Brake booster leak causing engine rpms to drop?

[veemexx]

Hi,

We just bought an 09 Rogue SL and have been getting engine noises when the car is brought to a stop with the AC on. Switching the AC off or shifting to Neutral or Park stops the engine noise.

I've also noticed that the engine idle speed decreases from 1000 rpm to ~700rpm when the brake pedal is depressed. Is that normal in a Rogue?

Thanks!

[philipa_240sx]

A/C operation does result in a change of engine noise and increased vibration. This is normal. Most 4cyl engines exhibit an increase in vibration at idle when the A/C is ON. It's the nature of the engine as 4cyl are inherently more 'rough' than a V-6, V-8 or I6.

As for idle speed, it depends. If the transmission is in neutral, idle speed will be around 1000 rpms or more. When warm and the transmission is in D with the brake depressed, it should be around 750rpms. The rpm's you are noticing appear to be normal.

[veemexx]

Hi Philipa,

Thanks for your input. We (my wife and I) traded in our Chevy Blazer (V6) for the Rogue (I4) and have been worried about the engine noise (with the AC on) ever since.

We were also worried about the CVT after reading so many negative reviews about it but we took the risk and got ourselves the '09 Rogue. After 1500 miles, it's running ok although on some days it sounds like it's got some piston slap going on under the hood even after 10-20 mins of running.

We're hoping that it'll run fine and give us minimum problems.

Thanks again!

[philipa_240sx]

I actually had to try this one for myself...

Transmission in D, brake depressed: 750-800 rpm. As soon as you release the brake pedal, rpms rise a bit to about 1000 rpms.

I have never noticed this or paid any attention to it. It's probably been like this since day one... and I have 42,000mi (70,000km) on my Rogue.

As for the engine tapping noise, it's normal. It's not actually piston slap, it valve train noise and is due to the design Nissan used. I have had this motor (QR25DE) on two Nissan's so far and the valve train noise was present in both of them.

Read this thread for an explanation:

zerothread?id=392210

[veemexx]

I'm glad that you took the time to try it out.

I own a 1990 Volvo 240DL (I4), do all the repairs and maintenance myself, and, if I remember correctly, have never seen the Volvo engine decrease in rpm with the brakes depressed. I'll need to check that but unfortunately I'm won't be back in CA for a month.

I can only surmise that the Rogue's brake booster must need a lot of vacuum for the rpms to drop like that.

[philipa_240sx]

Actually I think this is way it's the engine computer is programmed.

Years ago, idle speed was controlled via an adjustable stop on the throttle butterfly. This was typical of most carburetor's and early fuel injection system.

As cars became more computerized, mfg's added a computer controlled solenoid valve that admitted a small amount of air to the intake manifold. More air and more fuel resulted in an increase in rpms. Usually it was a preset amount and was active during startup, A/C operation etc.

The next step was quickly pulsing this valve open/closed (duty cycling). This allowed the computer to precisely control idle speed and is common on most cars produced today. A technician can actually adjust the preset engine idle speed via the computer.