1991 MAXIMA

When cold, and I start it up, it runs really rough, RPM drops to about
500, and it stalls. When it warms up, it runs better when idling.

I put a jumper across pin 29 on ECU (oxygen sensor) and it shows .2
volts (lean mixture). When it warms up, and I run it up to 2000 RPM
for 2 minutes, pin 29 still shows .2 volts, and it won't go into
closed-loop mode. When I race the engine, and then come back to idle,
sometimes the pin 29 voltage swings up to 1.3 volts, then back to .2
volts. When I race the engine, and then come back to idle, sometimes
the pin 29 voltage stays at .2 volts, and doesn't vary. Sometimes,
when I race the engine, and drop back to idle it goes into closed-loop
mode, and sometimes not. That is, I put the ECU into mode 2, and the
green light flashes. When it does go into closed-loop mode, and the
green light flashes in mode 2, the right light stays lit the whole
time (lean mixture).

When I disconnect the MAF plug, the pin 29 voltage (oxygen sensor)
goes up to 1.3 volts, and the car runs fine at idle, both when hot &
cold. I reconnected the MAF plug, and measured the pin 27 voltage
(MAF sensor), and it is within specs ( 1.3 volts at idle, and 1.8
volts at 3000 RPM ).

The car seems to run fine with the MAF plug disconnected, but the MAF
sensor doesn't seem to be bad. With the MAF plug connected, the car
seems to run lean, and sometimes won't go into closed-loop mode. When
I does go into closed-loop mode, the red light stays on the whole time
in mode 2, indicating a lean condition.

The possibilites I come up with are:
1) Bad ECU
2) ECU pin 27 (MAF sensor): bad connection between harness and ECU
pin, so that
ECU is not getting correct MAF sensor input.

Thanks in advance for any thoughts, ideas, or other diagnostic
suggestions.


Thomas

<morn237 (AT) hotmail (DOT) com> wrote

There is no such thing bad ECU. Computer software doesn't change, it's a
fixed logical processing unit. You're stupid "me" You like to scream on
other people, you don't know shit about car yourself.

You dipshits scream at the Chinese before the reach the moon. See now?
You don't have faces to face the world.

You may very well have a bad O2 sensor, there are only 3 components that
control the mixture feed back in those old J30 series Maximas.

The O2 sensor
The MAF
The Coolant Temp sensor

Ohm out the coolant temp sensor when the engine is hot, it should read about
300 to 400 ohms, it should read about 1.5k ohms when cold, if you disconnect
the cts when the engine is running do the cooling fans come on ? (quick and
dirty test)

My feeling is the O2 went south

Nice piece of diagnosing by the way






<morn237 (AT) hotmail (DOT) com> wrote

Kenny,

Thanks for your help.

I forgot to mention that I had replaced the O2 sensor and CTS.
However, they are after-market parts, not Nissan.

I also forgot to mention that I had checked the pin-28 ( CTS )
voltage. The FSM says the voltage should be 5 volts cold, and 0 volts
when the car is hot. I got 3.5 volts cold, and 1 volt when hot. This
is for a new, after-market CTS.

I'll try unplugging the CTS, as you suggested.

Thanks again,
Thomas

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On May 7, 5:49*pm, "Kenny" <HeywoodYablo... (AT) msn (DOT) com> wrote: