My AFM education

[hansh]

Today I received a new FPR. My AFM had been adjusted before the FPR went bad (by PO in some fashion - it was lean). The AFM had then been adjusted when the FPR was bad (lean under load although I had a mechanic check it). Installing the new FPR meant that I needed to accommodate the change in fuel delivery with an adjustment of the AFM.

I just spent the last 4 hours driving, adjusting, driving, adjusting, driving, adjusting, driving.... I thought that I fully comprehended Colin's explanation until I actually did it. Now I think I get it but, I have questions still.

The issue I had was that I could meet adjustment criteria regardless of the position of either the swiper or the cog. What I mean is that I could have the swiper really rich, lean out the cog and, get everything to settle into a nice idle without evidence of a rich or lean mixture at idle or 3k rpm found by slight adjustments of the sweeper. Vice versa was also possible.

How is the swiper position found without a sniffer? I can evaluate each setting via performance. Start at one extreme, going to the other, and then narrowing it down but, it takes forever! Any advice on choosing an initial swiper location given unknown adjustments to a cog wheel?

[Amskeptic]

How is the swiper position found without a sniffer? I can evaluate each setting via performance. Start at one extreme, going to the other, and then narrowing it down but, it takes forever! Any advice on choosing an initial swiper location given unknown adjustments to a cog wheel?

This is a 3D matrix. Although you can, as you pointed out, end up with the same mixture by swapping wiper position on shaft with spring tension on cog, there is only one true path to Mixture Enlightenment.

The spring is a critical adjustment. You can do a rough check of spring tension by blipping the throttle. If it is stoopit loose, it will wing on over to the left, and you won't perceive the fact that you have just richened the hell out of the mixture during the rpm climb. You might see a good blast of soot out of the tail pipe, though. If the spring is too tight, but the wiper has compensated for it by being adjusted rich, you will have a much tighter movement when you blip the throttle. Here, the fascinating indicator of too much spring is a wiper that dives shut when you release the throttle, sometimes causing a dip in rpms as the engine settles back to idle (the decel valve vacuum signal has to be disconnected for this test).

My adjustment procedure does assume that the presenting AFM has NOT been dorked with to a terminal degree before we even got there.

I have been very successful about 95% of the time with the
a) check idle first
b) check 3,000 rpm next
c) adjust idle parameters
d) adjust 3,000 rpm parameters * which will affect idle again, but last idle adjustment will be the
e) mixture screw for best idle

I think you can ask vwlover77 about our adjustment of his AFM, and who else was it? someone here found we had gone too rich on acceleration where it was stumbling under full throttle ( that is a *wiper* adjustment only, because at full throttle, the flap is open all the way no spring moderation whatsoever at full open).
Colin

[hansh]

If it is stoopit loose, it will wing on over to the left, and you won't perceive the fact that you have just richened the hell ou

Is this meant to mean on normal acceleration or a sudden push of the throttle?

I do have somewhat of a starting point but, there are a few marks. I was just curious. I can get the idle absolutely perfect, good acceleration, and good temps when it is at full operating temp. I have a rich-running issue that presents at restart and cold start though and, I am currently exploring those problems as likely unrelated to the AFM. I think that I've been able to get a grasp on the AFM based on your descriptions. Thanks.

[Randy in Maine]

I would suggest renting or borrowing a LM1 or equivalent device to get some numbers to document where you are at to start with and where you need to be going.

[hansh]

Thanks Randy. I do have a mark from a local VW mechanic who checked the mixture. It was slightly lean under load though but, I can work from his setting. I'm not completely in the dark on the setting but, was just curious about an initial setting of the cog. I've posted the cold start issue on The Samba as it seems to pertain more to the other components of my van. I could be wrong but, I shouldn't be so far out of the ballpark as the issue was present before I adjusted the AFM and I have it set very close to the mechanic's setting.

[hansh]

Just to follow-up on this. I dropped my neighbor's factory-sealed, known-good, AFM in and the mixture issues have straightened out. I'm having an odd issue with my O2 sensor though. Using one, I get a rough, stumbling idle, and an issue in which the van will get zero power when starting from a stop on even the slightest incline. It also surges under load. It became so dangerous that I've disconnected the O2 sensor so that we don't get struck while trying get moving. Without the O2 sensor, it idles great and has absolutely no issue getting up and going. This is the 2nd new Bosch O2 sensor I've installed in a couple months. It really has me baffled as the O2 sensor is working and there is no impedance to the ECU connector. The ECU has been exchanged with a known good one and there is no change.

The van is an '84 with Digijet FI.

[vwlover77]

Maybe one of your fuel injectors has gone wonky. If it's leaky at idle, it can make the whole mixture appear too rich to the O2 sensor. The computer assumes all 4 cylinders are performing equally, so it would lean out all 4 cylinders to compensate. One would still be a little rich due to the leak, but the other 3 too lean. Am I making sense?

[Randy in Maine]

I don't know the answer here but I think the problem is more likely to be in the idle stabilizer unit and it may just need to be adjusted and/or cleaned. We don't have those on our buses though.

Also I think that the O2 sensor is a voltage generator and that is what you want to measure, not the reisistance. The engine and the cat converter have to be warmed up to correct temps and you are looking for number is the range of 0.5 volts with more voltage = leaner.

I will gladly defer to someone that actually knows though....

[hansh]

I'm resigning to believe that this particular issue lies somewhere in the electrical system. I've been through the entire thing searching for brittle wires, corrosion, cleaning grounds, etc. The scary thing is that many wires have a stiffened and blackened area about an inch upward of its terminal end. Apparently, the entire system was shorted at some point. The ECU checks out, as has each stiffened, blackened wire that I've checked. I'm still searching though.

It seems that an electrical might make sense for this issue though since the O2 sensor feeds a voltage signal which affects the ECUs output. If worse comes to worse, my neighbor has a spare wiring harness for this van that I could use to test my theory.

PS VWlover - that is a good thought. I replaced all of the injectors a couple of weeks ago just in case...and also because there is rust coming from the tank and the van had only the square white prepump filter. Now it has a pre and a post to help save the new injectors.

[hansh]

To add resolution to this post, I've returned my AFM to a position that is identical to the one I borrowed from my neighbor. I found out today that my vacuum is low due to an issue with compression on cylinders 3 and 4. I've posted regarding that issue in the engine section in case anyone else is walking the same road.

I don't mind that I looked to the AFM to solve this problem though. I learned a lot about it and am comfortable listening to the engine to get the mixture set. If my problem was not confounded by the richness of startup compared to the relative leanness of a warm engine, along with low vacuum caused by compression issues,...I probably would have been right!

[airkooledchris]

is there a way one would be able to reset an AFM back to factory settings?

if you take one apart to rebuild it, which im guessing some vendor somewhere does, they must reset them back to a basic stock starting point. if thats true, how do you get there?

[Amskeptic]

is there a way one would be able to reset an AFM back to factory settings?

Nope. There is no real factory setting. They are all adjusted with bench equipment to the unique parameters of each casting and flap configuration of each AFM assembly.

With the O2 sensor out of the loop, you can adjust an AFM quickly and easily to best driveability, but the caveat here is that everything else must be in proper condition. Even a dipstick with a leaky little gasket under the "hat" can screw up your mixture. A valve cover gasket that has drooped inside of the cover can cause an air leak that leans out the mixture, so can leaky pushrod tubes that don't necessarily dump out oil.
Colin

[airkooledchris]

wow, im not sure where to start then. I had no idea so many different components would factor into the overall AF mixture... I guess its time to just make a big list and start crossing everything off before I bother even checking it again.

I have a slight exhaust leak im waiting on gaskets to repair and ill dig into the misc seals you mentioned as well just to be sure everything is nice and tight.