AFM spring tension vs output voltage
12-16-2018, 06:40 PM
#1
Drifting
Thread Starter
AFM spring tension vs output voltage
Recently, I cleaned the AFM and bent the little wiper as per various guides. Previously, the AFM tested fine on a scope (no drop outs) but I did see the tracks in the carbon and figured might as well bend the wiper. This service did seem to fix an odd cold-start hesitation issue and perhaps a *little* bit better response.
However, I have noticed a massive drop in fuel economy! That is, it's burning fuel more like it did before my headgasket-carbon cleaning-emissions tune-up work this past spring. After that work, I started getting 380-390 miles to the tank (fill at light turn on, about 60 litres fill).
Now, after servicing the AFM, I'm back to 300-320 miles range at the same fill (60 litres). This has been pretty consistent over a number of tanks now. I'm missing the extra miles!
The change happened immediately after the AFM work.
Could I have bent the wiper incorrectly? I opened it up again today and saw faint new lines in the carbon that correlate properly with the older ones above. But, I can't help but wonder if the wiper's start and end positions are off a bit. To keep it bent, I had to put sharp creases in the wiper and frankly there seems to me to be room for error that surprisingly no one talks about (?).
A recent DME refresh (re-solder of joints that solved a sudden stalling issue) hasn't changed the consumption.
Does anyone have the spec for spring tension vs. voltage output of the AFM? I'd like to confirm the wiper position and general accuracy of the unit
However, I have noticed a massive drop in fuel economy! That is, it's burning fuel more like it did before my headgasket-carbon cleaning-emissions tune-up work this past spring. After that work, I started getting 380-390 miles to the tank (fill at light turn on, about 60 litres fill).
Now, after servicing the AFM, I'm back to 300-320 miles range at the same fill (60 litres). This has been pretty consistent over a number of tanks now. I'm missing the extra miles!
The change happened immediately after the AFM work.
Could I have bent the wiper incorrectly? I opened it up again today and saw faint new lines in the carbon that correlate properly with the older ones above. But, I can't help but wonder if the wiper's start and end positions are off a bit. To keep it bent, I had to put sharp creases in the wiper and frankly there seems to me to be room for error that surprisingly no one talks about (?).
A recent DME refresh (re-solder of joints that solved a sudden stalling issue) hasn't changed the consumption.
Does anyone have the spec for spring tension vs. voltage output of the AFM? I'd like to confirm the wiper position and general accuracy of the unit
12-16-2018, 08:23 PM
#2
Rennlist Member
Recently, I cleaned the AFM and bent the little wiper as per various guides. Previously, the AFM tested fine on a scope (no drop outs) but I did see the tracks in the carbon and figured might as well bend the wiper. This service did seem to fix an odd cold-start hesitation issue and perhaps a *little* bit better response.
However, I have noticed a massive drop in fuel economy! That is, it's burning fuel more like it did before my headgasket-carbon cleaning-emissions tune-up work this past spring. After that work, I started getting 380-390 miles to the tank (fill at light turn on, about 60 litres fill).
Now, after servicing the AFM, I'm back to 300-320 miles range at the same fill (60 litres). This has been pretty consistent over a number of tanks now. I'm missing the extra miles!
The change happened immediately after the AFM work.
Could I have bent the wiper incorrectly? I opened it up again today and saw faint new lines in the carbon that correlate properly with the older ones above. But, I can't help but wonder if the wiper's start and end positions are off a bit. To keep it bent, I had to put sharp creases in the wiper and frankly there seems to me to be room for error that surprisingly no one talks about (?).
A recent DME refresh (re-solder of joints that solved a sudden stalling issue) hasn't changed the consumption.
Does anyone have the spec for spring tension vs. voltage output of the AFM? I'd like to confirm the wiper position and general accuracy of the unit
However, I have noticed a massive drop in fuel economy! That is, it's burning fuel more like it did before my headgasket-carbon cleaning-emissions tune-up work this past spring. After that work, I started getting 380-390 miles to the tank (fill at light turn on, about 60 litres fill).
Now, after servicing the AFM, I'm back to 300-320 miles range at the same fill (60 litres). This has been pretty consistent over a number of tanks now. I'm missing the extra miles!
The change happened immediately after the AFM work.
Could I have bent the wiper incorrectly? I opened it up again today and saw faint new lines in the carbon that correlate properly with the older ones above. But, I can't help but wonder if the wiper's start and end positions are off a bit. To keep it bent, I had to put sharp creases in the wiper and frankly there seems to me to be room for error that surprisingly no one talks about (?).
A recent DME refresh (re-solder of joints that solved a sudden stalling issue) hasn't changed the consumption.
Does anyone have the spec for spring tension vs. voltage output of the AFM? I'd like to confirm the wiper position and general accuracy of the unit
I'd focus on confirming that the car is still able to get to closed loop at idle and part throttle while driving at a steady rate. Bending the wiper is a last resort in my view because of the risk that you'll alter the output curve of the AFM. The O2 sensor should be able to account for minor changes and get you back to 14.7, but it can only adjust so much if the AFM is too far out of whack. If your mileage problem is a result of the AFM, it should show up as something richer than 14.7 while cruising down the highway. That said, you worked on a lot of things a while back, so while I agree the AFM is a likely culprit, I wouldn't rule out other causes.
12-17-2018, 07:34 AM
#3
Drifting
Thread Starter
Yet one more reason to get that wideband. Which reminds me: I've got a spare factory Cat with all piping in good shape. Now that emissions testing is disapearing for us here, would I be able to turn this Cat into a hollow "test pipe"? I ask because I'm more inclined to put the WBO2 into this piece, have it ready to go, and plan a day off driving to replace the installed Cat with this piece
12-17-2018, 11:40 AM
#4
If you have a good cat I would try find a bad one to hollow out. If the cat internally is good it is probably worth some money to sell to someone that needs a replacement.
12-17-2018, 12:00 PM
#5
Drifting
Thread Starter
How do you hollow it out?
12-17-2018, 01:39 PM
#6
Three Wheelin'
Unbolt the tail pipe move it out of your way... Get a piece of round stock or long prybay and smash.....after the bar goes all they way through start it and blow all the pieces out ....I did mine about 15 years ago ....made a mess in the garage. Lol
12-17-2018, 01:46 PM
#7
Drifting
Thread Starter
Maybe I can use some sort of plumbing pipe cleaning tool?
And.. I assume that a hollow cat doesn't have any performance issues, correct?
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12-17-2018, 03:27 PM
#8
Rennlist Member
Reading this reminded me of a something I had read a long time ago from FR Wilk.
I was able to find it, but not sure if it is much use to you at this point. I will post it here though in case it might help someone else looking for general knowledge on the topic.
http://e28-535i.com/technical/AFM%20Repair.pdf
I was able to find it, but not sure if it is much use to you at this point. I will post it here though in case it might help someone else looking for general knowledge on the topic.
http://e28-535i.com/technical/AFM%20Repair.pdf
12-17-2018, 04:29 PM
#9
Three Wheelin'
I'm fairly certain that you can see in with an flashlight. No issues. Dig it out ...did it on all my cars... My racecar has straight thru pipe and you saw how good that ran at 944 fest this summer...
12-17-2018, 04:31 PM
#10
Drifting
Thread Starter
Ben: Yes, thank you. I found and read the same. However, the article only gives a chart with Flap Angle vs Voltage; I have no idea how you would determine Flap Angle. What is the reference zero degrees point?
I think it would make more sense to have a torque value vs voltage. I did find one person's method using string and water, but that is just to reference his original findings, which I didn't take before bending
.
There must be some specific values or calibration method that Bosch used to set the initial wiper position. Or is there some correlation marker on the board I can't see?
I think it would make more sense to have a torque value vs voltage. I did find one person's method using string and water, but that is just to reference his original findings, which I didn't take before bending
.There must be some specific values or calibration method that Bosch used to set the initial wiper position. Or is there some correlation marker on the board I can't see?
12-17-2018, 04:33 PM
#11
Drifting
Thread Starter
I guess it won't hurt to start banging away at it. I can just blow it out with my compressor I guess
12-17-2018, 11:55 PM
#13
Three Wheelin'
I would skip straight to installing a wideband AFR sensor/controller. That way you can adjust your particular AFM to suit your particular engine instead of relying on some arbitrary specs for a 30+ year old component.
Plus the wideband will always be useful going forward!
Plus the wideband will always be useful going forward!
12-18-2018, 01:35 AM
#14
Rennlist Member
I would skip straight to installing a wideband AFR sensor/controller. That way you can adjust your particular AFM to suit your particular engine instead of relying on some arbitrary specs for a 30+ year old component.
Plus the wideband will always be useful going forward!
Plus the wideband will always be useful going forward!
12-18-2018, 09:21 AM
#15
Drifting
Thread Starter
Ok ok.. I know you guys are right 
Started looking at WB02s. Um... seems you can buy separate sensors and gauges (at $200+ USD EACH) or kits which appear significantly cheaper (ex $250 USD for both gauge and sensor: https://www.aemelectronics.com/produ...uge#buy-direct )
I guess I'm asking what is a good yet relatively inexpensive setup? I'm looking for an analog gauge for a stock-like look. Odd: I can't seem to find a VDO stock-style afr gauge. Don't they make them?
Started looking at WB02s. Um... seems you can buy separate sensors and gauges (at $200+ USD EACH) or kits which appear significantly cheaper (ex $250 USD for both gauge and sensor: https://www.aemelectronics.com/produ...uge#buy-direct )
I guess I'm asking what is a good yet relatively inexpensive setup? I'm looking for an analog gauge for a stock-like look. Odd: I can't seem to find a VDO stock-style afr gauge. Don't they make them?