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Really stupid question, but WHERE the heck do I attach the gauge to measure how much vacuum there is on my 84 C4?
Also, should the car running?
I've read all about WHAT the measure should be, how it is important, how to replace the lines etc. But tonight after I rented the vacuum gauge from AutoZone I just got stumped. Where do I hook it up? I'm guessing I plug up whatever I unplug to attach to the squeeze gauge?
Look for something that looks like this (vacuum check valve):
It will be on the back of the engine near the distributor. The vacuum hose on the pointy end will connect to something on the engine, probably the rear throttle body. Hook your vacuum gauge to that vacuum port.
yes, you are 100% correct. Pounds per Square Inch.
No....it's inches of mercury. How high, in vertical inches, a vacuum will "lift" mercury in a tube. That is the measurement. On the gauge, it will say "in HG" for inches of Mercury.
Pounds per square inch (aka "PSI") is a unit of measure for pressure. Not vacuum.
Vacuum Gauge:
Pressure (PSI) gauge:
Last edited by Tom400CFI; Jul 23, 2015 at 11:55 AM.
There is no such thing as vacuum. It is pressure in absolute rather than gauge pressure that is measured either referenced to current ambient or a standard day at 29.92 in. Hg. being widely accepted. A MAP sensor is Manifold Absolute Pressure and kpa. is a handy unit as 101 kpa is very close to 1 atmosphere in most places in the world so can be thought of as a percentage or a very close approximation. Nothing to do with your engine but just under 34 ft. is the max distance to draw water out of a hole and 26 to 28 dt a more practical limit due to changes in boiling temps at reduce psia.
It is pressure in absolute rather than gauge pressure that is measured either referenced to current ambient or a standard day at 29.92 in. Hg. being widely accepted. A MAP sensor is Manifold Absolute Pressure and kpa. is a handy unit as 101 kpa is very close to 1 atmosphere in most places in the world so can be thought of as a percentage or a very close approximation. Nothing to do with your engine but just under 34 ft. is the max distance to draw water out of a hole and 26 to 28 dt a more practical limit due to changes in boiling temps at reduce psia.
None of that really helps the OP, does it? He needs to measure his "vacuum". He is going to likely use a "vacuum" gauge....a readily available, and widely used tool. That gauge is going to read in "Inches of Mercury (HG)" of "vacuum"....representing how many vertical inches his "vacuum" would cause mercury to rise in a tube. Or to make you happy, how many inches atmospheric pressure would push mercury up that tube.
I posted to prevent the OP from being confused by the above post stating that he would be looking for "PSI"...which he will not.
None of that really helps the OP, does it? He needs to measure his "vacuum". He is going to likely use a "vacuum" gauge....a readily available, and widely used tool. That gauge is going to read in "Inches of Mercury (HG)" of "vacuum"....representing how many vertical inches his "vacuum" would cause mercury to rise in a tube. Or to make you happy, how many inches atmospheric pressure would push mercury up that tube.
I posted to prevent the OP from being confused by the above post stating that he would be looking for "PSI"...which he will not.
A 'vacuum' gauge is a 1920's device why perpetuate the myth?
Because it's a fantastic tool. Why confuse the OP and this thread with nerd-talk?
Exactly hes trying to do a test, to work on some issue or other. We need to keep itsimple, low pressure high pressure whatever its not time for an engineering analysis.
He mentioned a squeeze pump wonder if he rented a mighty vac instead of a gauge.I bought my gauge for $12.
Notice I didn't say vacuum gauge
I pull the vacuum hose at the brake booster plug the gauge into the open hose start the car and read it. Oops I said vacuum hose lol
Knowing how things really work usually make them simpler rather thn more complicated. When it is all pressure it is only a matter of how much rather than above or below normal atmospheric and changes according to where you live.
Sure. Show us the gauge the OP is going to use that illustrates "how much pressure" is in his intake manifold.
"Vacuum" is term. It is a term, used to describe that difference in pressure between atmosphere, and what ever lower pressure is in the plenum. Since that is the term used, the gauge is called a "vacuum gauge". That is the tool the OP will be using. That tool's unit's of measure are inches of vertical rise of mercury that the lower pressure (AKA "Vacuum") will lift the mercury. OR the number of inches that atmosphere will push the mercury vertically in that same tube when exposed to a lower pressure above. Doesn't matter how you "term it"...results are the same.
SO...in the interest of not confusing the OP, (if that is possible at this point)....he is going to use a VACUUM GAUGE. He can hook it to any nipple on the plenum. It will give him a reading in "Inches of Mercury", or "in. HG" and he should be looking for a reading of around 18" if it's a stock or stockish motor. Cool?
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Last edited by Tom400CFI; Jul 25, 2015 at 06:02 PM.
Knowing how things really work usually make them simpler rather thn more complicated. When it is all pressure it is only a matter of how much rather than above or below normal atmospheric and changes according to where you live.
But do you tell people to check their low pressure hose, or the vacuum hoses?
Cliff Harris-THANKS- I needed that simple pointer
Look for something that looks like this (vacuum check valve):
It will be on the back of the engine near the distributor. The vacuum hose on the pointy end will connect to something on the engine, probably the rear throttle body. Hook your vacuum gauge to that vacuum port.
TONI1960- Thanks! you are right- thanks for pointing that out to me in plain English. I went to Auto Zone and rented the squeeze pump. Guess I'll do my brakes with it though, I understand how to use it there at least. Hahah
He mentioned a squeeze pump wonder if he rented a mighty vac instead of a gauge.I bought my gauge for $12.
The rest, kinda lost me, but that's OK. I have to say that I'm really impressed with this forum. Most forums when they dive into technical stuff or people have dis agreements it gets nasty, and well on this forum so far I've seen nothing but polite and very forgiving answers to some of the oddball and easy questions I've asked. So thank you forum members for making this a very polite discussion forum.
Hi,
Thought I would close the loop on my thread in the event anyone else comes across it in the future.
Went to Harbor Freight and for $18 on sale they had a Boost/Vacuum gauge that comes with a T inside the box and some extra line. Plugged it up to my car and running about 18 mmHg at idle. Pretty happy with that. Thanks to all who helped me with this thread.
As a clarification, the Cross-Fire Injection (CFI) engine in the '82 and '84 is an L83, not an L98. The L98 designation started in '85, and has the Tuned Port Injection (TPI) engine.
Testing/Measuring Vacuum on L98
