O2 sensor voltage readings

thatbaldwinlife · Posted: Fri Feb 20, 2015 9:03 pm Post subject: O2 sensor voltage readings

1987 2.1L Digifant 2wd 4sp

New:
VC Fuel lines
Intake manifold gaskets and boots
TB gasket
vacuum lines
Temp II sensor
Bosch Fuel pressure regulator
Bosch O2 sensor
Bosch fuel filter
TP switch adjusted to factory specs (or as close as possible) Voltage checks out per Bentley

Trying to adjust my AFM per this thread:
http://www.thesamba.com/vw/forum/viewtopic.php?t=356377&highlight=afm+adjustment

Warmed up the van by driving for 15 min then brought back to the garage and shut it off. Disconnected the O2 sensor and Idle valve per instructions. Started van and adjusted idle to 850rpm and hooked the positive lead from the mulitmeter to the output of the O2 sensor and the black to ground. I was expecting a semi-constant voltage output from the O2 sensor when NOT connected to the ECM but mine would fluctuate from almost zero to .8 volts. Revved up the motor and the voltage seems to smooth out but shows a bit rich. I thought it was the O2 sensor so I put in a new Bosch. The voltage output was the same. It seems to run ok, maybe a sluggish from 1800rpm-3000rpm then is great.

What could cause the O2 sensor to have the voltage changes? Vacuum leak? Exhaust leak?

Idle is very smooth right at 800rpm or so.

What should I check next?

thanks
nate

And a pic for fun!

Image may have been reduced in size. Click image to view fullscreen.

1vw4x4 · Banned Joined: June 22, 2005 Posts: 472 Location: Pgh. PA

That's exactly how the O2 sensor is suppose to work. What to
do next is quit messing with it before really screw things up.

thatbaldwinlife · Posted: Sat Feb 21, 2015 7:52 am Post subject:

I was under the impression that when disconnected from the ECM, the O2 sensor should give a fairly stable reading if the motor is running correctly (at idle).

Is that not true?

davideric9 · Posted: Sat Feb 21, 2015 8:06 am Post subject:

The O2 sensor is sending information to the ECU, not getting information from the ECU. So being connected to the ECU or not will have no effect on its function. What you are describing implies that the ECU is telling the O2 sensor how to behave.
_________________
1987 Syncro Westfalia, stock (bought 1994)
1986 Syncro Westfalia SVX, 3 knob (bought 2008)
1987 Westfalia (bought 2010)
1988 Wolfsburg GL (bought 2012)

wcdennis · Posted: Sat Feb 21, 2015 8:14 am Post subject:

insyncro · Posted: Sat Feb 21, 2015 8:19 am Post subject:

Pin testing all wiring, checking sensors with a multi or DigiTool, making sure the shielded wire for the oxygen sensor is 100% intact and only changing one variable at a time will rule out non issues and help get to the root of the problem.

The FAQ section is full of awesome threads and advice.

thatbaldwinlife · Posted: Sat Feb 21, 2015 8:30 am Post subject:

Thanks, I have been reading every thread I can here and they have been so helpful.

I have checked that the wire going to the ECM is putting out a constant .5 volts with the O2 sensor disconnected and I will check again for the shielding ground though I thought I checked that thoroughly.

The Idle Control Valve is disconnected during this test so I wouldn't think it would cause the erratic voltage readings.

I agree that with the O2 sensor hooked up to the ECM, you should get a swing in voltage as the ECM changes the injector pulse to accommodate lean/rich readings. But at idle, the A/F ration should be fairly constant and that is what the narrow band O2 sensor should show.

I also have some worn spots on my AFM and am going to drill out the screws and move the wiper arm a bit. Maybe it is sending an erratic voltage to the ECM and that is causing it?

Thanks for all the help, I have learned a lot

nate

insyncro · Posted: Sat Feb 21, 2015 8:43 am Post subject:

In my experience, doing what you plan to do with the AFM is a waste of time.
Just my 2 cents.
When they are worn out, they are worn out.

Ahwahnee · Posted: Sat Feb 21, 2015 10:45 am Post subject:

crazyvwvanman · Posted: Sat Feb 21, 2015 10:47 am Post subject:

Many people have moved the board slightly to get the wipers out of wear tracks and thereby made the AFM useful for additional years of service.

Mark

thatbaldwinlife · Posted: Sat Feb 21, 2015 11:11 am Post subject:

At the least if it cures the problem, I will know before dropping $250 on a reman unit. If it doesn't cure it, I didn't waste the cash!

Might also grab the GW vacuum TPS as I have some GW bucks to use.

insyncro · Posted: Sat Feb 21, 2015 2:02 pm Post subject:

Its the bending of the arm, that I am referring to.

If moving the board works, let me know.
I will make over 50 AFMs available instead of scraping them.

I will search the subject to see what it entails.
Thanks.

thatbaldwinlife · Posted: Sat Feb 21, 2015 3:37 pm Post subject:

My screws are epoxied in or something so I am going to drill them out and retap the holes. I can then move the board the 1/8" or less I need to get clean carbon.

Might get to it in a few weeks. Gotta finish this first

the craigness · Posted: Thu Mar 12, 2015 9:32 am Post subject:

if you are sluggish on the low end of the rpm range but peppy at the high range, i'm betting either your timing is off or your distributor advance system is not working. Before you start diagnosing anything like an O2 sensor or AFM problem, time the engine to the factory specs. Use a timing light and bypass the idle module next to the ignition module. Take the two plugs off the green (or black) box and stick them together. Get the timing light out and time it according to the Bentley or search this forum for help. Get a hand vacuum pump on the negative side of the advance module and squeeze it a few times. The RPM should go up a bit. then put everything back and tighten the dizzy set screw. THEN start playing with the AFM and O2. If you must...

delta9007 · Posted: Thu Mar 12, 2015 9:51 am Post subject:

Does he have this stuff on an '87 Digifant?

the craigness wrote:

thatbaldwinlife · Posted: Thu Mar 12, 2015 10:22 am Post subject:

Yep, none of that vacuum stuff on my Digifant to go wrong. I did time it to factory specs before, but I will do it again.
I seem to get about 16-17 mpg on the hwy so I think it is a bit rich when warmed up also.

Recently the engine sounds loud at idle also, but not the typical lifter tick. Hope it is not a rod bearing.

thatbaldwinlife · Posted: Thu Mar 26, 2015 7:59 am Post subject:

Little update
figured out the knocking noise, it was a bad water pump. The bearing was bad but not weeping but the pulley was very loose. Glad I caught it when I did. A new German unit cleared that right up.

I also picked up a used AFM for cheap and opened it up when I received it to check out the carbon tracks. They were in much better shape than mine and the idle mixture plug was still in tact. Plugged it in and it fired up and idled just like my original, which is a good sign as it idled great before. Took it for a spin and the bog from 2-3K was much improved. Not totally gone but better.

Decided to mess with the timing a bit from stock setting and hooked it up and set my advancing timing light to what I thought was 38 degrees and took it for a spin. The bogging was worse! I was bummed. Then I realized the knob had moved during setup and i was running around 32 degrees full advance.
Re-timed it a bit and it was much better. Took it home and bumped it some more and it drives totally different! Much smoother from 2-5K and no knocking yet. I think I ended up around 40 degrees advanced.

Also just installed the NGK 1263 plugs but didn't notice much. Time will tell.

Thanks for all of the help!
nate


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