# Fast idle on start and Severe MPG drop

**URL:** <https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969>\
**Category:** Maintenance/Repairs\
**Created:** [July 5, 2011, 4:41am UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969 "2011-07-05T04:41:37Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![natparksroadtrips](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@natparksroadtrips](https://community.cartalk.com/u/natparksroadtrips)\
**Post date:** [July 5, 2011, 4:41am UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969/1 "2011-07-05T04:41:37Z")

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I have a 2000 Subaru Legacy Outback with 208,000 miles. Last Oct I had a complete tune up done on it: fluids, filters, spark plugs, etc. Ever since then, the gas mileage has dropped from getting around 390-400 miles off a full tank to 290-300 miles off a full tank. I have tried carburetor and fuel injector cleaners and nothing is working. At this same time, it also started idling at 1750 rpms every time I start it, dropping down to around 750-900 rpms after I put it in gear. The timing system was replaced about 7 months earlier. So far the repair shops can’t find anything wrong. I also just had the transmission rebuilt and it’s still idling fast. Any ideas?

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**Author:** ![natparksroadtrips](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@natparksroadtrips](https://community.cartalk.com/u/natparksroadtrips)\
**Post date:** [July 5, 2011, 4:47am UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969/2 "2011-07-05T04:47:07Z")

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Oh, the check engine light has not come on.

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**Author:** ![VDCdriver](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/vdcdriver/32/30_2.png) [@VDCdriver](https://community.cartalk.com/u/VDCdriver)\
**Post date:** [July 5, 2011, 10:31am UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969/3 "2011-07-05T10:31:46Z")

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A bad temperature sensor would lead to all of the problems that you described.

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**Author:** ![keith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/keith/32/5518_2.png) [@keith](https://community.cartalk.com/u/keith)\
**Post date:** [July 5, 2011, 5:50pm UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969/4 "2011-07-05T17:50:58Z")

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A vacuum leak could also account for the problems. It won’t set a code unless the idle is 200 rpm above normal for two minutes, and you would have to meet this condition twice within 3 drive cycles to turn on the light.

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**Author:** ![natparksroadtrips](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@natparksroadtrips](https://community.cartalk.com/u/natparksroadtrips)\
**Post date:** [July 18, 2011, 12:17pm UTC](https://community.cartalk.com/t/fast-idle-on-start-and-severe-mpg-drop/65969/5 "2011-07-18T12:17:15Z")

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Took it in and the shop could not find any leaks and there was no code when I had that checked. I was wondering if the timing could be off, the spark plugs were not put in correctly, or maybe something with the fuel pump or lines?