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Thread: New O2 sensor, and a strange burning smell

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    New O2 sensor, and a strange burning smell

    Hi all, so in the hunt for the cause of my high idle(2000-2500 RPM), I replaced the O2 sensor, as this was mentioned as a possible cause. I had a fairly easy go at the job, however, when I started the car upon completion, I got a strange burning smell(it also did not solve the idle issue). It was definitely not the usual smell to me; it was similar to burning plastic, as far as smells go. I do think I could notice a bit of gray fumes or whatnot that is not usual. Any thoughts on that? Is there a normal "burnoff" period or anything that I may not be aware of? As an aside, I have a hunch that the mixture is set rich. Thanks for any thoughts!

  2. #2
    Senior Member DMC-81's Avatar
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    I didn't notice a burning smell when I replaced mine. Mind you, I had some smoke and smell as I was curing Hi Temp POR -15 exhaust paint at the same time. But definitely not a smell of plastic.

    Was there perhaps a plastic cap on the sensor end that didn't get removed or was the wire routed too close to the exhaust pipe?
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
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    Quote Originally Posted by DMC-81 View Post
    I didn't notice a burning smell when I replaced mine. Mind you, I had some smoke and smell as I was curing Hi Temp POR -15 exhaust paint at the same time. But definitely not a smell of plastic.

    Was there perhaps a plastic cap on the sensor end that didn't get removed or was the wire routed too close to the exhaust pipe?
    Thanks for the response. There was not a plastic cap, and i know the wire was threaded through the frame properly, and isn't laying against the exhaust. The car only ran for about 30-60 seconds, so I doubt there'd be enough heat generated at the exhaust to melt anything....

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    The smell is probably related to the mixture being too rich.

    The original idle problem being up around 2,500 RPM is probably a bad electrical connection on the thermistor, which is located under the intake in the valley (which admittedly sucks to need to go in there to correct it).


    Sept. 81, auto, black interior

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    Quote Originally Posted by Jonathan View Post
    The smell is probably related to the mixture being too rich.

    The original idle problem being up around 2,500 RPM is probably a bad electrical connection on the thermistor, which is located under the intake in the valley (which admittedly sucks to need to go in there to correct it).
    This would, unfortunately, make sense as that problem introduced itself immediately after the thermister job. I distinctly remember doing my best to make sure the harness on the actual thermister secure before re-assembly. Is there a physical way to check that without doing a full disassembly, down to the valley? I keep wondering if there's a way to get a go pro camera down in there or something with my arm, just to check that harness. The idle jumps around erratically; if I let it sit, for a minute, it will occasionally just jump from 2000-2200 up to 3000-immediately-without any physical touch to the throttle at all. I think the prudent thing is for me to go back and double check all my connections....sigh...

    Thanks for the help!

  6. #6
    Senior Member NckT's Avatar
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    Assuming that the hammer on the throttle presses the idle microswitch, remove the white multi plug that has 4 wires on it. Use a multi meter, set the resistance k ohms and put the multi meter probes on the black/yellow and black/slate (grey) wires at the multi plug. If you get no reading ie open circuit, then you have a wiring problem to the thermistor. In this case, you could refit the white multi plug and put a 10 k ohm resistor across these wires effectively fooling the Idle ecu that it is referencing the thermistor.

    Let's hope that the idle motor doesn't have a short or that the ecu had not blown one of the switching transistors that operates the motor etc

    Good luck
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    Quote Originally Posted by NckT View Post
    Assuming that the hammer on the throttle presses the idle microswitch, remove the white multi plug that has 4 wires on it. Use a multi meter, set the resistance k ohms and put the multi meter probes on the black/yellow and black/slate (grey) wires at the multi plug. If you get no reading ie open circuit, then you have a wiring problem to the thermistor. In this case, you could refit the white multi plug and put a 10 k ohm resistor across these wires effectively fooling the Idle ecu that it is referencing the thermistor.

    Let's hope that the idle motor doesn't have a short or that the ecu had not blown one of the switching transistors that operates the motor etc

    Good luck
    Brilliant suggestion! Thank you so much. While I wouldn't put it past myself to make a mistake, seeing as how I did the entire VoD job just to replace the thermister, I distinctly remember being in there, and making absolutely sure the wiring harness was securely fastened to it after it was replaced. Will try this test, and report back. Thanks!

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    NckT- the white 4 pin harness, is it the one shown in this picture? If so, the colors on my harnesses are black/green on the left side, and black/black on the right side(the side that looks to connect to the idle microswitch). I get 0 ohms/full continuity across the left side(green/black), and open across the right side(black/black).

    Am I reading the wrong white harness? I've got a later car, FYI, it's apparently an '84 that was titled as an '83. If I'm reading the correct harness, what does this mean? Thanks.
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    Apologies(it's early for me!). Of course you weren't talking about that harness; you were referring to the 4 pin white harness at the idle ECU. Duh. I checked that; yes, it's open across black/yellow and black/slate. One follow up question: is it possible that this would be due to anything other than the electrical harness at the thermister? I will of course go back in there if need be, I just want to see if protocol would call for any other diagnosis, before stripping down to go back to the thermister location. Thanks.

  10. #10
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by rjd2 View Post
    Apologies(it's early for me!). Of course you weren't talking about that harness; you were referring to the 4 pin white harness at the idle ECU. Duh. I checked that; yes, it's open across black/yellow and black/slate. One follow up question: is it possible that this would be due to anything other than the electrical harness at the thermister? I will of course go back in there if need be, I just want to see if protocol would call for any other diagnosis, before stripping down to go back to the thermister location. Thanks.
    The two connections also go through the bulkhead connectors so you may want to check if a connection is bad there.
    Dave M vin 03572
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