My 2007 IS 250 instantly drops to 1k RPM after I let off the gas. I can rev to 4k RPM and it'll go without hesitation, but once I let off the gas it drops back to 1k, whenever the car idles it sits at around 500-600 (warmed up). I got a P0172 code three days ago, I went ahead and replaced my MAF sensor and cleared the code but my RPM still instantly drops, spark plugs and oil were replaced 1000 miles ago. Any help is appreciated! Thanks.

 

If you still have the error, you need to fix it or will will run horrible and likely be in limp mode... It needs to know how air is flowing or it will never run properly.

 

^ Yes reset the ECU. You must wipe the stored long term fuel trims. Otherwise it will take a long time to adjust.

Ecu Reset

 

My RPMs still dip abnormally fast after the ECU reset. My idle RPMs are sitting at 2k on cold start, driving around I can see my RPMs are jumping up and down.

 

The rpms will hold that high until operating temp. After a ecu reset it's normal. Then should settle down to about 700 at idle. If at idle the rpms jump you could have a vacuum leak.

 

My 350 never cold idles above 1400 even when below freezing. On a 65°F day it bumps to 1100, then settles 900 in minute, once moved from the parking spot it settles at 680 after maybe 3 to 4 minutes of run time.

OP, is it still throwing codes? Have cleaned the MAF sensor with MAF approved cleaner?
If yes, start listening for vacuum leaks as suggested.

 

Thanks for the help guys! Yeah I replaced my MAF and cleared the codes but still runs bad. I'll look around and see if I can find a vacuum leak.

 

I'm still working on my IS's idle speed. It's 1.5K to 1.2K after a cold start. It eventually drops down to 1K. I've never seen it idle less than 1K even with the AC on. I'm thinking vacuum leak but I have no codes.

 

Have you tried to clean the throttle body? There could be carbon build up not letting the plate close correctly.

 

Not directly but I did run a full can of STP GDI cleaner through it to get rid of the carbon buildup. I guess I could get some cleaner and soak the plate area.