Testing Manifold Vacum?

[Lazzman]

Does anyone know a good method for testing manifold vacum and what it should read? Where do I connecet the gauge?

Thanks

 

[BrooklynBay]

Put a "Tee" on one of the vacuum lines, and connect it to a vacuum gauge, or a vacuum pump tool that already has a gauge connected onto it. Manifold vacuum varies, and is not consistent. Turbos, and superchargers force air into the intake, so they have pressure, not vacuum when those things kick in. That is why they have a boost gauge mounted on the front hood of some race cars. They have vacuum, and psi on those gauges.

 

[aldive]

Does anyone know a good method for testing manifold vacum and what it should read? Where do I connecet the gauge?

Thanks

Take a look at http://64.78.42.182/sweethaven/MechTech/Automotive01/default.asp?unNum=2&lesNum=7&modNum=7

 

[BrooklynBay]

Here is a picture from the link that AlDive just posted:

 

[BrooklynBay]

Here is a reprint of the article:
VACUUM GAUGE TEST

When an engine has an abnormal compression reading, it is likely that the cylinder head must be removed to repair the trouble. Nevertheless, the mechanics should test the vacuum of the engine with a gauge. The vacuum gauge provides a means of testing intake manifold vacuum, cranking vacuum, fuel pump vacuum, and booster pump vacuum. The vacuum gauge does not replace other test equipment, but rather supplements it and diagnoses engine trouble more conclusively.

Vacuum gauge readings are taken with the engine running and must be accurate to be of any value; therefore, the connection between the gauge and the intake manifold must be leakproof. Also, before the connection is made, see that the openings to the gauge and the intake manifold are free of dirt or other restrictions.

When a test is made at an elevation of 1,000 feet or less, an engine in good condition, idling at a speed of about 550 rpm, should give a steady reading from 17 to 22 inches on the vacuum gauge. The average reading will drop approximately 1 inch of vacuum per 1,000 feet at altitudes of 1,000 feet or higher above sea level.

When the throttle is opened and closed suddenly, the vacuum reading should first drop about 2 inches with the throttle open, and then come back to a high of about 24 inches before settling back to a steady reading as the engine idles, as shown in figure 3-77. This is normal for an engine in good operating condition.

If the gauge reading drops to about 15 inches and remains there, it would indicate compression leaks between the cylinder walls and the piston rings or power loss caused by incorrect ignition timing. A vacuum gauge pointer indicating a steady 10 inches, for example, usually means that valve timing of the engine is incorrect. Below-normal readings that change slowly between two limits, such as 14 and 16 inches, could indicate a number of problems. Among them are improper carburetor idling adjustment, maladjusted or burned breaker points, and spark plugs with the electrodes set too closely.

A sticking valve could cause the gauge pointer to bounce from a normal steady reading to a lower reading and then bounce back to normal. A broken or weak valve spring can cause the pointer to swing widely, as the engine is accelerated. A loose intake manifold or leaking gasket between the carburetor and manifold shows a steady low reading on the vacuum gauge.

A vacuum gauge test only helps to locate the trouble. It is not conclusive, but as you gain experience in interpreting the readings, you can usually diagnose engine behavior.

 

[aldive]

Gee, and I thought that was the purpose of the link.....

 

[BrooklynBay]

Sometimes links change over time, and the information might get lost if somebody looks at this post a few months from now. The picture is linked from there, and not attached like the text. How did you find that link? It has a lot of useful information.

 

[Lazzman]

Thanks for the help guys .....

 

[gijoecam]

Also, keep in mind that they're talking about 'inches of something' there, not PSI. My mityvac has a vacuum gauge on it, but no matter how hard I try, it'll never pull more than 14.7 psi. (atmospheric pressure)

-Joe

 

[BrooklynBay]

Vacuum is measured in inches of mercury, not pounds per square inch.

 

[gijoecam]

Typically, yes, inches of water. It can also be measured in PSI, as well as inches of water, millimeters of mercury, millimeters of water, bars, or atmospheres. Vacuum is vacuum no matter what units you use. It's a negative pressure relative to atmospheric.

-Joe

 

[Lazzman]

With this in mind what is the purpose of vacum in an engine?

 

[gijoecam]

With this in mind what is the purpose of vacum in an engine?

Vacuum is a byproduct of the regulation of air intake by the throttle body. Close the throttle body, with a running engine, and there will be negative pressure, or vacuum, in the intake manifold. You can then use the vacuum to actuate things like secondary intake manifold runners, heater control valves, front axle disconnects, heater blend doors, EGR valves, and a host of other devices.

On some of the new foreign vehicles that regulate the intake air by controlling the intake valves electronically instead of using a throttle body, there is not enough vacuum generated in the intake to be useful, so they use other methods to actuate things.

-Joe

 

[Lazzman]

Thanks Joe

What problems can a leak in the vacum somewhere cause?

 

[BrooklynBay]

A vacuum leak could cause hesitation, a lean mixture, stalling, hard starting, and a rough idle. Keep in mind that combustion requires air. Air goes into the engine, mixes with sprayed fuel, ignites, drives pistons, then vents out of the exhaust.

 

[aldive]

Lazz, have you tested for as vacuum leak yet?

 

[Lazzman]

No I have not. I don't really have the tools to do it yet. No gauge and wouldn't know where to hook it up anyway.

I have heard of the blow torch method and the WD-40 method, I hear both could be dangerous. I think when I have the money I will have a mechanic check it out.

I seem to have a rich condition not a lean condition. Information I read said leaks cause a rich condition as the computer tries to compensate for the additional air leaking into the system with more fuel??

 

[aldive]

I seem to have a rich condition not a lean condition. Information I read said leaks cause a rich condition as the computer tries to compensate for the additional air leaking into the system with more fuel??

How do you know its lean or rich?

Lazz, you need to quit quessing and either fix what is wrong or determine nothing is wrong.

Good luck .......

 

[Lazzman]

It feels like it is bogging out and the gas gauge shows me that it could be true.

I want it fixed more than you know, but I can't find anyone who will spend the time to properly fix it.

I am getting tired of throwing money at this problem, not to mention broke.

 

[aldive]

Why don't you take out that "chip", disconnect the battery to reset computer and then drive it a day or two and see if there is a difference. And it won't cost you a dime.

That would have been the first thing I did to determine the root of the woes.