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Thread: Dwell reads 49 with o2 sensor grounded

  1. #1
    Stupid Newbie DaraSue's Avatar
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    Dwell reads 49 with o2 sensor grounded

    IMG_7943.jpg

    Running the checks on this list while attempting to root out a misfire. The normal operations test swings wide (28-60) and the test with the o2 sensor grounded consistently reads 49 instead of 87. I've redone it multiple times, even with a female connector on one end of the ground wire and a clip on the other to make sure it's actually attached, grounding it to the hook on the engine on the left side and to the exhaust, and I always get the same result. All of the other checks were within range.

    I asked Dave S. about it and he didn't even think that reading should be possible unless there's some weird electrical issue. Has anybody ever seen this before?

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    I remember a few other owner had problems doing the ground test. Don't remember if they got 49 deg. If yours is swinging that much for normal operation I would guess your O2 sensor is bad.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    Senior Member
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    Do you have your dwell meter on the right scale? Some dwell meters just don't work well on a Delorean, try another one. Sounds like your readings in closed loop are doubled. Swinging is a good thing but yours is a bit much. Might need a new O2 sensor. You won't find your misfire this way. Pull and check the spark plugs. Notice if any of the spark plug wells had water. Misfires often occur because of a bad ignition wire, worn plugs or bad patterns on the fuel injectors. Vacuum leaks can cause the nearest cylinder to run lean. If you have a lot of miles it may be time to do a valve adjustment. Do a compression test. The goal is to get all of the cylinders as close to each other as possible to make them all run smooth.
    David Teitelbaum

  4. #4
    Stupid Newbie DaraSue's Avatar
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    Thanks, guys. The sensor has about 24,000 miles on it. It didn't look super cruddy when I pulled it but maybe it's time.

    I'm using a Centech digital meter, on the 4 cyl scale. As I said, the other tests read in range. Dave S. said this is the kind of meter he uses so I assume it's good enough. (I bought an analog one on eBay but it doesn't work very reliably so I'm sticking with this one.)

    Plugs and rotor/wires were changed last year, about 3000 miles on the plugs since then. I did find a leak at the rear left injector seal and have since changed the seals but I thought I still heard some popping at 2500 when checking the dwell afterward. The car has 45000 miles on it (but 24000 of them were put on in the last 4 years and only 3000 were put on between 1989 and 2016). I did a mason jar test on the injectors, because of how the jars were sitting I didn't get a good look at the patterns but none of them seemed to be putting out dramatically less fuel than the others.

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    When I was using a wideband unit, I played with a setting which could slow down the response of the output of the wideband. Doing that made a very bad hunt at idle. So what I'm thinking is your sensor is slow to respond and that could be causing the new hunt. I think that is why some cars hunt (mine included) shortly after a cold start when the narrow band sensor has warmed up enough to put out an output not not hot enough to give fast output response. Now, new cars all use heated sensors which don't have that problem.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
    Senior Member
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    I find using an analog meter you can easily watch the needle swing, it is not as easy to see the rate of change on a digital meter. Different digital meters display changing values differently depending on the delay built in to "dampen" changes. Hunting is a function of how closely each cylinder matches the others. When a "low" cylinder comes around to fire, the motor slows down and then speeds up when the next one fires. By then the idle system tried to speed up the motor and because it fired a "good" cylinder the speed overshoots. You also have the Lambda system trying to correct the A/F ratio but because of the built in delay the effect is delayed and also overshoots.These systems are not in sync and sometimes they phase additive and sometimes negative. The way to minimize hunting is to get all of the cylinders firing evenly so the smaller the corrections the systems have to make, the smoother the idle. If you had one bad injector seal you should replace them all. You probably need the injectors cleaned too. Do a compression test and see how even the cylinders are. They should all be within 5% of each other.
    David Teitelbaum

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    The lambda and idle systems do not respond fast enough to follow each cylinder firing.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
    Stupid Newbie DaraSue's Avatar
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    I changed the sensor and the ground check still reads around 52. Weird. The normal dwell swing got a little better, around 33-59. Still think I'm hearing the misfire. I did replace all the injector seals and wipe the injectors down with some Brakleen. I'll see about getting a compression check next week but another DeLorean buddy of mine suggested checking the timing too so I might try that next.

  9. #9
    Administrator Ron's Avatar
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    $.02 I'd find the miss first. Pull the plugs wires one at a time and listen for no change in RPM...

  10. #10
    Stupid Newbie DaraSue's Avatar
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    How much is it supposed to drop off when you pull a plug wire? I didn't notice a huge difference between any of them but it fluctuates at idle anyway.

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