[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.

[GeorgeLeigh50]

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

[this8384]

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.

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?

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

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

[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?

[this8384]

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?

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

[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 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.

[this8384]

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.

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

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