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Fuel and Ignition Fuel Pumps and Systems, Ignition and Spark Systems

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Old 07-17-2017, 04:02 PM   #1
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It does start good.

My 96Z has an intermittent miss at low rpm acceleration and cruising below 2200 rpm. The SES light comes on after several miles of driving. When I had Auto Zone read it, it gave "oxygen sensor". But when I told the attendant I had replaced both and the MAP sensor he brought up "vacuum leak at the throttle body". How likely is that? It really seems to me like a bad spark plug or wire although the wires are MSD and were new about 12 years and 60,000 miles ago and the plugs Iridium and replaced about 6000 miles ago. Any thoughts?
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Old 07-17-2017, 08:07 PM   #2
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Re: It does start good.

What was the exact numeric code for the O2 sensor? You have four sensors (unless you've deleted the after-cat sensors), and there are multiple codes for each sensor, and codes related to running rich or lean overall. Knowing the exact code would be necessary to evaluate the validity of his suggestion.
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Old 07-20-2017, 09:10 PM   #3
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Re: It does start good.

The agent at McGuier's got several codes: P0131 Low Voltage - Bank 1 sensor 1, P0171 Bank 1 sensor too lean, and P0300 multiple misfire. What should I be looking at?
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Old 07-21-2017, 12:40 PM   #4
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Re: It does start good.

Three possibilities 1) Bank 1 is actually running lean, due to a plugged injector or vacuum leak specific to that bank of the engine (e.g. - vacuum leak at the o-ring on an injector); 2) Both Banks are running lean, but the long term fuel trims on Bank 2 just haven't reached a level high enough to set the corresponding codes for Bank 2. Low fuel pressure, vacuum leak common to both banks, faulty/dirty MAF sensor; 3) the Bank 1 O2 sensor is faulty or shorted to ground, providing a false indication that Bank 1 is lean, causing the PCM to add extra fuel the engine doesn't need.

Then there's the question of which came first - the lean condition, or the misfires? Misfires, or exhaust leaks before the O2 sensor cause a false lean condition at the O2 sensor.
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