Low interstate MPG

Firstly it's not really smoke, it's likely just a vapor that's a combination of steam and unburnt fuel. The amount of this that you see will depend upon the recent trip history (like lots of cold starts and short trips). It can be a sign of a blown head gasket, but to be honest if you're not seeing any other signs of a bad engine then I wouldn't worry about it.

BTW. One little test you can do is to check your oil level immediately before and after a good freeway drive that will keep the oil a full operating temperature for around an hour or more. If you're not seeing any sudden drop in oil level then you're probably haven't got overly large fuel (or water) contamination.

 

It could mean that you have a little bit more blow-by from worn rings......but that's not really a concern if it isn't using oil.

I think it most likely was water vapor from the partly warmed up engine.......and absolutely nothing to worry about.
You really should warm the engine thoroughly before draining the oil though.

One does NOT consider rebuilding an engine that is running good and not using oil and not making any outrageous noises.

P.S. You gotta be careful about that stuff you read online.......except here, of course !!

 

sounds like your car is a lot like me, 'reduced performance due to age'.

 

I replaced mine at 124K miles. No reason really. Just felt it was about time. Didn't notice any changes in performance or MPG afterward.
In regard to your MPG, do you press the MFD reset before every trip? Short term averages are really affected by other variables, outside air temperature, wind, etc. A longer period average would be more representative of overall vehicle health. I drive all interstate miles at 68-78mph and never reset the MFD MPG value. It reads consistently between 47.4 and 47.6 MPG, and I confirm this with a periodic manual MPG calculation. Anything less all of the sudden and I know I've got a problem somewhere.

 

That does seem rather low. So you were getting 48-49 and then on the last tank the MFD value dropped to 38? I've never seen such a radical change in my vehicle. Just looking over the thread, didn't see any mention of the health of your 12V auxiliary battery. This battery is always the prime suspect when things aren't working properly or your MPG is low. If you keep track of such things, you might notice the ICE spending a lot more time running when it shouldn't be. Kind of hard to notice this if you are mostly at highway speed. The computers will keep the ICE running in an attempt to bring up the charge level of the 12V battery, and if your battery is toast then all that extra burned fuel is wasted. If you search the site there are various threads on the 12V battery and a process to check the level of charge and general health. It also helps to have a clean ICE air filter and to remove and clean (with MAF cleaner ONLY) the mass air flow (MAF) sensor on the side of the throttle body. You said your ICE noise was loud, like an exhaust leak maybe? This could be compounded if your catalytic converter is plugged up and could also be causing poor mileage. If it were me, I would probably take it to a dealer service department and pay for a proper diagnosis. This might give you (and us) more to wonder about.

 

the cat should get tested by way of your annual emissions test. but the info is stored and available anytime.

 

have you done compression tests, valve drawdown etc.? maybe it's sludge or carbon build up.

 

okay, i'm just grasping at straws.