sudden drop in gas mileage

1996 SL2, 200'000 miles. A few months ago soaked rings per 'saturnfans forum" post to cure excessive oil consumption; it did not work but fuel consumption went from 38 MPG to 25 MPG! Plugs are black, front O2 sensor reports LEAN, code for intermittent misfire, check engine light on. Replaced plugs & wires, O2 sensor, coil pack (measured open but still ran fine),MAP sensor, and PVC valve with no change. What is next?

 

Coolant temp sensors are a common failure point on these cars. Unplug the sensor and look at the pins in the connector. If they are green replace the temp sensor and pigtail connector.
The fuel mileage is directly affected when these fail. Also will cause hard start and smoking.

 

Thanks, I had forgotten that I did replace the coolant sensor with no change but did not check the pigtail. Is this the same sensor that drives the temp gauge (the gauge still reads normally) or is there another one; I have seen some references to two different sensors on some models but I find only one on mine?

 

Have you tried replacing the crankshaft position sensor? Not sure if related but I noticed a drop in mine, replaced some of the same things, then my car would die but start up again 20 min later like everything was normal, only to die less than ten minutes later. Would up being crankshaft position sensor. I think I gained back 3-4mpg but still missing another 3 or so from my old normal.

 

Update: MPG still terrible. note that new O2 sensor reads 850 mV when unplugged but 0 when plugged in. No it is not shorted to ground. PCM connector shows 3.5 k ohm between O2 sensor wire (14b) and next wire (13b) which is grounded. Is 13b supposed to be ground? where can I find a PCM pin-out for 1996?

 

Black usually indicates carbon buildup, the cylinder is most likely running Rich. Some of the old Chilton Manuals for individual models used to have a picture guide for spark plugs to aid in diagnosis. I would suggest running a compression test on each cylinder. 25-mpg for City driving (all city) seems normal. I have a 2001 SL2 DOHC and I average 25 mpg city and 35-38 mpg highway (all Highway) and mine is an automatic transmission. I had been getting some pre-ignition (pinging) under hard acceleration so I switched from 87 octane (regular unleaded) to premium 93 octane. That and changing the spark plugs solved 99 percent of the problem. If you get too much pre-ignition or pre-detonation (igniting the fuel before the piston gets to the top of the cylinder) you can burn a hole in the top of the piston which will result in a significant drop in fuel economy. The compression test will reveal which cylinder or cylinders have an issue. I did that to a 2.0L Dodge Motor and found the hole when I pulled the cylinder head. The GAP on the spark plugs also plays a part. For my 2001 SL2 1.9L DOHC the spark plugs gap is 0.040". You don't need the fancy Platinum spark plugs. Changing the heat range of the spark plug, depending on the type of driving you do, may also help. They put heat range guides in the back of the spark plug application guide catalogs. If your Parts Store doesn't have a paper Catalog look the Spark Plug brand up online to locate the PDF version of the catalog.

 

New Old Guy here...
Good call on the coolant temp sensor.
Not sure what to say on the O2 sensor other than maybe the exhaust is full of fuel and possibly the cat too?
I wonder if one (or more) of the fuel injectors may have failed?
It does happen eventually. They are just solenoids, right?
Think about how many times an injector has to open and close for 200K miles?
Hope you find your excessive fuel consumption issue.

 

If you did an MMO soak, throw out the plugs and get some stock in GK plugs in there.

Your O2 sensor was no doubt poisoned by all of the oil residue that did release from the rings and got burned in the follow-up ignition cycles of the engine. When the O2 sensor gets poisoned, it does not measure as much oxygen as is really present.
Thinking there is not enough oxygen present, it richens the fuel mixture to increase the amount of oxygen detected at the front sensor. So you think you would get a rich code, but you don't because the PCM, in turn, sees the rich mixture driven by the O2 sensor and says Hey that's not right and drives to drive the fuel air mix so it is More lean. So the two are fighting with each other, the PCM can't lean out the mixture any further than it already is and you get a check engine light. Please post the codes so that we can independently confirm what's what. You should have also gotten a code for the front O2 sensor being whack.

Coolant temperature sensor has nothing to do with the procedure you performed. It's sitting into the corn flow in the head. If it has failed, it is total coincidence. It is however true that this sensor and the feedback it gives to the PCM of critical with respect to driveability and mileage per gallon.

I also doubt that anything has changed with respect to the O2 circuit unless the wiring was already damaged before you started. Or you disturbed the wiring while doing the soak which doesn't seem likely based on where it runs. At least under the car part.

Replace the front O2 sensor with the denso equivalent of the AC Delco sensor. They're identical as the denso was the manufacturer for the OEM part for Saturn. Take the 12 bucks and buy a cup of coffee.

If you're getting a ground where you do not expect one to be when you hook things up, chances are a wire is chafed somewhere. I cannot remember if the front '02 is a heated '02 sensor, but if the heater for that sensor is an issue, you should receive an O2 heater code.

Do not attempt to use a Bosch or universal '02 sensor for this application. They do not play well with Saturns in my experience. At least the s cars.

Again please post all codes as PXXXX, not the definition of the codes.

Thanks.