Fluctuating Idle in 3.5L
 

Hello
My 00 Acura RL starts and runs at a normal rpm but when it is cold outside and the car gets to operating temperature if you put the car in neutral or Park the rpms will jump up to 2000 rpms and if you tap the accelerator will go from 1000-1500 rpm. When it is hot outside it does not fluctuate at all, all summer it idled normal both at start up and at operating temp. At about 50F outside it starts to show these symptoms but sometimes the idle smoothes out after 2 minutes. It seems the colder it gets (now 5F) the higher the rpms will go and I haven't noticed it stopping yet but I will check. At current temps the idle goes from 2200 rpms to fluctuating without me touching the gas pedal. When I first had it checked at a shop it had IAC codes (will have to check) and both IAC valves have been replaced with OEM new. Coolant flushed and new radiator cap. What else can it be?

That is because of the temperature difference. If your transmission is automatic it is a very common thing and nothing to get worried about.

I can't see what was the premier car of their car line was designed to sit in Park with the engine constantly going from 900-1500-900rpms all the time. If it is common is their any solution to this. It affects the drivability because the car now accelerates from a stop without pressing the accelerator and when slowing down it affects the trans shifting.

From my searching it seems to be a common thing among Hondas and Acuras but the faults are air pockets in coolant, vacuum leaks or dirty/faulty sensors.