Everytime I do a cold start on my is250. It starts up on about 1500rpm then within you can see the rev counter starts going down. It goes down to about 1100rpm and then shoots up for a second almost like someone pressing the throttle pedal. Then it stabilises. Is this normal? It's been happening for a long time

 

Well if it's cold out it'll start with a high idle rpm to warm the engine up faster and drop as the engine temps rise

 

I understand that but what I am referring to is a instant drop and then the car corrects itself when it realizes it has dropped so then surges up like a shiver. All this within the first 30 seconds. Definitely nowhere near warm yet.

 

well if you're having rough idles like that all the time I'd look into potential carbon buildup symptoms

 

2013 350 behaves pretty much the same way. 11 to 1200 rpm but takes a 15 to 20sec to drop to say 800 vs it 666 warm idle.

The added kicker is turning on the heat even tho its too cold to blow anything warm and its right back to 1000 rpm but it cycles on and off which blows as you stop at a light rpm at 800, then it bumps to 1000 and if brake pressure is low you start creaping... Should do wonders on the ice...

Anyone else get this heater on induced rpm?

 

OP turn the heater/fan OFF during warm up. Tell us if the RPM still jumps around.

 

Tried this. Still same thing. As soon as I start engine in cold within a few seconds the rpm starts to fall and goes to I think 900 and then it surges up for a split second to about 1800rpm and then back down to 1000rpm and then it starts to climb back up gradually from 1000 to about 1500. I've tried resetting by removing battery but no luck

 

Something is definitely wrong if it's dropping to 900rpm. My car does it on extremely cold mornings, such as today (-25c, -31c wind chill) but drops from 1700rpm to about 1300rpm then goes back to 1700rpm until it warms up. I'd look into the alternator and spark plugs if you haven't already. My old Mazda 3 used to do this. Replaced the alternator and everything was smooth from there on.

 

Try cleaning the MAF sensor. While your at it check for vacuum leaks. Maybe a hose came loose.

Clean only with CRC MAF cleaner only. Your local Auto parts store should carry it.

 

oh wow, that worked for me. Thank you!

 

what does it mean if the heater/ac/fan is off during warm up and the rpm jumps around less frequently / violently?