Sudden gas mileage decrease after long idle!

[WDoherty]

My car (sc400) always averages around 22mpg per tank at fill up, mostly highway driving and some city driving. Well, a couple of weeks ago i had to wait almost two hours to cross the border and idled the whole time. As soon as i got across i filled my tank and these last two tanks have averaged 18mpg and one was almost all highway. This is a huge sudden decrease! Did i fry my sparkplugs by idleing so long or something else?

 

[Peewee]

See how it goes on the next tank.
You may have gotten an old batch of fuel.

 

[a.Lpha]

really there's not THAT much of a difference idleing and driving at a slow cruise... nothing should have happened there... the octane rating could easily have been too low in that gas, or something like that.

 

[Warm Pudgy]

the only thing that is can think of is that the 1uzfe is an oversquare engine
and oversquares have a tendency to over heat
usually though its not a problem

but idling for 2 hours certainly could have caused something to warp
perhaps your pistons, valves, or injectors
if you read the manual it clearly says do not let the engine idle for long periods of time, just shut it off

alpha: while there wouldnt be much difference in rpms there wouldnt be nearly as much air flow over then engine

 

[Warm Pudgy]

i thought of another possibility

did you have it in drive the whole time or did you slip it up into neutral while waiting?

sufficient idling in drive can cause the torque converter to produce a significant amount of waste heat thru friction

you could have warped the turbines inside maybe

but i think this would affect power and acceleration more than fuel economy

but just a thought

 

[WDoherty]

I was in neutral and park most of the time. I didn't want to turn off the engine b/c you end up moving about every 30 sec. I think carbon buildup is to blame for the mileage decrease. I just seafoamed my engine and will be changing spark plugs tomorrow (amongst other things) and i will see if it returns.

i thought of another possibility

did you have it in drive the whole time or did you slip it up into neutral while waiting?

sufficient idling in drive can cause the torque converter to produce a significant amount of waste heat thru friction

you could have warped the turbines inside maybe

but i think this would affect power and acceleration more than fuel economy

but just a thought