well i did a search on vacuum idle and found some stuff but not exactly what im looking for.

a car im looking at has vacuum at idle at 15-16... is this bad? I was told it should be at or around 20.

thanks

 

get a compression test, then you'll know if it's O.K. for sure.

 

i second that.
the car im looking at has only 11hg in vacuum...

 

depends. what rpm? if you are idling around 800 rpm, you will probably see vacuum levels from 14-20 depending on seals, compression and port. if you idle higher, the vacuum levels will increase to 18-20.

i think 15-16 is fine and you shouldn't worry about it. maybe change your plugs and see if that changes anything. i've read about some people seeing slightly better vacuum after changing plugs, but have never experienced it yet. havent' changed plugs yet thou.

 

Actually, it's the opposite.

The higher the idle rpm, the lower the vacuum reading. So, if you car is idling at 1200 rpms, stock ports and good compression, it would read around 15-16 inHg. At 750-800 rpms it should be around 18 inHg.

More engine load is less vacuum (as it transitions over to boost).

jj3kgt,

Idle rpm, port sizes, altitude all effect your vacuum reading. If your engine is ported, it will have a lower vacuum reading than stock ports.

If you can provide the rpm your car is idling at when warm and whether or not your engine is ported, that will help in possibly determining if you have a problem or not.

 

ive noticed with a higher rpm idle i get more vacuum... with the throttle closed your not even coming close to building boost and since the engine is moving more air you should have more vacuum..

just my .02

i pull around 16 when it idles at 800 and about 18 when its idling at 1200

 

well its not mine, im just looking at it...

New Mazda Remanufactured Motor Less than 20k, ngk plugs, vacuum at idle is 15 - 16, intake, downpipe, catback, power fc, (dont know if this would affect it ) that is all it says

 

also the compression test was 75 psi on each rotor... isnt it supposed to be a bit higher?

 

Provided the compression test was done correctly, then that reading is below minimum specs. I believe 85 PSI (average) is the minimum spec. That doesn't mean the engine is worthless. Many people have put thousands of miles on low compression engines. However, it does mean that a rebuild is near in it's future.

 

mahjik I believe this is wrong...if your idle is set by itself higher you will get more vacuum...you can also try this out by slowly depressing the gas pedal when the car is idling the vacuum will increast...now if you rapidly slam the gas your vacuum will decrease...in my experience I got the highest vacuum after the car was warm and I drove it hard for a little bit or if it was cold and the idle was slightly higher.

its a very confusing deal...compression #s are helpful though on a stock motor