I've read a number of posts where a change in vacuum readings signaled problems.

Just wondering, what kinds of information can a vacuum gauge give you.
Also, is it just low or lowering vacuum that is a concern, or does a high or rising vacuum reading signal anything?

thanks

 

Vacuum getting stronger could signal a new engine breaking in, probably wont be many other times vacuum would go up. A decline is bad, probably signals a weakening motor, lower compression... unless you just have a vacuum leak somewhere.

 

If your compression is low will it show up as low vacuum?
Or does vacuum and compression corelate most of the time?

thanks

 

It basically depends on how well you know your engine, and even then it's not an accurate indicator of the general health of your engine.

Vacuum readings fluctuate drastically with very small changes in rpm, meaning that changes in idle speed can be deceiving. A span of 300 rpm can take you from "my engine's healthy" to "OMG, my engine's popped".

A compression test is the only sure way to determine the health of your engine.

 

I think many discussions about vacuum stem from the fact that so many owners have a boost gauge. If you're familiar with your idle vacuum, a change in idle vacuum can show a change in compression.

 

One thing to watch out for is a rhythmically 'ticking' needle in vacuum at a steady rpm. Usually signals a chipped or cracked seal, esp if the exhaust note sounds irregular.

 

...as long as you're familiar with exactly what rpm and idle settings your idle vacuum was measured at, and you're sure those haven't changed or that you haven't developed any intake leaks whatsoever. It takes very little change in rpm to change the vacuum reading at idle.

 

agreed. this is what my vacuum was doing. It ran but was on the way out.