rich at idle problem on a 2000 sentra 1.8

[mmm_2000]

My car after its warm drops in rpm or sometimes dies every 30 seconds of running (at idle only. Otherwise runs fine) The problem progressed over a period of 3 years from barely noticeable at the beginning, to engine dying at idle many times for the last one year.

Never had a DTC light on.

I checked the fuel trim with an obd2 interface for my laptop and it turns out my short fuel trim is at about -25% at idle and sometimes goes up to -10% but its mostly aiming for -25%. The exhaust gas smells like unburned gas. Today I did a little test:

After driving the car for about 20 mins from work I disconnected the o2 sensors one by one and I noticed that if I disconnect both top o2 sensors the car seemed to run fine at idle without shutting down or drops in rpm every 30 seconds. If I disconnect all four it also runs fine at idle.If I disconnect only the two on the bottom then the car still has the idle problem every 30 seconds.

So once the top two are disconnected the car runs fine! What does this mean?

Can the fuel pressure regulator still be the problem? Or is it 100% sure my top 2 o2 sensors are bad?

Tomorrow I will buy a o2 sensor socket and a torch and I will bench test the o2 sensors just to be sure.

If the sensors turn out to be good then I'll have to test the fuel pressure and the fuel pressure regulator. Is there anything else I should try?

The MAF works fine, the air flow changes with the acceleration.I also noticed all 4 o2 sensors show between 0.8 and 0.9v at idle all the time. Under load, the same o2 sensors show changes in voltage like they are supposed to from 0v to 0.8.

Here is some data recorded over a period of a few mins at idle:-all 4 o2 sensors show voltages between 0.85 to 0.94v-MAF reading is 0.63 lb/min-engine temp at about 200f and the air charge temp at about 70f

I have a total of 4 o2 sensors.

Any ideas ?

[mmm_2000]

K, I tested all 4 o2 sensors and they are all fine. I unplugged the booster vacuum line and once I do that the voltage drops to zero. And I can control the voltage with the amount of vacuum I let slip into the intake. That's how I tested them.

My voltmeter showed the voltage drops when the line was disconnected.

Now what? I am still running very rich and the o2 voltages are steady on all 4 o2 sensors at around 0.9v

Any ideas?

[mmm_2000]

Anybody ?

[nametakennow]

How's your water temp gauge acting?

If it's reading cold, the ECU thinks you need more fuel than you do.

[mmm_2000]

How's your water temp gauge acting?

If it's reading cold, the ECU thinks you need more fuel than you do.

while warm it reads 200f so it looks k. The gauge in the dash alsoshows a normal temperature. I never had a problem with the cts.

I'm thinking it's maybe my fuel pressure regulator.I'll have to test that.What else can it be?

[thereal_speedjunkie]

The MAF works fine, the air flow changes with the acceleration.I also noticed all 4 o2 sensors show between 0.8 and 0.9v at idle all the time. Under load, the same o2 sensors show changes in voltage like they are supposed to from 0v to 0.8.

Here is some data recorded over a period of a few mins at idle:-all 4 o2 sensors show voltages between 0.85 to 0.94v-MAF reading is 0.63 lb/min-engine temp at about 200f and the air charge temp at about 70f

I have a total of 4 o2 sensors.

Any ideas ?

First of all your cat converter is shot if your bottom o2 sensors are reading the same as the top 02 sensors, or your ses light is on TELLING you your bottom o2 sensors are shot. What are you getting for a % of load? If it is high then i would suggest you test your vacuum at idle and clean your MAF (almost all drive ability problems with the series 1 & 2 qg motors are caused by a dirty maf, ask me how I know).