Hi! I recently purchased a 2008 Mercury Mariner from a Ford Dealership with 41,000 miles, put on about 4,000 miles and boy do I seem to have a ton of issues with this guy. Anyway, my latest issue came when I was driving to work I notice rough idle during a coast. It felt like I had a bent rim, but went away at higher RPMS. I pulled to the side and popped the hood and I can hear a puff it seems like 2 times each cycle at idle. I’m assuming at this point it’s a vacuum issue, but with this being a new car I’m not even sure where the vacuum hoses are located. I have a V6 2 wheel drive. Any help would be appreciated.

Too add to the issue, no check engine light...

Also, I noticed my rear window glass leaks just a little bit. I am going to try spraying some silicon on to it, but was curios to see if this is a common issue.

 

I don't know your engine, so I can't help with specifics. But if you are hearing a repetitive sound, you should be able to zero in on it with some hearing help.

Take a 3 foot long or so length of 5/8" or 3/4" heater hose, or a similar length of garden hose if it is flexible enough. Hold one end up to your ear. You need to really wrap your hand around it and press it hard to your ear, need to make a good connection there so sounds don't come in that ear except what comes through the hose. Move the other end of the hose around slowly close to different points. Use it as a stethoscope. Watch out for the rotating belt!

By working carefully, you should be able to pin-point the noise source.

 

Great idea about using the garden hose to localize what appears to be a vacuum leak which might explain the rough idle. Believe it or not it could even be a loose spark plug, especially if some rocket scientist at the dealership worked on the car. I have a 3.0 liter mariner and the spark plugs went out "according to the dealer" with less than 74,000 miles on it. They had opened to .074 inches and the engine misfired eventually. I was amazed it ran at all. Ford plugs- I now use Bosch double platinums. The rear glass which opens separately may have come a little off line and a readjustment may be all that is needed. I am assuming the vehicle had not been in an accident, but a new rubber seal may be needed. Assuming that fuel pump pressure is normal (atleast35 PSI) and the injectors are all clean and working properly the other thing that will affect the idle is something called the throttle position sensor. It is mounted at the end of the throttle shaft and sends a signal to the computer (PCM) according to Ford. A clean signal is essential especially at idle. The computer readers may not indicate a fault on this part if it is intermittent since they don't do well detecting electronic noise. It should cost about $100.00 to have it replaced, an easy job, but you will certainly get another Chinese part. In any case a clean idle is most indicative of a well running engine since many problems appear to disappear as the RPM increases.