95 Geo Prizm rough idle and SUPER poor gas mileage (10mpg)

[95GeoPrizmGuy]

Hey guys so I bought a 95 geo prizm not too long ago and when i bought it i noticed it idled rough (vibrates the interior-is loud)...i did a quick analysis and saw that the motor mounts needed replacing, So i thought thats all it was and bought it...the person I bought it from didnt take care of it except for changing the oil on time so I figured a good tune up would get things straight...So far I've replaced:
Spark Plugs,Plug Wires
Cap and Rotor
Air Filter
PCV Valve
Timing Belt
Crank and Cam seals
water pump
valve cover gasket (i asked the shop to adjust the valves if need be but they said they were fine)
Front and rear motor mounts
exhaust flex pipe
oil change/filter
coolant flush
idler pully
fuel filter
and cleaned the throttle body (REEEEALLY DIRTY)
i also put some premium gas and some fuel injector cleaner in it

AND STILL...its idling rough and gettin terrrible gas milage (10mpg)
the exhaust breathes well from what i can tell and theres no black or excessive white smoke after warm up. (its cold here in ny)...no sweet smelling exhaust...and when i bought the car if you put your finger in the exhaust it came out like a diesel! now it doesnt, so i can assume its not burning oil after the tune up and the oil level on the dipstick is consistent...

so any ideas?
IACV
EGR
EGR vacuum
vaccum leak (i checked but couldnt find any)
faulty fuel regulator?
messed up cat or o2 sensors?
dead fuel injector?
goblin in gas tank drinking gas?

the idle is really rough when cold but calms down when warm (but still bad) and btw i drive like a grandma...people honk as i leave a stop light cause i take my sweet time lol

its starts easy and hasnt stalled ever

PLease Help!

[pdq]

I'd check your front O2 sensor or for vacuum leaks. Make sure if you do your O2 to get the OEM Denso model.

If fuel injectors, you probably would have bad acceleration but not bad mileage.

[pdq]

Actually, the only possibility with fuel injectors would be if the seals are leaking gasoline externally. That's harder to diagnose unless it's bad enough that you see gasoline spilling out.

[panama2005]

I have the same issues with my Coro 96 Automatic AE101, bad mpg, terrible rattling in cold start, overheats after 10minutes, i could say mine´s is a lemon, i´m working hard on it, i even believed the gas tank has hole, fixing the exhaust pipe at moment

[Bitter]

faulty coolant temp sensor?

[speedy25]

I would suspect leaky injectors, bad fuel regulator (easily tested!) or your neighbors are siphoning gas while you sleep.

Good maintenance on the other items. After you fix the current problem you should have lots of troublefree miles.

-SP

[Rich L]

Replaced the motor mounts a few months ago on my 96 with 230k. Made a big difference in the perceived vibration at idle. Vacuum leaks can cause all sorts of issues, but there really aren't that many lines. Pretty straight forward to chase all of them down and verify issues.

Coolant temperature sensor and o2 sensor should not be related to your rough idle when the engine is cold. The computer isn't reading the o2 sensor until the car is warmed up. Either or could be responsible for the poor mileage. Just got a new front Denso direct fit o2 sensor for around $35. Never had issues with the coolant temperature sensor on my car.

I had EGR issues about 180k and they exhibited themselves as minor hesitation under certain circumstances and eventually a CEL. Never had issues at idle and did not cause significant changes in mileage.

Is the car reaching operating temperature? Short trips in the winter kill mileage, although I have never heard of a Corolla getting 10 mpg.

[speedy25]

Originally Posted by Rich L

I have never heard of a Corolla getting 10 mpg.

And of course all the other things you listed would decrease mileage but not to 10!

Either the OP is exaggerating or if really has a horrendous fuel leak somewhere.

-SP