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Changed fuel filter & O2 sensor; now blowing black smoke
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devesvws
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PostPosted: Fri Jun 01, 2007 1:31 pm    Post subject: Changed fuel filter & O2 sensor; now blowing black smoke Reply with quote

The extra fliter I use on my 87 is added after the fuel tank and before the fuel pump and traps a lot of crap from the tank, so when the motor starts to stumble and the pump starts to buzz louder its time to change it. But this time I thought that I would change the oxygen sensor at the same time, no big deal right? Very Happy wrong Crying or Very sad Found the green wire unpluged when I removed the old sensor Shocked put the new sensor on and started it up . In less than 2 min it started loading up and started to stumble with black sooty smoke Confused I thought, bad sensor, I will put the old one back on no luck at all. I get the same thing. Ok, no big thing, I will just use one of my good spare fuel pumps see if that helps, still no luck at all. So I take it for a drive praying that helps. Nope, runs like crap. Ok guys, what have I done to the beast? It was running well before I changed those items, it just bucked a few times but no sooty black smoke at all. Whats going on here ?
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Dogpilot
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PostPosted: Fri Jun 01, 2007 1:37 pm    Post subject: Reply with quote

Check and see if your O2 sensor wire is not shorted to ground. Measure it from the connector (unplugged from the sensor) to ground. It may be shorting out to the shield on the green wire (it is actually a shielded wire). The other question, was it a 'Universial' type O2? The type you crimp together? If so are you sure you did it right?

Your small amount of maintenance may have caused the short, which barely existed before to come back like some sort of zombie.
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devesvws
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PostPosted: Fri Jun 01, 2007 2:16 pm    Post subject: Reply with quote

the one i removed was a bosch but was spliced together, the one i replaced it with was from the vw dealer but not bosch it was german made . i have one more spare and its a bosch all three have the proper plugs.
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Dogpilot
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PostPosted: Fri Jun 01, 2007 2:31 pm    Post subject: Reply with quote

Check the green wire to the ECU for a short to ground.
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tencentlife
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PostPosted: Fri Jun 01, 2007 4:15 pm    Post subject: Reply with quote

Yep, that's almost definitely where the problem lurks. There can be damage to the O2 sensor signal coax wire, and if there is any contact to ground you'll get exactly the symptoms you describe. Do the basic grounding check Dog mentioned, and as a visual check, slide the little rubber boot that covers the connector back up the green cable and look carefully for frayed ends of the shielding braid. One tiny wire that manages to connect with the signal core wire will make that rich running happen. Use a razor knife to trim back the coax wires a half-inch so none of them can reach the signal connector. What often happens is, in order to connect the new sensor, you slide the boot up the cable a bit, and it disrupts some of the braid ends, then when you slide the boot back down to cover the connection, it pulls a strand with it and it makes contact.
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devesvws
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PostPosted: Sat Jun 02, 2007 7:03 am    Post subject: Reply with quote

yep it was the ground shield wire shorted to the green wire and it was me who did it Embarassed thanks guys its runing much better now Wink
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MsTaboo
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PostPosted: Mon Jul 14, 2008 4:22 pm    Post subject: Reply with quote

Hi guys, I have just found this post after searching oxygen sensor. Heres the rub. My 90 Carat is doing the black sooty thing after running a few minutes (starts fine, runs good, then gets crappy, stop, start, runs fine, gets crappy even sooner, etc.)
I just swapped my brand new Cat. Oxy. sensor, muffler from Carat to Multivan. Put the older but serviceable parts from Multivan (keeping) to Carat (selling) Now I have this problem. I found this post, went out and performed tests.
The green wire on Carat is not grounding and looks in good condition, however if I unplug green wire and test drive, problem disappears. I have swapped oxy sensors. The brand new oxy gives best performance, but still not great.
This is driving me nuts. Both cars ran fine before swapping exhausts. The older oxy looks in pretty good condition, but I also tried a leftover Oxy from when I bought new Cat, muffler, etc. It fails also, but worked before I removed.
All this points toward the coax wire, but it passes tests.
Can oxygen sensors fail just by removing? Anything else I missed?
Thanks!!
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tencentlife
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PostPosted: Mon Jul 14, 2008 4:44 pm    Post subject: Reply with quote

A quick check of ground shorting on the green signal coax often misses the problem. You really need to isolate and work that wire, wiggle and bend and tug on it with your ohmmeter set in audible mode, if it has a beeper. The shorting is not going to be something constant and solid; it's going to be something barely making contact. Most times when this problem surfaces it was right after work was done in the engine bay, or the sensor was just replaced, and in the process the wire got moved and the short occurred.

You also have to do a close visual inspection of the end where the sensor connects. Slide back the little rubber cover and make sure the grounding-sheath braid wires aren't able to reach the signal wire in the center of the coax. It's a good idea to take a razor knife and trim off some of the outer insulation 1/2" back, and trim off every single fine braid wire to that point. Then cut off and crimp on a new female connector. Usually if you go to that much trouble you'll get it. The fact that yours starts fine and turns bad after a minute of running, then repeats the pattern after every restart, says definitively that the problem is in the o2 sensor circuit.

And no, the sensors are quite rugged and won't be harmed merely by removing and replacing. Do avoid silicone sealers anywhere in the exhaust tract, as the silicone releases chemicals as it cures that will damage the sensor element.
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Last edited by tencentlife on Tue Oct 28, 2008 9:10 am; edited 1 time in total
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MsTaboo
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PostPosted: Mon Jul 14, 2008 6:08 pm    Post subject: Reply with quote

Thanks Tencentlife.
Yeah I really did try wiggling and tugging on green wire while hooked up to meter, pulled back the rubber, checked for stray wire shards, cleaned wire and hood. Also compared to Multivan, looks same. The thing that gets me is it runs ok (not great) with the brand new sensor. I will try the peeling back and soldering deal now. Hope that will fix. If not, then maybe I'll have to chase short up the wire harness. Any idea about rerouteing new wire along side harness? What type of wire, and where end points are? The Bentley manual is a little vague.
The oxy sensors are clean, no silicone, just a little carefully applied anti-seize Thanks again.
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PostPosted: Mon Jul 14, 2008 9:04 pm    Post subject: Reply with quote

OK going nuts here,
I have peeled back the coax outside wire and trimmed, checked signal wire and found a tiny- micro crack in insulation about 1/4 inch down wire. Ah ha! I thought this has got to be it. Must be a very tiny stray current. Cleaned up wire, cut all loose strands carefully away, and taped the wire and connector to truly insulate (in case of dirt, wire bits, etc. and test drove.
Still no good!
The Van will not run with the green wire connected.
I have carefully checked for grounding of the signal wire. Nothing.
Now have I got it wrong? This is what I've found: The signal wire and the surrounding shield wires are not grounding to each other. The signal wire is not grounding to chassis. The shield wire will ground to chassis.
My other thought is can the catalytic converter be at fault? I know the oxygen sensor is upstream from the cat, but can back pressure in an old cat cause bad readings for the oxy sensor? The catalytic converter is old, but I gave it an inspection per Bentley and it looks good. However it may be full of soot by now! ( the van is not spewing clouds of black, but the oxy sensors have been covered when I pulled )
My other question is What the hell is the coax wire for?? I have limited electrical experience but all coax, or shielded wires I've seen have connections on both ends of both wires. The green wire has shielding that terminates just before the signal wire connector and dead ends. What is it's purpose? The other end of the shielding goes to the ECU per Bentley, but I can't discern how it is collecting information.
Thank you for any and all help.
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PostPosted: Tue Jul 15, 2008 10:00 am    Post subject: Reply with quote

The shielding is used for the same reason as shielding an antenna lead; to ground out induced currents (noise) from all the RF in and around the engine bay. The o2 sensor is actually a voltage generator, which grounds thru the exhaust tract. It makes a 0-1V signal driven by the difference in free o2 at the inner and outer surfaces of the sensor element. That signal is carried to the ECU on the core wire. Being that it's such a low-power signal, and there are several strong sources of RF energy in the engine bay, it would easily be overwhelmed by the RF inducing currents on the signal lead. So, the shield intercepts that noise, and leads the induced currents to ground.

The subject came up here recently about replacing the coax with an alternate to bypass the stock one that was suspected of being damaged. Symptoms were similar to what you're having. It would be easy to do; any TV coax would be fine. The signal lead just needs to go from the o2 output to the appropriate pin 2 on the ECU, and the shield grounds out at one end only. The stock one is grounded at pin 19. But the pin 19 ground isn't at the ECU, it leads back to the cluster of engine grounds in the engine bay, on the block and beneath the coil. Make sure you've cleaned up and reconnected those grounds anyway, because they are the source of a lot of FI errata. You could try taking the brown lead at pin19 and making it common with the pin 13 ground at the ECU as well, to eliminate any ground offsets that might be between those lines.

I just went and checked two known-good o2 sensors I have. An ohmmeter showed 4.1 ohms between the heater connector pins (that's the resistive heating element); no continuity between either of the heater pins and the sensor body threads; none between either heater pin and the signal lead; no continuity between the signal lead and the threads, and it doesn't test as a semiconductor either with a diode check. So the only ground for the sensor is thru the exhaust tract.
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PostPosted: Tue Jul 15, 2008 11:01 am    Post subject: Reply with quote

Thank you Tencentlife, excellent reply. Comprehension dawns!
At this point it seems my best bet is new wire. It there was one fracture in the signal wire many more might exist. My meter is not the cheapest, but still not pro level. It may not be sensitive enough to catch transient grounding. Also, perhaps, the new oxy was generating a stronger signal? Older units were too feeble to overcome voltage loss?
Anyway, new wire. I'll post back success I hope!
THX.
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Last edited by MsTaboo on Tue Jul 15, 2008 11:40 am; edited 1 time in total
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PostPosted: Tue Jul 15, 2008 11:24 am    Post subject: Reply with quote

What I've been able to gather about the sensors as they age isn't that the signal gets weaker. After all, voltage is voltage, and the current is already microscopic, which is why it needs to be shielded. But what happens is the response time slows gradually. The lambda loop involves the sensor telling the ECU that the mixture is off, either too lean or too rich, but not by how much. So the ECU corrects and the mixture swings toward the other end of the scale, at which time the sensor tells it when it's crossed the stoichiometric line, the ECU responds, etc. Back and forth over the point of correct mixture, always seeking that point but never able to achieve it and stay there. A narrow-band sensor is more like a switch than an actual sender of information, the response is not linear.

So it's easy to see that how close the ECU can stay to stoich depends entirely on how quickly the sensor responds when the mixture transitions from too rich to too lean and back. Older sensors respond more slowly, but the actual signal voltage and strength doesn't change.

When sensors fail, they no longer produce any voltage, so no signal. When that happens the mixture tends to go a bit rich, since the basic tuning settings before integrating lambda feedback are for it to be a little richer than it will run with lambda. This depends on how the engine was manually tuned but if it's done by the book, it will be a bit rich in open loop. I've never heard of a sensor failing and producing too high a voltage, and I don't think it's possible. So if an engine is running too lean, it pretty much can't be caused by a failed sensor, but when an engine runs a bit rich, that's one of the first places to look.
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PostPosted: Tue Jul 15, 2008 4:45 pm    Post subject: Reply with quote

OK first, THANKS for not roasting me for asking dumb questions!
I have since done a search for oxygen AND wire, found a lot of the info you have been kind enough to retype and expound upon.
A little background: I have been driving and working on 64-71 buses since 1978. I bought my first VW on recommendation that as a woman of limited means I would be able to afford, and work on air cooled VW's. (they forgot to mention how much fun buses are!) I've gone through seven splits and bays since. I can pull engines, do body work, and most wiring.
I bought my first vanagon two years ago and love em! However, this is the first vehicle with "black boxes" I've worked on. So...I'm a little leery of making matters worse.
Heres my question. I've got the ECU loose and have pulled back the wire harness shielding in preparation for stringing that new coax wire. Can I get that #2 terminal out of the brick? Or should I just cut into the green wire and splice the new signal/shield wires, leaving the original green/white wires attached to their terminals? It would be a cleaner repair to match new wire with new terminals but are they available?
THX.
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PostPosted: Tue Jul 15, 2008 7:17 pm    Post subject: Reply with quote

The terminals can be removed from the multi-pin connector. I'm not sure if they're exactly the same as the ones that fit in the other AMP Junior Timer connectors used elsewhere in the FI harness. If they are then you can get new crimp-on female connector pins, and you could do a proper job of connecting a new coax signal core there just as it was done at the factory. I have some of those pins, and a spare harness, so I guess tomorrow I could take a look to see if they're the same type.

You can order the pins from here:

http://www.eagleday.com/ampconnectors.html

or this helpful fellow who posts here:

http://www.kyleautomotivespecialties.com/home

In fact, if you emailed Kyle, I'm sure he could answer your question directly. But I'll take a look at it tomorrow.
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PostPosted: Wed Jul 16, 2008 5:44 am    Post subject: Reply with quote

IF not already recently done, i believe i would do as 10cts suggested and DO ALL grounds 1st. amazing what intermittent problems will go away.
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PostPosted: Wed Jul 16, 2008 1:45 pm    Post subject: Reply with quote

Thanks morymob, I'll do that also.
Tencentlife, I have been in contact with Kyle. He has the terminals. $1.20 each. He is also purposing to run up some plug-an-play cables to replace O2 sensor signal wires, complete with appropriate ends from ECU to sensor plug. I'll post back any info he supplies.
This whole O2 thing seems common enough that a "sticky" might be called for??
Also, I'm still fighting to remove the terminals but now that I've seen a drawing of the little beasts I can see where to probe Confused
Thanks all.
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PostPosted: Wed Jul 16, 2008 6:30 pm    Post subject: Reply with quote

OK heard back from Kyle,
He will do plug-and play O2 sensor wire complete with all appropriate terminals (ECU to engine bay) for 50$
I'll just be getting the terminals and crafting myself!
Anyway if you need...thats www.kyleautomotivespecialties.com
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Last edited by MsTaboo on Mon Jul 28, 2008 1:09 pm; edited 2 times in total
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PostPosted: Wed Jul 16, 2008 8:22 pm    Post subject: Reply with quote

Right on.

NOW this needs to be a sticky.
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PostPosted: Fri Jul 18, 2008 9:04 pm    Post subject: Reply with quote

Let me know guys if this is something that you would want.
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