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Stumble on accelaration 200 1989

I have developed a persistant stumble in acceleration around 1500 rpm. Stumble appeared after hammering off the loose heat sheilds and also replacing a siezed up air cond. commpressor. Car starts and runs fine otherwise. Normal tune up measures have not helped and dianostic codes are normal. My suspicion is O2 sensor needs replacement, (may have been damaged when I was beating on those heat shields). Digital volt meter readings on O2 sensor output range from .05 to .7 volts at idle, .8 volts when operating at higher rpm with induced richness, and .015 V at idle with small vacuum leak. Does replacing the O2 sensor sound right? Any other suggestions?

Thanks TG
89 240 auto W/179K (his car)
88 240 5 sp W/209K (her car)








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    Stumble on accelaration 200 1989

    The O2 sensor should toggle between .1 and .9 volts about every other second when the engine is in closed-loop operation.

    Some digital meters in particular are usually not the best for measuring varying voltages; the rate they sample at, how much they average (integrate) the reading voltages, and how often they update the display all have an effect on what you read. The ideal reading for an O2 sensor is .45V, but due to the imprecise processes of the combustion engine this reading is impossible to maintain, and the ECU is constantly trying to compensate.

    More to the point, however, is that your sensor 'seems' to be working from what you've said so far. If it goes toggles high and low that is a good sign. (If it was stuck high or low you would get an ECU code about fuel trim, etc.) They don't like shock, though, so yes it is entirely possible you ruined the sensor in some way by banging on the exhaust, like maybe the heater is broken.

    Anyway, I would look for something that might have been part of the A/C work, like maybe a spark plug wire not seated or broken, etc. first.








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      Stumble on accelaration 200 1989

      Spark plug wires was one of my guesses too, but that didn't fix it when I replaced them. The stumble occurs In a very narrow part of the acceleration curve. The O2 sensor voltages actually drop to zero and sometimes show a few thousanths of a negative reading at this point, probably indicating very lean combustion. Just for kicks I unplugged the AMM, which had no effect on the stumble. The stumble is consistant, occuring on all low rpm acceleration from a cold start to a full operating temp engine on a hot day. When I press down harder on the accelerator, it will break out of the bad spot and accelerate smoothly right on up to cruising speeds, (if I keep the rpms up). If I accelerate slowly, the car stumbles and hesitates at that same rpm spot through each of the gears.








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        Stumble on accelaration 200 1989

        Sounds like it could be a bad or misadjusted throttle position switch. If the computer mistakes the combination of throttle position and rpm for a "trailing throttle" condition, such as coasting off the freeway, it may shut off the injectors in a fuel-saving mode. I would check out the TPS.








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          Stumble on accelaration 200 1989

          The coasting fuel cutoff only works if the throttle is closed and the engine is above a certain RPM, like 1500 or 2200 (forgot the exact number); below that minimum RPM there is no cutoff.

          But hey, wouldn't hurt to check the switch anyway.








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    Stumble on accelaration 200 1989

    I had a similar problem under acceleration and it turned out to be my ignition wires. Not sure if this is the culprit in your case, but if you haven't replaced these in a while, I advise you do. Replace them with Bougicord from either IPD or FCP.

    Cheers,
    Raj
    --
    90 BMW 325is, 98 BMW 528i, 90 Volvo 240DL







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