Oscillating idle / shudders with HEAT ON - 09 hatchback

[James in Denver]

Howdy Yaris experts,

My first post :)

I have a Bayou Blue 2009 Yaris Hatchback 5 speed. Just got it 3 months ago used, but with only 8500 miles.

Anyway, its gotten colder in Colorado and I've noticed that the car idles very roughly when I turn on the heat. It oscillates from normal say 5 secs to vibrating the front of the car for maybe 1 sec, then back to normal. This happens when its warmed up usually.

Outside temp is 30s to 50s, car temp light is off (although it might happen when its still cold, never checked).

It idles normal when the heat is off. Haven't tried the AC yet as its cold outside. No difference with headlights on or off so I don't think its power/alternator related.

I have the remaining factory warranty, but I want at least a little idea of what might be wrong so I don't get screwed by the service techs.

Any ideas?

James in Denver

[xaero_81]

Mine shudders when the cooling fan kicks in then it levels out. I wouldn't worry about it.

[annn]

They all shudder when fan comes on and when ac runs...ac does not have to be pushed in for ac compressor to run some heat settings run it also to keep refrig moving.

[R2D2]

Sounds pretty normal. If it runs "smoothly" with the heat off try moving the selector off one of the defroster selections such as lower vents only. I suspect the idle/vibration is from the AC comp as stated above. Try turning the AC on for a few seconds also for comparison to confirm.

[bankrobber]

Mine did same thing. I thought it was going to stall out sometimes. I replaced the spark plugs with some Denso Iridums and it has never idled low again. The old plugs were very worn down at 104,000 miles. It may start bogging down again after 20K miles on plugs. Have to wait and see.