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 Post subject: Help with O2 sensors
PostPosted: Sat Jun 09, 2018 1:13 pm 
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Joined: Tue Sep 29, 2015 11:36 am
Posts: 6
Year and Trim: 2002 se
I have had a decrease in MPG to 22, it used to be 27 and on long highway drives 30. I then changed upstream O2 sensor, spark plugs, map. Cleaned injectors and downstream O2.
Checked for brake drag and tcc slip was ok.
I always use OEM parts or premium brands when replacing parts.
Here is my test of O2 sensors today.
KOEO: both 4.5v, upstream declines to .1 after about 40 seconds
Engine on: upstream goes to .9, downstream goes to .1
Closed loop entered: upstream oscillates between .4 and .8, downstream .2 small oscillation of .05 not in time with upstream.

On the highway yesterday the downstream stayed at .1 or 0.

Is this normal for O2 sensors?
Should I replace a O2 sensor? Both or which one?

Thanks.


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 Post subject: Re: Help with O2 sensors
PostPosted: Mon Jun 11, 2018 3:58 pm 
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Posts like an L67
Posts like an L67

Joined: Thu May 01, 2014 9:37 pm
Posts: 1277
Year and Trim: 1993 SLE
1998 SE
2001 SSEI
2002 SSEI
AFAIK, both at idle closed loop readings seem fine.

There are many possible causes of poor gas mileage.
Did you check for any stored codes?
Are there any problems besides lower gas mileage?
Did the problem start right after you cleaned or changed something?

What brand spark plugs are you using?
If you're using Bosch spark plugs, change them out for ACDelco plugs.
For whatever reason, the 3800 engines hate Bosch spark plugs---and Bosch sensors.
ACDelco is the only way to go on the plugs and the O2 sensors.
For a MAF sensor, Denso made the original and is still the best to use when replacing.

Before you start the car in the morning, see if the intake air temperature sensor and the engine coolant temperature sensor are giving you readings very near the actual air temp in your location.
Borrow a fuel pressure tester from Autozone, etc. to check fuel pressure at idle.
Next, pull the vacuum hose off the fuel pressure regulator and record the fuel pressure.

Since you have a scan tool, on a warm engine, check the STFT and LTFT at idle and then hold the RPMs at 2000 and record that reading.

What readings is the MAF sensor giving at closed loop idle?

The downstream oxygen sensor does not impact fuel consumption.

Here is a good series on what else to check:
https://www.ericthecarguy.com/faq/findi ... f-poor-mpg


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 Post subject: Re: Help with O2 sensors
PostPosted: Mon Jun 11, 2018 5:08 pm 
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Joined: Tue Sep 29, 2015 11:36 am
Posts: 6
Year and Trim: 2002 se
No stored codes. Spark plugs are iridium autolite. AcDelco MAP and O2 sensors. Denso MAP. Temp sensors are correct. Tried fuel pressure once but the gauge popped off the rubber hose so I thought it was good. Checked fuel pressure regulator, no gas smell. LTFT is about -5, STFT goes 1 to -3 at idle. Will try LTFT and STFT AT 2000 next. MAF at idle is 3.8.
I feel a slight miss when cold. A very very slight non smoothness at idle hot. No misfires on any cylinder recorded.


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 Post subject: Re: Help with O2 sensors
PostPosted: Mon Jun 11, 2018 8:53 pm 
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Posts like an L67
Posts like an L67

Joined: Thu May 01, 2014 9:37 pm
Posts: 1277
Year and Trim: 1993 SLE
1998 SE
2001 SSEI
2002 SSEI
I wouldn't count on the fuel pressure being OK.
If the gauge popped off the rubber hose, then it was likely the wrong type of gauge (old kind for non-electric fuel pumps.)
Ignition wires are another item that needs replacement as these cars age.
You can mist them with water when it's dark and see if there's a "light show" as the sparks jump to unintended places.
This would , of course, indicate poor insulation/need for new wires.


Last edited by MKMike on Tue Jun 12, 2018 7:48 am, edited 1 time in total.

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 Post subject: Re: Help with O2 sensors
PostPosted: Mon Jun 11, 2018 11:20 pm 
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Joined: Tue Sep 29, 2015 11:36 am
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Year and Trim: 2002 se
Wires are belden 3 years old. Will redo fuel pressure test. Tonight while stopped at a light MAF was 5.8, rpm 750, LTFT was -7. I thought the MAF was high. The downstream O2 sensor was 0.3, better than during the weekend which was 0.1.


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 Post subject: Re: Help with O2 sensors
PostPosted: Wed Jun 13, 2018 1:49 pm 
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Joined: Tue Sep 29, 2015 11:36 am
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Year and Trim: 2002 se
Mike, You were right about the pressure gauge. It does not go high enough. I will find a gauge that goes to at least 60. Did a vacuum test. At idle, 750 rpm, vacuum was 19. The needle moved about the width of a hash mark line. My elevation above sea level is 500 ft.


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 Post subject: Re: Help with O2 sensors
PostPosted: Wed Jun 13, 2018 8:41 pm 
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Joined: Tue Sep 29, 2015 11:36 am
Posts: 6
Year and Trim: 2002 se
Fuel pressure is 48.


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