Idle Problem?

Hi,

we have 2 Luminas 98 3.8L and one have on idle if the engine is warm 1000 rpm and the other 900...
I did changed the sparkplugs and wires but nothing change...
The only diffrent is the 1000 rpm vehicle have changed fuel filter... Air, Oil and other stuff is made...
So why is the diffrent?

Oh the 1000 rpm Lumina have platinum spark plugs the other have regular TR55 NGK...

Maybe the tach is inaccurate on one of the cars, 100 rpm's? Try cleaning the throttle body and Iac on both of them.

Can anybody look which is correct? 1000 or 900? On both failure the engine soon light must go on or?

The pcm presets determine the idle speed, both 900 and 1000 are high, based on my scanner readings, my 98 LTZ 3.8 idles at about 750.

The pcm presets determine the idle speed, both 900 and 1000 are high, based on my scanner readings, my 98 LTZ 3.8 idles at about 750.

Warm or Cold start?
But the 1000 rpm Lumina runs since 3 years ago fine, the gas milage is good and the rest is fine, is the tachometer not correct?

Only way to tell if the tach is in accurate is using a scanner.