vacuum test

[Jeanluc]

About my 1990 TPI, when I connect a vacuum gauge on intake, the pointer have important and fast vibration between 16/18 hg at idle (800 rpm).....
When I open throtle, the vacuum increase, but pointer have still vibration, until 2000/2500 rpm and become stable at 2500 rpm, (vacuum value is 20 hg)
What is the problem???
Could it be an intake leak??
Thanks
Jean-Luc

 

[RonJ]

Some vacuum gauges vibrate at the best of times, it's just their nature. Most automotive gauges have something like a small jet inside the inlet so just a small amount of vacuum is being applied to the gauge. This tends to dampen the needle vibration.

Maybe your gauge has lost this dampening "jet"? Try squeazing the hose coming off the gauge with a small c-clamp or vise grips to restrict but not block it, see if that takes the vibration out of the needle.

With the vacuum readings that you are getting, if the engine is idling and running well, I wouldn't worry about it. My two cents!!

Ron ...

 

[G Winter]

Most engines have a high freq. pulse in the vacume, some of it comes from valve timing. Piston is moving up before valve is closed causing a reversal in the air flow. Higher rpm ,more air velosity, less pulse. A bad valve would give you a larger pulse. As Ron said , the way the guage is build is a factor on how it reads.

 

[BlackNBlue95]

Guess we should ask are you having a drivabilty problem?
-=Rick

 

[Jeanluc]

Yes, I have a drivabilty problem: oils on 2 sparks plugs treads...and bad cold idle...
unstable idle seems like a intake manifold leak!!

I follow the IAC and TPS procedure: problem is same.
I have removed and control many time my intake manifold...
In fact my engine is a complete new one with a 1990 TPI...

So I make a test with a vacuum tester....
and it's going to make me crasy !!!!