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Stop Leak & Gas Mileage

My car: 1999 Subaru Legacy (Outback wagon), 150,000 miles.
Climate: Austin Texas Baby

I had observed a drop in my gas mileage of 30%, down to about 15mpg for the last few tanks. I took the car in for a free ‘multipoint inspection’ at the dealer and was told it needs a few things: a bulb in my blinker, a brake flush, fuel system cleaning, and it has an oil leak at the separator plate (expensive to get to & repair). I replaced the bulb and I’m going to get the fuel system cleaning and brake flush, but I haven’t done those yet.

Instead of getting the expensive oil leak repair I decided to try Oil Stop Leak, a Blue Devil product recommended by my local AutoZone guys. Note that I have never seen any evidence of the oil leak (no drips, no smoke, never noticed the oil was low though I admit I don’t check it much), but I did see online that it’s a fairly common leak on this car because Subaru used to use a plastic part that eventually fails.

I put the stop leak product in and followed the directions (had to run the car continuously for a couple of hours), and now, on the very next tank of gas, I’m getting my normal mileage again.

Could the stop leak have impacted my gas mileage by that much?

Nope. Sounds like you burned through the last of a batch of bad gas. Happens.

just check your oil once a week and see what happens…

+1 for @BustedKnuckles since the stop leak would not have affected your fuel economy one way or another.

Thanks for the replies. I had kind of ruled out a bad tank of gas since the poor mpg had gone on for several tanks, but with the remaining skunky gas mixing with the new gas each time I filled up maybe I just finally got to the point where the gas is running clean again. I’m about to take the car in for a fuel system cleaning & am hoping that too will improve my mileage.

I do plan to keep checking my oil frequently now. So far I’ve not seen the level decline since the dealer noted the leak a couple of months ago.

then the dealer was upselling you. and that s being polite. you can find a drip of oil on most cars

A dealer is remiss if they spot an oil leak during any kind of inspection and do not point it out to the customer along with providing a repair estimate if needed.

Those tiny leaks that are often dismissed as trivial have led to the ruin of many engines. The same goes for transmission, rear axle, power steering, coolant, and brake fluid leaks.

it’s possible they’re telling me a lie, but i suspect there is a leak of some small or large magnitude. this leak might be hard for me to spot & may not drip. this video helped me get my head around what might be going on under there & why :

Any seepage or wetness is considered a leak by the dealer, even if it is not enough to create a drop. A '99 will have signs of seepage and wetness just due to age. Most of the time, it is nothing to worry about, but the dealer will always call it a leak. My 2000 Explorer has wetness all over the oil pan and drips about a drop a day from the rear main. I cannot detect the leak on the dipstick, so it’s not a problem.

Probably by running it continuously for 2 hours, you cleaned up the ignition system and injection system. Need to get it out on the road more often.

Just keep checking the oil level and all levels for that matter at evey fillup. An old car needs to be watched and if a level drops you’ll catch it.

I say go for the brake fluid flush . . . brake fluid absorbs moisture and should be flushed every 2 years, anyways

As far as the fuel system cleaning . . . unless you have a driveability complaint which can be directly attributed to a partially plugged fuel rail or injector, skip it

Stop leak products should have no effect on fuel mileage.

If the fuel economy has really tanked that badly then there should be some symptoms; CEL on, codes set, maybe a hint of black smoke, and possibly some slightly sluggish acceleration.

How are you checking the fuel mileage?

The low mileage could be a bad thermostat.
Should be changed anyway if it’s over 10 years old.