Best treatment for old gas?

[Northwoods]

Getting the Forester sort of running.   It's had the same fuel (a little under a quarter tank) in it for almost a year.   What's the best way to handle that old gas without draining it?  Fill with fresh gas?  Add Seafoam and/or a fuel injector cleaner?  Both?  Something else?  I'd rather not drain the gas if possible mostly because disposal will be a pain.

[dogmush]

A year isn't that long in terms of gas. It's probably degraded a little, but not enough to cause issues.

Drive it to the gas station and top it off with fresh gas.  If you're feeling flush use 93 octane.

[230RN]

i was always kind of curious as to what the mode of "decay" there would be in gasoline besides the faster evaporation of the lighter fractions and possibly some reaction with residual water in the tank. Topping it off with fresh gas and maybe a water-absorbing treatment with methyl alcohol might be advisable, but in my ignorance, if it burns at all, the vehicle's computer should be able to compensate.... yes... no?

Terry

[Northwoods]

i was always kind of curious as to what the mode of "decay" there would be in gasoline besides the faster evaporation of the lighter fractions and possibly some reaction with residual water in the tank. Topping it off with fresh gas and maybe a water-absorbing treatment with methyl alcohol might be advisable, but in my ignorance, if it burns at all, the vehicle's computer should be able to compensate.... yes... no?

Terry

It'll varnish the innards of the engine. 

[Hawkmoon]

It'll varnish the innards of the engine.

This.

Siphon it out and replace it with fresh fuel.

[Nick1911]

i was always kind of curious as to what the mode of "decay" there would be in gasoline besides the faster evaporation of the lighter fractions and possibly some reaction with residual water in the tank. Topping it off with fresh gas and maybe a water-absorbing treatment with methyl alcohol might be advisable, but in my ignorance, if it burns at all, the vehicle's computer should be able to compensate.... yes... no?

Terry

To expand on the sumpnz answer, I think it polymerizes over time.  I recall reading something a while back that noted ethanol tend to accelerate it.  Some mechanism involving free peroxides.

[230RN]

Written before seeing Nick1911's post:
-------------------
OK, by "varnish", does that mean residual longer fractions of the gas?   Fuel systems nowadays are pretty much almost hermetically sealed, so where would any smaller fractions go except back into the larger body of fuel?

Or does the gasoline re-polymerize to longer chains, and that's "varnish?"
------------------------

I never had the problem of long-term sitting of a vehicle and never added anything but drier to eliminate freezing of fuel lines in winter so I never looked into it.  (I'm also asking for the benefit of others who might want to know.)

Terry

[HankB]

I would siphon out as much of the old gas as I could, fill the tank with some name brand "detergent" gas, and pour in a name-brand additive (maybe STP gas treatment and its "powerful cleaning agents"). Then I'd drive for a while.

I would probably dispose of the siphoned out gas improperly. Bearing in mind the obvious hazard of extremely flammable vapors.

[charby]

A year isn't that long in terms of gas. It's probably degraded a little, but not enough to cause issues.

Drive it to the gas station and top it off with fresh gas.  If you're feeling flush use 93 octane.

I concur, after a year of sitting in a car, at most probably a little drop in octane.

My neighbor has a classic car (with a supercharged SBC) and a bunch of classic motorcycles, because he isn't able to drive them all the time, He typically just buys fuel once a year for everything and he doesn't even go through a full tank in most of his vehicles. He just mixes old fuel with new fuel.

[Tuco]

I wouldn't worry about it.  Siphoning gas from a modern car is difficult by design. The biggest hose you can get through the sphigmoid filler neck  is 3/16 or 1/4, and you can't really tell when the end is submerged.

Unless you are willing to open the pressurized fuel system or have access to a peristaltic pump, it's more trouble than it's worth.  Maybe not such a big deal if you can screw a bleed hose into an existing Schrader on the fuel rail.

Any varnishing has already occurred.  I'd get it on the road and run a couple tanks of fresh high octane detergent fuel. No seafoam or snake oil until its on the road and the current problems are put to bed.

 

[Northwoods]

Right now I'm getting cyl 2 & 4 (both on the driver side bank) misfires.  My consulting mechanic (really need to bring him a 12 pack each of a couple beers he likes) says ignition coil is the most likely culprit.   He's going to loan me a known good coil to test with. 

[JTHunter]

sumpz - back in the 80s, I had a Toyota 4WD with the 22R 4-cyl. engine.  For the first 75K miles,. I would mix my own "cleaner" out of various petrochemicals and add it to a nearly full tank in the late fall.  I used about 8 oz. each of xylene, toluene, MEK, and mineral spirits, along with a 12 oz. bottle of methyl alcohol based gas line antifreeze.
After 75K, I did it in the early spring as well.
Truck cracked the block under the rear cylinder at ~150K but it finally threw a rod at ~190K and died.

Never had a problem with the fuel system lines or filter.

[HankB]

. . . I would mix my own "cleaner" out of various petrochemicals and add it to a nearly full tank in the late fall.  I used about 8 oz. each of xylene, toluene, MEK, and mineral spirits, along with a 12 oz. bottle of methyl alcohol based gas line antifreeze . . .

Sounds like it worked well for you. Me, I'd be concerned about various solvents attacking gaskets and such, but maybe if they stand up to gasoline (and all the additives already blended in) small amounts of additional petrochemicals are harmless.

[charby]

I'd be worried about the MEK dissolving any rubber.

[Jim147]

Once you get the misses figured out, I would dump in can of Seafoam and topit off with fresh fuel. I've gone longer than a year more than once.

[JTHunter]

I'd be worried about the MEK dissolving any rubber.

Never did in the 15 years I ran that truck.