It's had this problem for a while, but with a new A/C season starting here in Alabama, I'd like to fix it if I can afford it. Last summer I'd switch the A/C off because the idle was surging from about 700 to 1000 RPM, then restart the A/C once the car was moving. These days it surges a little less w/o the A/C, but sometimes it's smooth as silk. Doesn't seem to correlate to warm or cold engine, air temp, relative humidity, or phases of the moon.

It's got the manual trans, 122K, Bosch platinum plugs, and I run regular gas. I use Techron with each oil change (every 4-5K miles). The car accelerates like a scalded dog, but the rough idle is annoying. Any ideas?

01 T5, 98 V90 (RIP: 91 740T, 86 240 wgn, and my favorite bronze 88 740 turbo wgn that choked on its second turbo at 240K)

First, use premium gasoline ONLY. That crud is fouling your plugs, detonating like crazy, and the timing is retarded.
Second, use only copper plugs with a high pressure turbo. Platinum will melt in these engines at high rpm.
Stop using Techron.

Then after a couple of tanks of gasoline, let us know if it is still surging. If it is, it is probably the ETM.

Klaus
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Praying for Cynthia Amendt, heaven bound