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Idle problem when blower operates - after tune up

Just got my timing belt replaced and a tune up on my 2005 subaru impreza wagon (80k miles). Car runs great, except that the idle now pulsates from 1000 to 2000 and back every 5 seconds when the blower is on. usual idle is smooth 750 rpm steady with blower off. Confirmed that it is not AC compressor by disconnecting electrically and mechanically (belt). The mechanics are stumped. What the heck happened? Any ideas? This was not an issue before the work.

Did the dealer or a reputable shop do the work? I bet they got the timing belt on tooth off and it is causing this.

its a good local shop that i’ve been going to for a while. can you elaborate more on your theory? it runs really fine with the blower off. smoothest I’ve had in years. How can one tooth of misalignment affect blower-idle relationship?

Perhaps the throttle body needs to be cleaned and relearned.

interesting thought on the throttle body. Do you mean that the car’s computer is basically confused because the tuned up engine is running well? That sounds plausible. But how does the blower come into the picture? When it is off, no problem. When it is on (at any setting, 1-4) that’s when the idle problem happens.

Sounds like a draw from the alternator is causing the idle change. Do headlights or any other current demanding device cause the same thing? Sounds like more troubleshooting is in order

no, just the blower.

I wonder whether the fans (that’s the blower you speak of, right?) is the effect but not the cause. They come on when the car gets to temperature and it is sitting still. Since they had the belt off, I take it they replaced the water pump because that’s a no brainer. So they drained the coolant.
It could be that you have an air bubble in the cooling system now that throws the temperature reading off, making it believe the car is hotter than it actually is. The computer turns the fans on and adjusts air/fuel mixture accordingly. Since it really isn’t that hot, the car runs like a hooptie when it is idling with that fan on. It will hunt for idle and never quite find it.
That sensor on a Subie is towards the top of the engine. That is also where air bubbles tend to collect when the car is sitting still.

Of course, it could be other things but that’s where I’d look: an air bubble in the system or maybe a bad temperature sensor. It is easy to check for and add coolant so it is also a cheap way to see if this is your problem.
Subarus like to be burped: It is often best to fill the engine first with coolant using the top radiator hose, then quickly move the hose back onto the radiator. Very often, when you don’t add coolant that way, you will get a bubble in the system that very often messes with the temperature sensor.
Park it under a slight incline or on a ramp, and run the engine with the radiator cap off. Rev the engine up and down a little. Watch bubbles come out the radiator neck and, when you see them, notice the level dropping a bit. When you see that, add more coolant.
Do this until you see no more bubbles.