Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Fast idle on start and Severe MPG drop

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?

Oh, the check engine light has not come on.

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

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.

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?