mirelesjp

Have my Civic since new (2008), it has 99,500 miles.
In the last year or so, when you come to a stop in a traffic light or waiting for somebody and the A/C compressor kicks in, the car vibrates and the rpm vary up and down (about 100 rpm up and down).
This happens either in Drive or in Neutral but it is worst in Drive.
Changed the spark plugs about 20k miles ago.

I've heard it could be some vacuum valve, but not sure.
Any suggestion what can I do?

Thanks

ezone

A mild vibration change, noise, and very slight RPM change is normal due to additional load on the engine while the AC compressor is running. The RPM should recover immediately though and return to normal, usually the RPM change isn't even noticeable.

Make sure the AC is not overcharged, and both radiator fans turn on whenever the compressor turns on. Either of these can lead to high head pressure that causes the engine to struggle as the compressor cycles on.

Otherwise...
An idle relearn MIGHT help.
Throttle cleaning then idle relearn might help.

A harsh loud rumble vibration is not normal, you have to use your senses to locate a source area such as the hood rattling or pipes contacting the body or loose parts.

Vibration that shakes the entire car and may sound like a fart can muffler, may disappear during acceleration....is usually a collapsed engine mount, check pics in this post: https://www.civicforums.com/forums/1...ml#post4577741

mirelesjp

Shaking is not extremely strong, but I believe is more than the "normal" you mentioned.
I have another "not new car" (2006 Sienna) with the same miles on it and it does not move at all when A/C kicks in.
Will look into your recommendation
Thank you

ezone

You can't compare apples to oranges. You need an identical Civic with similar mileage to compare with.

steinfamily01

i have a 2011 civic hatchback and am in the same bucket. if the A/C is off, nothing happens, everything is normal, revs stay the same etc.

if the a/c is on, sometimes when stopping at a traffic light, the revs will go up and then down and the settle after that ... vibration is very slight when it does it ...

ezone

"Slight" is expected and normal just because the AC puts a significant load on the engine.

steinfamily01

cheers.

brags

Yup i notice the same thing when i turn on my A/C there is a slight
variation in RPM wich Yes is normal BUT i have noticed when its
very hot outside the RPM"s do seem to kick down and back up just
a little more mind you the engine never stalls but i have noticed that
on very hot summer Temps.