Rod Bender

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?

Injuneer

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.

Rod Bender

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?

Injuneer

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.

Rod Bender

Thanks Injuneer for the suggestions. Where can I get information on how to check these things?

Injuneer

Data log with OBD-2 software might help reveal the source of the problem. Free data logging software, you buy a cable:

Free OBD2 scanner software - ECM/PCM - Firebird Nation

Swap the O2 sensors side-to-side. If the codes switch to the other side, it’s a sensor problem. Ditto with the injectors. If the codes switch to the other side, it’s an injector. If you switch the injectors one at a time, it would even point you to the faulty one.

Check fuel pressure with engine under load (test gauge with long hose, taped to outside of windshield). Check all hoses and gaskets for vacuum leaks, using carb cleaner or propane. Do this OUTDOORS, COOL ENGINE, FIRE EXTINGUISHER handy.

Rod Bender

Sounds like more than I want to get into. What do you think a dealer would charge?

Rod Bender

I had an appointment with my local dealer and all of a sudden the day before the car was to get probed it started running ok. I drove it to work today with no problems, so I canceled my appointment. Does that narrow down the issue?

Rod Bender

Any chance a dirty throttle body could be the issue? I have a K and N air filter that is serviced with oil.. I have read where if you get too much of the ac oil on the filter it can cause issues. Also, I do not believe the throttle body has ever been cleaned.

Injuneer

Over-oiling the K&N filter may coat the wires on the MAF sensor, and throw the readings off. Dirty MAF sensor was mentioned as a possible cause in post#4, item 2).

K&N recommends inspecting the filter at 25,000 miles, and cleaning the filter at 50,000 mile intervals. It can only be cleaned 20 times, when it will be worn out. K&N denies over-oiling the filter can cause MAF problems, but some car manufacturers have issued warnings and technical service bulletins.

Rod Bender

Thanks for the reply. I took it into the local dealer this morning. They just called and said it was missing across all cylinders. They said there was an oil leak and it appeared to be leaking into the Opti. That would have to be addressed before going further. They are going to call back with a quote and that it would be costly.

Rod Bender

What about rebuilding the opti?

Rod Bender

The engine has an oxygen sensor in each header but it should have another one. Therefore the pcm does not know whether to send more or less fuel and that could also be causing the misfire.

Injuneer

Oil leaking into the Opti would seem to indicate the seal that pushes into the timing cover has failed. Not the Opti. May need to be cleaned.

Injuneer

No. You should have one pre-cat O2 in the header collector on each bank of the engine. Those are the two sensors that help the PCM control the A/F ratio. There is no "third" O2 sensor required to accomplish that.

In OBD-2 there is also an after-cat O2 sensor after each of the two cats. Those do NOT help the PCM control the A/F ratio. They are only there to tell the PCM if the cats are working. If you don't have cats, you can delete the two after-cat sensors, and tune the catalytic converter diagnostic codes out of the PCM. Will not cause misfires.