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Vacuum Readings
02-14-2012, 12:22 AM (This post was last modified: 02-14-2012 12:24 AM by cferry7.)
Post: #1
Vacuum Readings
1990 Cadillac Deville 4.5 L FWD 180,700 mi.

Rupe wrote

Re: Thank You
To: cferry7
It's ok to post stuff like this on the message boards.

While you are on the topic of vac readings, if you are from the old school you may recall seeing 18 - 20 inches as the norm. These days (and for the past 20 years) it's more like 15 - 18 inches max on a good running engine.

cferry writing :
Vacuum Readings : T fitting bet. Intake Manifold and MAP Sensor.
First Start of Engine ( Engine Cold ), idle : 15 - 16 psi , steady needle.
As Engine Warm's up, idle : Slowly changes from 16 - 13.5 psi, steady.
Engine Warm idle : 13.5 psi steady.

From Idle - The "accel to 2500 rpm then close throttle abruptly" Test :
Needle movement is 13.5 -> 5 -> 21 -> slow return to 13.5 psi.

During City Driving , idle at stop lights : 13 - 20 psi (varies) , steady.

Suggestions as to what these readings are possibly telling ?
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02-14-2012, 11:55 AM
Post: #2
RE: Vacuum Readings
Not too far off considering the mileage. Guessing some internal wear, some timing chain wear, and possibly slightly retarded ignition timing, which is about the only thing you can adjust easily. Of course the gauge could be off a bit too.

You sound as if you want to play with this so as an experiment why not jack up the timing a bit to see if the vac readings go up as well. If you really have the urge to play you could dial in the exact TDC the old fashion way just to be sure the timing mark on the crank pulley is correct. I have seen them off by quite a bit and they can slip with time too. Years ago I had a truck that nobody could get through an emissions test because the timing mark had gone south and I set the timing by vac reading while also using an exhaust gas sniffer to verify a clean burn.
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03-02-2013, 07:48 PM
Post: #3
RE: Vacuum Readings
I have just noticed that my Vacuum guage is reading approx. negative 3 when it should be zero - no hose hooked up.
I am not sure how long it has been like this, nor whether it was similar
during the original post.
Rupes' post about Guage Accuracy !
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03-02-2013, 08:09 PM
Post: #4
RE: Vacuum Readings
I suspect the guage you are talking about is a combination vac / fuel pressure unit that reads from about 5 psi positive down to 25 inches on the negative side.

That said, the fact that it "zeros" several marks off the money means at one point it either got dropped or someone subjected it to excessive pressure and sprung the internal works. On the better units you can remove the glass and reset the adjustment screw to zero. After that compare readings to a known good gauge to confirm things. If it's off too far you may have to pop for a new one. It's a handy thing to have even though most people don't use it very often these days.
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