Gas Mileage O2 Sensors and Spark Plugs

[TheDoughboy]

Hello All,

I have a 1997 Park Ave with 76,xxx miles. I am the second owner have have the service records, it's had the needed services at the correct mileage.

My problem is that my fuel mileage has dropped from an average of 25-26 down to 19-20, this has been the 3-4 tanks.

My driving habits have not changed, my route to work has not changed and I am using the same gas from the same 2-3 stations. The motor stil has plenty of power and I do not notice any strong fuel smells or leaks.

First thing I checked was the air filter and it's almost new still. I even pulled some plugs to check them and they look ok, still look like the original ones. I'm thinking O2 sensors, but I have no check engine lights and the spark plug wires are not near the O2 wire.

I had the parts store check for stored codes and the handheld unit said no codes found.

From what I know O2 sensors get slow and lazy as they near the end, but I cannot find the recommended mileage on when to chage them. Some cars are 60K some are 100K.

I am thinking about getting some new plugs even though I'm not at 100K yet. A fuel filter might be nice as well but I can't find the mileage on replacing that either.

Anyone have any info on when GM says to replace the O2 sensors? I could use any other tips you might have to.

Thanks,

Ron

[HotZ28]

You need to look at short & long term fuel trims first. Also, check EGR % while driving. The life of an 02 can range 60-100K, fuel filter should be changed @30K or less.

[TheDoughboy]

Ok, will be doing a fuel filter for sure. How do I check the trims and egr while driving?

What tool would I use for that? Back when I had my Grand National I used a Scanmaster to read all the values in real time.

[C man]

An Actron obd II scan too or any OBD II scan tool that reads live data will work. you should also make sure your not having a plenum problem. I read somewhere that someone had gas mileage drop as a symptom of plenum failure. at 75k you should worry or at least check that out and keep an eye on fluid levels.

[HotZ28]

When the gas mileage drops, that is an indication that the PCM detects a lean condition, so therefore it commands fuel enrichment. That is where you will see the fuel trims on the + side. Under normal operating conditions, the fuel trims should range from -5 to +5. The STFT will move up & down based on accerlation, however if it remains high for a predetermined period, it will set the LTFT up one notch. Mileage will suffer when the LTFT approaches +10. BTW, be sure to check all vacuum fittings & hose. Any un-metered air will cause fuel trims to go positive.

[TheDoughboy]

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So the latest is I unpluged the front o2 sensor and drove to work. The SES light never came on in the 18 mile drive. I even started the car a couple times before the drive, no light.

On the way home, still unplugged it finally came on. So I plugged it back in and disconnected the neg battery cable to clear the code. The SES light did go out this time.

On the other hand I did get plugs, the front o2 and a fuel filter to install this weekend. I am still looking at getting a scan tool.

[TheDoughboy]

I have the shutter when hitting 4th gear, uphill or not. BUT when I drove to work with the O2 sensor Disconnected it was Gone. Smooth as silk on the way to work that day.

Re-connected it for the drive home (then it threw the code) and as usual the 4th gear shutter came back.

I still have yet to figure that one out, first the mileage issue.

Today I installed a new O2, plugs and fuel filter. Double checked my tire pressure as well. I run at 35 psi.

I'll top the tank off tomorrow and see the mpg of this tank full then report back.