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akls400

1990 Ls400 Erratic Idle

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I have a 1990 LS400 with 125k miles. Starts fine. Runs well cold but idle seems a bit high. 800 - 900 rpm. Once it warms up the idle becomes erratic in gear or in neutral/park. It will range from 1000 rpm to 0. Eventually it dies, but will start right back up. If i hold the pedal down a bit it will stay running and the rpms stay even. Hwy mileage is 23mpg. I have replaced plugs, fuel and air filters.

Any suggestions?

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When the engine is at temperature, the idle should be 800 RPM's. When cold it should be around 1600 RPM's. Sound to me a lot like carbon buildup in the throttle body. The easiest and cheapest way to try to fix the problem is to add 2 bottles of Complete Fuel System Cleaner to a new tank of top tier (major brand) fuel. I have used both Valvoline and Slick 50 CFSC with excellent results for similar problems. You need to make sure you spend around $10 for each or you are getting junk.

If it fixes it, you will be happy, if not you spent $20 and only a few minutes. I always start with the easist and cheapest first then go from there.

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