General LSX Automobile Discussion - LS1 Gas mileage/temperature problem

Man, I haven't logged in here in a looong time. I miss this place.

Anyway, I've had a gas mileage problem for a while now. Normally my 2000 M6 SS gets around 24mpg; with this issue it's down to around 16-17. It's been going on for some years now; between life things and other mechanical things with the car I'm only really getting around to trying to figure it out now.

There have been no driveability issues; everything feels absolutely normal. No codes, no gas smell, all emissions measures are normal. I cleaned the MAF sensor, no change. I haven't tried changing out the MAP or O2 sensors, but with no codes, no weirdness in emissions or how the engine's running, and no degradation over the past four years, I'm skeptical that they're the problem. I had the spark plugs and wires changed about 1.5 years ago, no change. The other strange thing is that the fuel gauge appears to be dropping while the car's parked. I'll drive the car and leave it and look at the fuel gauge, and when I start it later the gauge will be 1/32-1/16 tank lower than it was. I have no idea if this is just the gauge being weird or if I actually have a fuel leak somewhere or what.

In the past six months or so I've noticed the engine heating up a lot faster than usual, at least according to the dash gauge. It isn't overheating, just warming up significantly faster than before and then staying at a stable temp. Again, no codes, no driveability issues, coolant level is fine and I can't find any evidence of leaks external or internal. I don't know what's going on, but my idle thinking makes me suspect that something's making the car run extra-rich, causing both the temperature and gas mileage differences.

Searching for these various symptoms hasn't gotten me anywhere. Anybody have any insight for me? Thanks in advance!

have you done a tuneup recently? how many miles on the car? i would do new sprak plugs, wires, o2 sensors, and clean the maf as well as throw some seafoam in the tank. all of those little things can have a big hit on mpg's especially lazy o2 sensors. as far as the the temp goes how fast are we talking. weather, accessory load and many other factors attribute to that I'd say as long as it isnt going past the 210 mark youre ok. but if you like to be safe get a scanner and monitor the pcm engine temp. also for the gas gauge between my friends and i we have had about 7 fbody's and the gas gauge is about as accurate as just sticking a dipstick into the tank yourself it will move around when parked, on hills, turning, etc etc

I think your mpg and "heating up" problem are unrelated. I would think that the richer the engine ran the cooler it would run. It obviously wouldn't be a huge difference in temp but my rc car and weedeater both run hotter the leaner i make them go and visa versa. I could very well be wrong because the car isn't an air cooled or a 2 stroke. If I'm wrong please disregard this whole post.

You might find a tuner to log some data to see if anything weird is happening that isnt obvious.

X2 on sefoam, search for the write up and do the oil, fuel and top end

Have you tried to clean the injectors? How did you calculate your MPG? Also keep in mind your temp gauge is a dummy gauge, mine will show about 205, but it is really around 190 according to my OBDII scanner.

I think your mpg and "heating up" problem are unrelated. I would think that the richer the engine ran the cooler it would run. It obviously wouldn't be a huge difference in temp but my rc car and weedeater both run hotter the leaner i make them go and visa versa.

I agree on both counts. An engine will run hotter with a leaner mixture and/or retarded timing. An engine will run cooler with a richer mixture and/or advanced timing.

If the gas gauge is actually dropping even when the car hasn't been driven inbetween noting the readings, then the most likely reason would be a leak. The only thing that makes this unlikely is the fact that the leak would have to seal up when the engine is running, otherwise you'd be getting EVAP codes.

The faster warm-up time might just be a matter of the temp sending unit aging, or maybe there is a minor blockage somewhere in the cooling system that is restricting flow enough to allow faster warm up but not enough to actually over heat. You could flush the system (if it hasn't been done recently) and see if that changes anything.

That's a pretty big drop in gas mileage. But if the engine was really running that much richer, I would expect there to be a code or emissions failure by now. Other things that could have caused a decrease in mileage without an increase in emissions nor a sensor/component failure nor noticeable driveability issue would be under inflated tires, additional weight added to car, change in driving habits, change in fuel blend or octane used, air filter blockage, or any combo of the above.