I have a stock 99 mustang with a 4.6 V-8. It has been stored for about 3 months. Now upon start up idle quality is poor. 500 to 1000 rpms. Then when warmed up the idle keeps climbing. It has went as high as 3000. I have checked the basics, tps voltage, ect temp., maf readings, etc. Nothing seems to be standing out. Any ideas?

 

I'm guessing its the gas. "When gasoline is left for a period of time, gums and varnishes may build up and precipitate in the gasoline, causing "stale fuel". This will cause gums to build up in the fuel tank, lines, and carburetor or fuel injection components making it harder to start the engine. Motor gasoline may be stored up to 60 days in an approved container. If it is to be stored for a longer period of time, a fuel stabilizer may be used. Stale gasoline can be detected by a colorimetric enzymatic test for organic peroxides produced by oxidation of the gasoline. Users have been advised to keep gasoline containers and tanks more than half full and properly capped to reduce air exposure, to avoid storage at high temperatures, to run an engine for ten minutes to circulate the stabilizer through all components prior to storage, and to run the engine at intervals to purge stale fuel from the carburetor." -wikipedia, because they know all.

Could try one of those fuel system cleaners.. I believe if you are storing your car you should start it and let it run for awhile each month to help clear out anything that may build up in the lines. I don't believe that running the engine with that old gas will harm the engine but you could also try removing all the gas and filling up with fresh.

 

I'm thinking vacuum leak, all the hoses are 10 years old.

 

Thank you very much I will try the gas additive. I figured gas is at least 6 months old.

 

Thanks for the info I replaced all vacuum lines already