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Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Procedure
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t3 kopf
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PostPosted: Thu Aug 06, 2015 3:51 pm    Post subject: Reply with quote

jetpoweredmonkey wrote:
OK guys, I'm a little stumped here. I popped the lid off of my AFM tonight to see if I could effect any positive change on my stumbly idle. If I unplug the O2 sensor, the idle is just as smooth as you please. Checking the O2 sensor output, all seems normal. Sitting at idle, I can dial the CO screw up and down and get it up to 0.8-0.9V and down to maybe 0.2V. The O2 has a pretty steady output and changes right away when I tweak the CO screw.

With the O2 unplugged and CO set so that the O2 output is about 0.8V or higher, the idle sounds great. As soon as I plug the O2 back in, within a few seconds you can hear the ECU trim the fuel back down and the stumbly idle returns. If I probe the O2 connector while plugged in, I see the normal voltage swinging back and forth across 0.5V.

If I adjust the toothed wheel in the AFM, I can clean up the idle *a little* but it's still not nearly as smooth as with the O2 disconnected. Likewise if I reach in there and manually tweak the position of the air flapper, with the O2 connected I still have that dang lean stumble. The AFM voltage output is nice and smooth with no gaps or drops across its range.

Although the O2 sensor seems fine and isn't all that old (maybe 20-30K miles?), I am thinking about replacing it. I wonder if it's possible that the overall voltage output from the O2 is too low. I guess that's my next move - unless someone else has a better suggestion?


This is almost always an intake leak somewhere after the AFM. It can be pretty hard to track down.
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jetpoweredmonkey
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PostPosted: Sat Aug 08, 2015 12:27 am    Post subject: Reply with quote

Thanks guys for the replies. I did a lot more checking tonight and read all of the threads I could find regarding O2 sensors and the green coax wire. Output from the O2 sensor looks good. It goes into closed loop and the voltage there looks good. I installed a new O2 sensor, no change.

I went back and ran all of the Digifant checks in the Bentley (not the injector resistance, but that does not seem to be an issue). TPS is functioning well, all grounds are in great shape, O2 sensor coax wire checks out including the sheath grounding and infinite resistance between sheath and wire, including while moving the wire around. I can't find a darn thing wrong electrically.

Moving on to the air intake, I pulled off the ISV and crankcase breather, removed and inspected the air boot, reinstalled it and plugged the ISV and breather holes. I had already vacuum tested the charcoal canister ports and those are working fine, but I disconnected them and plugged the port on the TB. Also plugged the ISV port on the plenum. Took the brake master vacuum line off, it holds vacuum fine. Plugged that port also. With all of the above removed and plugged, I tested with propane and solvent for any air leaks all over the intake system, down at the injectors and heads, main manifold boots, no problems. The TB seems fine, I can idle it down pretty low by closing the air bypass, I can't quite kill it, but almost. There is a small whistle with the air bypass screw open maybe 1/2 turn from fully shut, but it seems to be coming from inside the TB. Unless there is actually hole in the bottom of the plenum, which the propane seems to rule out, my intake is solid.

Revisiting my symptoms, the issue is a lumpy idle with the O2 connected which smooths out when disconnected. Adjusting the CO screw to 0.9V makes the idle like glass. Closing it down to 0.8V gets a slight stumble at idle. Leaner than that and it really gets lumpy again. Best running is always with the CO screw fully closed and the O2 unplugged. The off-idle transition seems to suffer a bit, though - small hesitation. With the O2 plugged in, closed loop voltage sweep starts up quickly, the lumpy idle returns. There is no rich fuel smell. With O2 connected, the idle is always lumpy but is still best with the CO fully closed and gets worse when opening.

Adjusting the AFM toothed wheel one or two notches CCW improves the situation both with and without the O2, however the CO screw loses its effectiveness and the voltage can only be cut down to around 0.8V even when opening the screw many turns. I could try adjusting the wiper position, but I'm a little reluctant to mess with it just yet.

It seems like everyone else with a similar problem was experiencing rich running, while I'm lean. Tomorrow's plan, fuel pressure test, because I can't think of anything else to check!
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ALIKA T3
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PostPosted: Sat Aug 08, 2015 2:19 am    Post subject: Reply with quote

Good luck man!

I just came across your post and I feel your pain. I'm having issues too.
I'm reading and hopefully will see what your issue was!

A gallon of gasoline and a match could solve it though.... Twisted Evil

Cheers!
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jetpoweredmonkey
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PostPosted: Sat Aug 08, 2015 1:49 pm    Post subject: Reply with quote

ALIKA T3 wrote:

A gallon of gasoline and a match could solve it though.... Twisted Evil


That's about where I am today!! Very Happy

I obtained and deployed a fuel pressure tester. At idle, the pressure is 32PSI. Removing the vacuum hose bumps it up to 39PSI. If I open the throttle, it drops to 29PSI.

Bentley spec is 29PSI with the hose connected and 36PSI when disconnected.

I did not invest in a super high quality pressure tester, but I did compare the gauge reading to my air compressor source and it reads the same as two of my other good quality gauges.

So, the pressures are off a little bit at idle...enough to matter?
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PostPosted: Sun Aug 09, 2015 9:34 pm    Post subject: Reply with quote

It could be your ECU Shocked

I refused to acknowledge it could be an issue for many weeks. Kept chasing lean running gremlins thinking I could figure it out if I just kept at it. Installed a brand new AFM. New fuel injectors. Installed a fresh O2 sensor. Repeated smoke-test of exhaust and air intake/vacuum. Cleaned grounds and acid-cleaned terminals, replacing many connectors where possible. Even a new throttle body & gaskets.

Went to a local VW mechanic where he put it on the exhaust sniffer and went through all the usual suspects including grounds, AFM CO2 and wiper adjustments, fuel pressure, O2 sensor & it's shielded cable with no improvement shown on the CO2 sniffer.

Eventually swapped out the Digijet, and lean condition went away. California SMOG tester told me it was the best pollution numbers he's seen out of a Vanagon in many years.
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PostPosted: Sun Aug 09, 2015 10:55 pm    Post subject: Reply with quote

I had thought about that, too. The good news is that a rebuilt ECU is about the same price as a rebuilt AFM! Is it possible for the AFM to fail in some way that can't be detected by the Bentley tests? Seems like if those outputs are all OK, it's OK...? I may have to check around with the local Vanagon gurus to see if anyone has a known good ECU to swap in. The ECU does seem like a candidate since the wiring checks out, and the lean condition *appears* only to be a problem at idle.

I did a bit more checking today. I ended up putting the AFM back to the standard toothed wheel setting. I advanced the wiper adjustment just the tiniest hair richer. Set up like so, it is idling better with O2 connected, but not perfect. Transition is clean and I do believe that it has a bit more midrange, but that may just be wishful thinking after all these hours of work! We will see how the MPG does with this small change. I do think it picks up a little cleaner than it did before, though drivability has really not been a complaint.

I also stopped to check and re-adjust the hydraulic lifter setting. I have read about 300 different ideas about where to set them. I ended up setting them to 1 turn. The engine builder had used about 1.5-1.75 turns. I didn't notice any change from this, but it's good to know where they're set.

That concludes my pre-trip fooling around with the Digifant and AFM - I'm leaving for the PNW on Friday. The van made the same trip last year with the same issue and no problems arose, so I'm crossing my fingers that the same will be true again this year! I'll report back if I ever conclusively resolve this issue.
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ALIKA T3
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PostPosted: Sun Aug 09, 2015 11:19 pm    Post subject: Reply with quote

As long as you're not too lean, you should be fine I guess...

I swapped back and forth my 2 air flow meters.

I either get a nice idle but extreme bucking if I press the accelerator too much, like I feel I'm gonna loose the engine on the road, or a bad idling,and stalling at idle but nice power when driving.

I ordered a new AFM from Van-Café, besides a used coil, used fuel pressure regulator and used injectors and ECU, everything is new. Man, those injection systems are so much trouble Evil or Very Mad

What I don't understand is prior to the rebuild I did, the engine was happy (rebuilt twice because a head stud snapped while driving Rolling Eyes )

good luck guys!
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PostPosted: Sun Aug 09, 2015 11:22 pm    Post subject: Reply with quote

Hmmmm...Alika, assuming you have checked out and found those two AFMs in spec...perhaps there is more going on inside of those little guys than we can discover via the usual checks. I sure would be interested to know if your replacement AFM tidies up the running issues.
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PostPosted: Mon Aug 10, 2015 5:35 am    Post subject: Reply with quote

Applause

This is a really useful discussion! I suspect that the AFM on my Oettinger 2500E has been "tweaked" because the top cover has been re-sealed and there is a tell-tale scratch on the spring tension wheel, from where it has been adjusted by about three notches.
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PostPosted: Tue Aug 11, 2015 1:36 am    Post subject: Reply with quote

jetpoweredmonkey wrote:
Hmmmm...Alika, assuming you have checked out and found those two AFMs in spec...perhaps there is more going on inside of those little guys than we can discover via the usual checks. I sure would be interested to know if your replacement AFM tidies up the running issues.


Sure is, I left the van parked over the week end, it stopped doing it...

I'm waiting on the AFM, if it runs good or not, I'm gonna send my 2 bad ECU's to be rebuilt so I have some spares. One of them has the ShoeBox diagnostic connector, I'd like to use it Rolling Eyes
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PostPosted: Mon Dec 28, 2015 5:14 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

Some excellent info here thanks. We have a 89 2.1 Camper and the AFM has a bent flap and is jamming in the AFM body. I want to disassemble the AFM and see if I can straighten the flap out. Does any,one have an exploded parts diagram of the Digfant AFM that they could post here. Thanks Smile
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PostPosted: Wed Feb 03, 2016 9:23 am    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

The info in this thread is great, but I had a few issues/questions when attempting this procedure. My new engine has a rough/stumbling idle and most of my other options have been exhausted so I'm giving this a shot.

My main question: is the voltage reading from the O2 supposed to be completely steady? What would cause it to go up and down?

The green wire of the O2 and the plug on the valve are disconnected. If I turn the CO screw all the way in it is steady at .8-.9v. It will stay at that spot for another full turn out. But then, instead of gradually decreasing as the screw is turned, it just goes all wonky. After about 1-1.5 turns out, it goes from a steady .8 to going crazy between .1 and .7. So basically I can't get it to remain steady at anything below .8.
Is that a problem? I was hoping to get it steady at a lower reading so I wasn't running too rich.
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PostPosted: Wed Feb 03, 2016 12:00 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

I had a similar problem and I may have a solution for you. My digital VOM was doing that same dance and it did not seem to get better no matter what I did with the CO screw. I hooked up an analog VOM and I found that once in a while, the meter would peg at 0 volts. You will not see it on the digital since it only happens for a moment. The only thing that would cause that would be damage inside the O2 sensor. I replaced the sensor and all was well. My bad sensor wasn't that old and had not been abused, so I'm not sure why it failed.

You should also check your VOM test leads, I worked for hours chasing a "problem" that turned out to be a bad lead.
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PostPosted: Wed Feb 03, 2016 12:21 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

Thanks for the help! My O2 sensor is new (<2000 miles), though it's possible I have a lemon.

I think the test leads is the more likely problem and I'll look into that. I have an analog meter too, but the lowest setting is 10v, which makes it impossible to accurately read lower than 1v so I was only using my digital meter. I will test both and see if I notice a drop or any inconsistencies. Maybe I just need new leads for my digital meter.
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PostPosted: Wed Feb 03, 2016 12:31 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

Roger that, I had to go find an analog VOM with the correct 1V range. They are pretty cheap.
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PostPosted: Mon Feb 08, 2016 8:07 am    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Proce Reply with quote

So far I don't have an analog with a 1v range, but I have one with a 10v range that has marks for every .2v from 0-10. I'm getting pretty much the same results as with my digital VOM, except when it is finally steady the analog reads around .6 and the digital reads .8.
Either way, I'm still seeing the voltage dance on the analog that I saw with the digital. Anything below the .6/.8 (depending in which VOM I'm using) and it's all over the place. I got a new digital VOM too because the leads were flaky on my old one but no change.

Maybe it's worth it to find somewhere with a CO meter to get this done right....either that or just put the settings back to what they were before.
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PostPosted: Sun Sep 04, 2016 1:18 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Procedure Reply with quote

Super helpful thread--thx!

I ran into some issues while doing the procedure. Wondering if anybody can help.

1. I had trouble keeping it running with the idle control valve disconnected. Although this was done with the van nice and warm, the issue seemed to improve slightly the longer it ran--until i could get a somewhat rough idle with the ICV disconnected.

2. The multimeter readings were all over the place from step one (idle mixture screw adjustment).

3. Moving on to the next steps of getting readings at different RPMs, it was almost impossible to hold the throttle at the middle range RPMs. I could get it to hold at about 2800--anything below that couldn't be held long enough to take a reading.

I'm assuming that my AFM is pretty far out of whack due to the erratic behavior of the throttle response. Any thoughts?

Thx!
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PostPosted: Sat Sep 10, 2016 8:21 pm    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Procedure Reply with quote

A local mechanic working on my engine pried open my AFM to deal with a misfire issue, ended up being plugs! I could of fixed that, but since it was there for other things I said check it out. I was disappointed when I got it back and I saw that a newly installed re-build AFM I put in 6 months ago had a mangled cover and the Idle mixture screw cover was missing as well. Anyway, I was digging into some 1K-3K RPM hesitation when I decided to replace my O2 sensor. Suddenly I have a bunch of power at low end but now it stalls and smells very sulfur like. Anyway, I followed this AFM adjustment procedure (thanks!) was excited I could do this without a CO sniffer. The previous mechanic had the Mixture screw out 7 turns, I started adjusting there but no matter how far out I came the V reads .9-.98. So I reluctantly took off the freshly sealed AFM cover and adjusted the wiper, slightly, still no drop, then the spring, still no drop in voltage. (did negatively effect run-ability though) On a side note, I heard a rattle in the exhaust at idle and still trying to figure that out but it sounds like maybe the cat might be loose/broken, that would suck cause it's new with only 17K on it and the new GoWesy SS system. Drivability really sucks now and it smells more suffer like ever since I put in the new O2 sensor. Exhaust pressure seems stronger and more even than before but the van doesn't idle well and power is erratic now. What would cause the constant .98v? Sometimes it would be at .1 and running crappy after a re-start and then after about 30 sec. it would go back to 8.8-9.5 and smooth out. Ideas?
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PostPosted: Sun Sep 11, 2016 6:22 am    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Procedure Reply with quote

sounds like your adjustment screw isnt doing anything if the V doesnt change when you adjust the screw. When I have this type of issue I start looking for vac leaks. Thats the type of thing that negates the adjustment screw, already having lot of air leaking into the system. Have you ever run a smoke test on the motor? A cheap way to sleuth out vac leaks is to add pressure to the system w a bicycle pump and spray the joints and hoses with soapy water while looking for bubbles. My 1st attempt at doing this revealed a lot of leakage. But beware! keep the pressure to below 5 psi or you could create new leaks by blowing out a seal.
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PostPosted: Sun Sep 11, 2016 7:11 am    Post subject: Re: Vanagon (Digifant) Air Flow Meter (AFM) Adjustment Procedure Reply with quote

Thanks, .95 V means too rich, so I thought if I had a leak it would read lower V -lean, but with all the feed back loops in the system, who knows. It does stall out if I turn down the mix screw, so the engine is responding just not showing up in V readings. I just wish I had a baseline I could set this AFM to to start from. How many turns out on mix screw, appx where wiper should be at engine off, and were at idle, what's the normal turns out in idle speed plug. The whole thing is such a cats cradle it would be nice to know where to start from with a system that has been tinkered with buy someone else I'm not confident with. Ugh, and I'm supposed to be heading out on a 3K vacation road trip in a week and tomorrow is one of the last days available to work on it, Murphy is at it again!
AZ Landshaper wrote:
sounds like your adjustment screw isnt doing anything if the V doesnt change when you adjust the screw. When I have this type of issue I start looking for vac leaks. Thats the type of thing that negates the adjustment screw, already having lot of air leaking into the system. Have you ever run a smoke test on the motor? A cheap way to sleuth out vac leaks is to add pressure to the system w a bicycle pump and spray the joints and hoses with soapy water while looking for bubbles. My 1st attempt at doing this revealed a lot of leakage. But beware! keep the pressure to below 5 psi or you could create new leaks by blowing out a seal.
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