It doesn't do it all the time but all of sudden it starts with a loud noise (like a fog horn) under the hood then the throttle goes up and down quickly.It's a 94 Mercury Marquis LS with 45000 miles on it.

Sounds like your automatic transmission is slipping or may need the fluid level checked

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Originally Posted by rusty14

It doesn't do it all the time but all of sudden it starts with a loud noise (like a fog horn) under the hood then the throttle goes up and down quickly.It's a 94 Mercury Marquis LS with 45000 miles on it.

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Is your check engine light on? Are you saying it doesn't start loud all the time, or it doesn't have a fluctuating idle all the time?

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Originally Posted by GeorgeLeigh50

sounds like your automatic transmission is slipping or may need the fluid level checked

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Where did he say that the vehicle had an automatic transmission? Why would low fluid levels cause the engine idle to rise and fall?

I took it to mean it shifts gears with a loud bang. If it had a clutch it could not be shifting gears without him pushing the manual shift lever, his question title says it does this by itself. A tranny going out will run in low gear longer than necessary then shift up with a bang, won't it?

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Originally Posted by GeorgeLeigh50

I took it to mean it shifts gears with a loud bang. If it had a clutch it could not be shifting gears without him pushing the manual shift lever, his question title says it does this by itself. A tranny going out will run in low gear longer than necessary then shift up with a bang, won't it?

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He said the throttle goes up and down, not that the transmission shifts hard. And a "fog horn noise" isn't "a bang."

I paid the fee and asked the pros which was well worth the money.The problem was what they call a HOOT TUBE, because that's what it does, hoots. It runs from the back of the intake to just in front of the throttle body.Anyways, I had it replaced and the car works fine.

Quote:

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Originally Posted by rusty14

I paid the fee and asked the pros which was well worth the money.The problem was what they call a HOOT TUBE, because thats what it does, hoots. It runs from the back of the intake to just in front of the throttle body.Anyways, I had it replaced and the car works fine.

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So the tube was leaking vacuum, causing the fluctuating idle.

Glad to hear it worked out for you! Thanks for updating us :)