in summary: if you mess with the tension by loosening it you effect the overall air flow curve such that it results in mixtures that get more and more rich as RPM and air flow increases. And if you tighten the spring it goes leaner and leaner. If it's slightly off you really should use the wiper arm adjustment instead. My doc explains this in detail.

But also be aware that a lot of times you are lean because of some air leak, be sure you have no vacuum leaks in the intake, else you are simply tuning around them.

Measuring the voltage at the O2 sensor is a decent approach for setting mixture and it works like this:

Unplug the O2 sensor and measure the signal with engine warm and at idle, in your case you are at .2vdc and lean.
Before you adjust anything let's richen the mixture for a quick sanity test by simply unplugging the vacuum line on the Fuel Pressure Regulator, this will increase pressure across the injector from 30PSI up to 36PSI and thus the mixture goes rich and the signal at the O2 sensor MUST saturate at > 0.7vdc. This test helps prove that the sensor is responsive.

Other alternative way to force rich mixture is to push the Barn Door in the AFM with a pencil while at idle. As you push the door slightly it tricks the DME to see a higher AFM volatge and mixture richens.

One more trick for rich mixture is to unplug the CHT sensor and mixture also goes rich.

Once you know the sensor can detect lean (<0.3vdc) and also rich (>0.7vdc) then you can proceed to setting base mixture via the AFM mixture screw. Simply turn it out (lean it) till sensor reads <0.3vdc and stop. Then start tuning it in (richen it) and count turns till sensor reads >0.7vdc and stop. Then divide number of turns by 2 and then back off that many turns. You will be in the ballpark for 14.7AFR.

If you can not richen it with screw all the way in then simply move the wiper slightly up the track using the wiper adjustment, no not mess with the spring.

And also please verify fuel pressure before you even mess with the base mixture. At idle the FP should be 29-30PSI with FP Req vacuum hose attached and abut 35-36PSI with hose disconnected. If the FP is off you can NOT get base mixture correctly set.

[Smoove1010]

Thank you Sal for the response. You'll be interested to hear that I noted many, many of your posts in that 3-page Word document I put together while doing my research on this topic!

I don't plan to adjust the spring tension any further, I'm confident that I put the tension wheel back where it belonged after the PO (or other person) changed the setting.

I've diligently looked for vacuum leaks, replaced many hoses and lines, tested those that I didn't replace, installed new intake manifold gaskets and spacers, etc. She passes the remove-oil-cap-stumble test, for what that's worth.

I will, however, repeat the test and try to force the mixture rich (as you suggest) by pushing on the door while my voltmeter is hooked to the sensor, if for no other reason than to validate that the sensor is really working. It's got just over 1,000 miles on it, and was installed as part of the top-end project, so it ought to be good. What isn't making sense is that I was getting a .19-.21 reading before making the AFM changes, now I get +/- .04 with periods where the voltage just jumps around. I'm attributing that to my using an old Radio Shack digital multi-meter. I'll be borrowing something a little more substantial to repeat the test. (As I understand it, I need a good quality, high-impedance digital unit.)

PM with Email address is on the way - thanks for your help!
GK

[scarceller]

You need a meter that's very accurate in the 0-1vdc like 0-1000 millivolts range. I use a Fluke digital meter for most of this work. But even a decent cheaper meter should work.

If possible have the O2 sensor's 2 heater wires powered up and the 3rd signal wire disconnected from the harness. Read the voltage between the signal line and the main DME ground screw on intake runner #1. Put meter's '-' on the ground screw and meter's '+' on the signal line. Put meter on it's lowest DC scale, some meters have 1000 millivolt scale.

Originally Posted by Smoove1010

Thank you Sal for the response. You'll be interested to hear that I noted many, many of your posts in that 3-page Word document I put together while doing my research on this topic!

I don't plan to adjust the spring tension any further, I'm confident that I put the tension wheel back where it belonged after the PO (or other person) changed the setting.

I've diligently looked for vacuum leaks, replaced many hoses and lines, tested those that I didn't replace, installed new intake manifold gaskets and spacers, etc. She passes the remove-oil-cap-stumble test, for what that's worth.

I will, however, repeat the test and try to force the mixture rich (as you suggest) by pushing on the door while my voltmeter is hooked to the sensor, if for no other reason than to validate that the sensor is really working. It's got just over 1,000 miles on it, and was installed as part of the top-end project, so it ought to be good. What isn't making sense is that I was getting a .19-.21 reading before making the AFM changes, now I get +/- .04 with periods where the voltage just jumps around. I'm attributing that to my using an old Radio Shack digital multi-meter. I'll be borrowing something a little more substantial to repeat the test. (As I understand it, I need a good quality, high-impedance digital unit.)

PM with Email address is on the way - thanks for your help!
GK