1993 Honda Prelude bad gas mileage with slight exhaust smok

Hi napri266,

Thank you for the donation.

Was any repairs carried out prior to this occurring?

Is the ignition timing correctly set?

Did you check the throttle position sensor?

vehicle was recently purchased and has a new egr valve , checked timing and looks good, have not checked throttle position sensor. But would that cause the drastic consumption that is occuring.Its getting about 12 mpg Hiway. I replaced cat converter also because it looked like it was time. Still no improvement

Yes, improperly adjusted TPS can cause a lot of difference.

Remove the spark plugs and check if they are sooty.

If the performance of the vehicle is poor, I wold suggest getting the timing belt checked for proper synchronising. If the Exhausr camshaft is in time you would get the ignition timing correct but if the Inlet camshaft is out of time, it would cause a drastic power loss and bad fuel consumption though the ignition timing is correct.

Is the Check Engine Light indicating when driving?

DId you try to retrieve for trouble codes?

Engine does not have any power loss runs and idles great. Check engine light comes on and off periodically . When I check code using a jumper in the service check connector it shows 2 long blinks and 3 short blinks = code 23 bad knock sensor. But no pinging or acceleration problems. The only symptoms it has are dramatic fuel consumption , slight white exhaust smoke and rich exhaust odor. Spark plugs are good. The engine is an H23A1 non Vtech

A bad knock sensor or trouble code from it indicates either the timing belt is out of sync, timing is too advanced or the fuel octane is too low for the setting.

Pinging is seldom audible for Honda engines and the knock sensor fault would not result in noticeable difference in the engine performance.

If the exhuast smoke is completely white, that is vapor and can be due to the weather or some problem with the head gasket which I doubt is the case as you mentioned it idles well.

If the smoke is slightly grey, it is oil burning and could be due to the valve seals.

Rich exhaust gas could be due to the EGR system not working correctly as the spark plugs don't seem to run rich.

For the high fuel consumption, it is a little puzzling as the spark plugs do not indicate a rich condition.

Any possibility of a faulty odometer reading?

yes , I was lied to ,mileage is 248k with new timing belt

Have you found the cause of the problem?