Lowering idle control speed

[M1Accord]

My 1992 Corolla idles rough while in D at 700-800 rpm. I boosted it up to 1100 rpm and it idled fine but gas mileage went from 35 mpg to 18 mpg. Therefore, I dialed it back down to the 550 and it seem to idle much better; no vibration. However, it does act strange, as if it wants to cut out but it never does. Gas mileage went back up to 32 mpg but that's because it's winter so I do expect 35+ mpg in warmer month.

Is it bad to have the idle speed too low. I believe the factory setting is 700, which create loud vibration that sends a head jarring ache when I hold the steering wheel.

Thanks.

 

[Quest]

Is it carb'ed or EFI?

If EFI, check for possible vacuum leaks, IAC valve passage carboned up, etc. also replace/service PCV valve with factory OEM unit.

check/replace spark plugs, distributor cap and rotor and/or spark plug cables.

Last but not least: put in a bottle of PEA based FI cleaner in the tank and see how it goes afterwards.

Q.

 

[M1Accord]

It's EFI. 92 Corolla LE.

These items were done very recently:
check/replace spark plugs, distributor cap and rotor and/or spark plug cables.

It idles almost fine at 550 rpm and perfectly fine at 1100 rpm but I think the book calls for 700 rpm, which cause vibration.

 

[mechtech2]

Going from 35 to 18 from changing the idle is bizarre.

Trying to stop a harmonic vibration at the 700 speed? Going up or down a bit is fine.
It sounds like you have no idle control speed on that model.
The idle will therefore change when cold, hot, in gear, or out of gear.
Find a compromise.

 

[IndyIan]

Sounds sorta like my motor mount issue, my front mount(controls most of the rocking) is not happy, so if the car is in D but stopped, at certain rpms the steering wheel starts to vibrate.
Also I can feel the motor vibrations a bit more under hard acceleration. Have a look at the mounts anyways, if your motor is running well then it might be the motor mounts.