MOTOR PURR
WE Dissolve PROBLEMS
Since 1967
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Rough Idle - Engine Shakes, Vibrates, Hunts at Stoplight
Your engine is running unevenly. You feel it as a shake or a vibration, and it has a lot of possible causes — ignition, vacuum leaks, fuel delivery, the PCV or EGR system, and carbon deposits in the intake, injectors, or timing solenoids. This page is about telling them apart, because the most expensive mistake in a rough idle is replacing parts that were never the problem in the first place.
When carbon deposits are a likely contributor
Deposits build up on intake valves, in injectors, through the EGR, the air intake, timing solenoids, and more. That produces a shake at idle that looks exactly like ignition failure. These are the symptoms that point toward deposits:
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The idle smooths out above about 1,200 RPM. Deposits restrict flow most at low airflow.
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It's worse when the engine is warm, not better.
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It improves for a while after a long highway drive, then comes back.
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Fuel economy has dropped over the same period.
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The engine is gasoline direct injection -these designs don't wash the back of the intake valve with fuel.
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High mileage, or a history of short trips, or both.
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You already replaced the spark plugs and it didn't fix it. This is the single most telling one. New plugs in a clean engine fix a rough idle. New plugs in a dirty engine don't.
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No check engine light, or a light with no single-cylinder misfire code.
Our Solution - The 3-Step Deep Engine Clean