Hello guys
i have an 03 RL (c35a engine, 200k miles). it has a rough idle, almost feels like a slight misfire, no codes. I've replaced the fuel filter, cleaned the throttle body, cleaned the EGR port, changed all 6 spark plugs with OE Denso 3275's, checked for vacuum leaks to no avail, the car runs fine under load - cursing down the highway and in town. happens in neutral and in drive. Any ideas?
I've just replaced the timing belt and water pump, so I'm hoping to keep this car for awhile, but the idle is driving me crazy...maybe ignition coils?

 

idle air control valve perhaps needs cleaning

 

Have you removed the intake manifold disassembled it cleaned all the gunk out, cleaned the egr pipe behind the engine These engines are sensitive to dirty passages and a dirty intake manifold. Is the pcv valve good? I have a 2000 with 256k and have performed this service twice in 2012 and 2020

 

Put gallon of laquar thinner with ten gallons of fuel! As it is major ingredient in fuel injector cleaner! Run till empty! Sounds like a clogged injector/intake path, also do a relearn! Pull bot ecm ecu fuses out replace after a couple minutes idle at 3,000rpm. For 3 minutes, as cleaning tb may have taken it out of wack. Also using the laquar thinner will help with cleaning the ice! Or open it and manually clean it, good luck.