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I have a car ('96 Lexus ES 300) that's been parked non-operational for over eight months, except for one drive down the block last month (it started up just fine). It's in Los Angeles, so not too much cold or humidity. Outdoors (not in a garage). I did disconnect the battery before parking it, but I did not put in fuel additive (I didn't learn of the need to do so until just recently). The gas tank is about 1/4 full. I still don't need to drive it, and am not really sure when I'll need to, but it will definitely remain in storage for at least "months."

I understand that the best way to clean up the fuel system of a car stored this way would be by topping off the gas tank with new/clean fuel, and running Sea Foam through it (per Reviving a vehicle that has been idle for a long time).

My question: at this point, now that it's already been sitting for 8 months sans-additive, does it still make sense to add STA-BIL before letting it sit even longer? Or the fuel has already oxidized at this point, so I should just leave it as-is & use the Sea Foam once it's actually time to drive it again?

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The gas will continue to oxidize for years, eventually turning into a sticky varnish. Definitely add more Sea Foam and take the car for a drive, so the treated gas gets into the fuel injectors, intake valves, top piston rings and exhaust valves. You almost can't add too much Sea Foam, so I'd put 4 oz. per gallon. Adding STA-BIL simultaneously will only help, without any chance of harming the engine.

Different brands of gasoline will oxidize at different rates. Cheap, no-name gas can begin to oxidize in three weeks. Name brand, "top tier" gasoline is usually good for one year without any extra treatment.

Why not burn off the 1/4 tank and start with fresh gas? Also, store the car with a full tank because the gas tank will rust in the exposed areas.

Back in the early '90s, I drove an F-150 pickup truck that had been sitting for over a year with an untreated tank of gas. After 200 miles, the oxidized gasoline contained enough varnish to stick all the intake valves in the open position. The engine of course would not run until a mechanic removed the valve covers and hammered the intake valves back-and-forth with solvent to free them again.

By contrast, I stored a 1980 Cutlass Supreme in a barn in the mid-'90s. In that case, I added an entire bottle of Marvel Mystery Oil to a full tank of gas and sprayed a little down the throat of the carburetor until blue smoke came out the tailpipe. After two years undisturbed, she started-up immediately and ran perfectly for an entire Summer before I sold it.

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  • Thanks! Great answer. Just to clarify: when you say "top tier" gas, do you mean brand-wise, or i.e. 91 vs 89? – Metal450 Jul 19 at 18:00
  • Also: I read in some places they suggest changing the oil. Any thoughts on if that's actually necessary/helpful? The oil I have is way under miles-wise (i.e. 2k since last change), but over time-wise (last change was 2 years ago). Do I need to change the oil before storing too? – Metal450 Jul 19 at 18:28
  • And also, re burning off the tank: is it bad for the engine / counterproductive to burn it off by just letting the car idle for a few hours? Or do I need to actually drive it around? – Metal450 Jul 21 at 0:55
  • Yes, "top tier" is name brand, such as Mobil, BP, etc. – Carguy Jul 30 at 4:16
  • Yes, change the oil. Condensation in the crankcase, due simply to weather changes, could theoretically contaminate the oil. – Carguy Jul 30 at 4:18
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If the car runs fine you should be OK-just put some fresh fuel in soon.

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