Strange O2 sensor voltage

[blackglasspirate]

Recently I've been pulling my hair out chasing a hesitation off idle. I'm no newbie to troubleshooting that, as it's been a reoccurring issue, especially since installing the RMW SS exhaust and constantly having small leaks from the clamps.

All that aside I've been adjusting the AFM per the Samba instructions (here : https://www.thesamba.com/vw/forum/viewtopic.php?t=356377 ) and noticed strange voltage from the O2 sensor. This is with the engine fully warmed, idle stabilizer valve unplugged, O2 green wire unplugged and the sensor side connected to an analog multimeter on a low voltage setting.

So what's happening? There's really no adjustment between 0 and .7 volts. Either it's at .7v or 0, with a very small in-between section where it just constantly bounces between 0 and .5ish. This is all within a 1/4-1/8 turns of the adjustment screw. There's no real "adjustment" per-se. It's all or nothing. Screw it in all the way and the voltage remains at .7. Screw it all the way out and it's 0.

The kicker? The hesitation is remarkably worse when it's in the .7 range than it is at 0. If I can get the screw juuuuuuuust right in between where it's at .7 and where it drops to 0 then the hesitation is nearly 100% gone.

I should also note that the O2 is new as well as a lot of the other likely culprits for this sort of thing, so it's not the O2 failing. If I hook everything back up and watch the sensor voltage while idling (sensor plugged in) then I can see the normal bounce between 0 and .8.

Has anyone else every experienced this or have any idea why the voltage would behave like that? Every adjustment thread I read makes it sound like you should be able to see the voltage change all throughout the 0-.8 range as you rotate the screw but for me it's all or nothing. I'm also unsure why the hesitation would be worse when at .7v than at 0. The Vanagon gods be strange.
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'87 Vanagon GL Westfalia
IG: @holidayatsee
FB: https://www.facebook.com/holidayatsee

[13b Bro]

I had an intake manifold leak on one cylinder that would miss unless I richened it up but my readings were opposite. If i richened it way too rich then it would run smooth then try to lean out and it would stumble.

Not exactly your symptoms but thought I would throw it out there in case it helps.

[13b Bro]

Forgot to explain that in that case my O2 readings were at the extremes too...no middle

[DanHoug]

[Ronzo_volvo_guy]

bgp;

Dan has explained it well...Lambda sensor voltage at Lambda point exhibits a fairly abrupt switching action, and typically the ECU keeps the engine at that operating point, so if you were to measure the voltage, all you'd see was this bouncing V (that's why "wideband sensors", which spread out this display are popular for guys who really want to monitor whats going on for the purpose of tuning Carbs etc.)

...these two pages from the C. Probst book Bosch Fuel Injection & Engine Management (Fantastic and highly recommended in every Bosch FI owner's library!), shows it well.

I have scanned and posted these as excerpts under fair-use...I hope this is not a copyright problem with the admins/sys ops.

Note that last time I looked at the graphs, it seemed like the CO and NO curves were swapped, so I penciled it in, but I have yet to confirmed this [Dan, maybe you could double-check me on this and chime in, please!]

Hope that helps!

[blackglasspirate]

Thanks guys, that helps a lot. I guess I never really noticed how steep that curve is in the enrichment voltage. I could have sworn on other AFMs I had more of an adjustment range but I currently only have one other to test with and I'm not 100% sure it's a good one.

In the end I adjusted the spring tension 3 clicks lean, then adjusted the screw. Oddly enough, I didn't notice a big difference in performance between 2 and 3 clicks. I also had to adjust the stop screw on the throttle body to get 0 vacuum on the port to evap because it wasn't closing all the way to cover the hole, and that made a difference as well. I probably need to verify the timing after those adjustments too but, seeing as I just set it, I doubt it will make any different.

For now it's running pretty well with only the slightest of stumbles sometimes if I quickly floor it from a stop. I'm fairly certain someone else driving my van wouldn't notice any hesitation - I'm just very familiar with my van's "normal". Either way I've spent enough time chasing this issue that I'm ready to just call it good and start enjoying the van again.
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'87 Vanagon GL Westfalia
IG: @holidayatsee
FB: https://www.facebook.com/holidayatsee

[MarkWard]

If you changed the throttle stop, you need to go back through and readjust your tps switch per the manual. That will really affect off idle acceleration.
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[blackglasspirate]

Thanks - I plan to do that, just haven't yet. I backed the screw out 1.5 turns. The TPS still closes at idle, it just takes slightly more movement to engage off idle. I had *just* adjusted it and put it back in when I noticed the screw needed to be adjusted too.

Unfortunately I've got the GoWesty throttle body and TPS, which means the entire TB has to be removed to adjust the switch. No adjustment from the top like the stock switch :/
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'87 Vanagon GL Westfalia
IG: @holidayatsee
FB: https://www.facebook.com/holidayatsee

[DanHoug]