1999 SC2 142k miles. Gas mileage has dropped to about 23mpg average of both city and highway driving. I have recently installed new plugs (NGK) and checked all the wires and coils for corrosion. ECTS is new about 2 years ago and is of correct brass type. Oil and filter changed recently. Using a scan tool I find that the coolant temp is about 170 F and the needle on the gauge is just below 1/4. I'm guessing that the coolant may be too cool and the PCM is providing a rich mixture, does that make sense? If so my next move would be to replace the thermostat with a Stant 195 degree.

I should note that the car runs nice and smooth and has good acceleration...

Also, the scan tool reports that the TID $07 $0D and TID $0C $20 are high and fail. What is that all about and would that impact the gas mileage?

Any and all help gratefully accepted!!!!! Thanks in advance!!!!

Tom

 

First, I think you've got the wrong plugs in it. I would recommend going back to whatever the sticker under the hood recommends. Also, yes, it's running too cool. You definitely need to put an OEM temp thermostat in it. Are you running a 50/50 blend of anti-freeze to distilled water? If not, correct that too. As far as the rest of your post, I can't make heads nor tails out of it. BUT, I'm not a mechanic.

 

Well, I am a professional mechanic and the last part makes no sense to me either. Yes put a "superstat" in it not the regular Stant thermostat there is a difference. Select the "Oem" temp.

 

I see you are looking at generic OBD2 data streams, I have the actual OEM software for what I work on at work. Please list the description of the codes and I will try to help. I only use Mode 6 because it has a great misfire monitor as part of my diagnostics. There may be other problems with your car other than the wrong thermostat. You may have a lazy oxygen sensor also. Correct the thermostat first and report back.

 

usually throws a rich code if that rich. Any SES codes at all?
Have no idea what that jibberish at the end is about

Also:

Significant drops in gas mileage are almost always attributable to either the ects or the front o2 sensor since these have the greatest direct impact on air fuel mix.

Brass ECTS AND CONNECTOR should be sourced from GM dealer due to calibration differences with aftermarket parts and the connector due to corrosion issues which may contribute to inaccurate readings made by the pcm.

Have you ever changed the front O2 sensor? If not, you're overdue. Usually get around 100K out of front on an S car. If you do change it, I recommend going with the denso equivalent of the OEM part (GM used a denso as OEM). A bit more costly and usually special order, but my two s series coupes run like crap on the bosch front O2's.

And the NGK's are indeed stock, Rube (stranger than fiction, I know)

Don't mess with the thermostat for now. (My opinion anyway).