Slight Shake at Idle

[paulyjsob]

My 07 Mountaineer V8 with 101,000 miles has a slight shake at idle... Especially when at a stop light in Drive. On 2 occasions in the last year, RPM's dropped below 650 and car almost felt like it would stall. Only happened twice and cannot repeat problem.
Car had new plugs and coils at 85,000 miles along with new O2 sensor and cat converter.
This past week I replaced to MAF sensor and had the throttle body and fuel system cleaned (new fuel filter as well). No change. I also did the idle relearn. No change.
An ideas?

Thanks!

 

[paulyjsob]

BTW, no check engine light and just had emissions done, so no codes.

 

[Flag Gibby]

So if you put it on a scanner while running it shows no misfires? Do you have an exhaust leak on the manifolds? Sounds like you could have a vacuum leak to me.

 

[paulyjsob]

They had a computer hooked up while doing emissions (car running).. no codes.

 

[Flag Gibby]

Okay, so look to see if your manifolds are leaking and see if you can track down a vacuum leak anywhere. Those are the only two off the top of my head that I can suggest that could cause a rough idle if the plugs/coils are good. You know, replacing the PCV valve wouldn't hurt since you are at 100k miles.

 

[paulyjsob]

Thank you!
Any suggestions on tracking down a vacuum leak?

 

[Flag Gibby]

Spray some carb cleaner or starter fluid around the major vacuum connections under the hood (PCV, Power Break line, intake manifold, TB Gasket ... etc..) while the engine is running and listen for the RPMs to change. When they do you found your leak.
If you prefer to watch youtube videos then here you go: https://www.youtube.com/watch?v=9CPqbaSgcok

 

[paulyjsob]

Thanks!

 

[Explorer_PL]

Does it do it only in D or R, or also in Park ?
If it's in D or R (more violent), and not in Park, then your torque converter is not disengaging when you apply brakes, and it should.

 

[fritz24]

Can you feel the shaking underneath or on driver seat mainly? If it does possibly you have a transmission issue. Could be low on fluid or a small leak on the tranny lines.do you feel hesitation while cruising like on 80kph? Other than that like they said a vacuum leak, cylinder leak, fuel delivery, or ignition. Can you come up with a live data or PID's of the short term and long term fuel trim on each bank?

 

[paulyjsob]

Actually noticed the shaking in Park today. Hmm