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I found this column:
http://expertscolumn.com/content/how-use-vacuum-gauge
and thought it was fairly comprehensive. I am posting it here so our experts can add/edit their feedback.
james
=========================
How to Use a Vacuum Gauge
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Published by AaronCoates | August 24, 2011 - 45 weeks 5 days ago
As the piston in your engine pulls down, it draws air into the cylinder. This creates a vacuum in the intake manifold, and monitoring this pressure can give you a good look into the health of your engine. That is done with a vacuum gauge. It is a must have for any mechanic's tool box, and anybody who tells you different does not have a clue what they are talking about. A vacuum gauge is nothing but a gauge with a piece of vacuum hose on it, that is used to measure the amount of pressure in the intake manifold of your engine. This is very important to monitor when diagnosing an engine, because you need to know how the air is moving through the engine as it runs, in order to diagnose many different problems. The only thing you need to know about hooking a vacuum gauge to the engine is to make sure that everything is hooked up. This means you will have to tie it in to the lines that are there. If you 'bypass' one of the systems that run off the vacuum hose you are tied into, then that can affect the readings of the gauge and ultimately lead to a false diagnosis (e.g. waste of time and money).
There is no mystery to how to read a vacuum gauge, and they even come with excellent instructions. But if you have one in your toolbox, and just do not know how to read it, then this article will prove to be an invaluable resource to you. So now you have the vacuum gauge hooked to the engine, but what do you do now, right? Well, about all you can do is watch the needle, and play with the throttle. Depending on what the needle does and how it reacts, you can determine what is wrong with the engine. There are 15 different ways the vacuum gauge can read, and here is a list of them, and what they mean.
1. A normal engine will have a vacuum reading of 20in/hg at sea level, and that number will drop 1 in/hg per every 1,000 feet of elevation above sea level. The needle will be steady and not moving at all on an idling normal engine with good compression, no vacuum leaks, and no ignition system issues.
2. When you open the throttle to make the engine accelerate quickly, the needle should drop down close to 0, very quickly. Quicker is better. Then as you close the throttle the needle should go up past the normal 20 in/hg to something like 25-30, then return back to normal after about 1/2 a second.
3. Racing engines with big camshafts will have lower levels of vacuum, and the needle will shake while idling about 1 in/hg. This is from the valve overlap on the big race car camshafts causing turbulence in the intake manifold at idle, since they are designed to run at high RPM, instead of for maximum fuel economy.
4. A normal engine with worn rings, or in bad need of an oil change will have the same 'lower than normal' reading like the race car engine, but the needle will still remain steady while the engine is idling. Then when you rev it up, the needle will drop all the way to 0. Then when you let go of the throttle to let it close, the needle will rise up to only about 23in/hg instead of the 25-30 of a normal engine. This is because the rings are not sealing good enough to pull the proper amount of air into the engine. At this point, you would change the oil, and try again.
5. If the needle has the right amount of vacuum while the engine is idling, but fluctuates 1-2 in/hg down, then the problem is a sticky valve. Valve jobs are not cheap, either.
6. A bigger fluctuation than the plain sticky valve issue would be a downward fluctuation of about 6-8 in/hg. If you see a big fluctuation like this, then the engine has a burned valve that is leaking the whole time the engine is running. To find this valve, you would do a compression test of each cylinder, and remove the head to change the burned valve.
7. A slow fluctuation of 2-4 in/hg means that there are some valve seating problems, and the best way to fix that issue is (once again) a valve job.
8. A needle that fluctuates 4-6 in/hg while idling, and gets bigger as you accelerate it, is an indication of worn valve guides. If this is the case, you would look for which spark plug was fouled, and either do the valve job, or 'try' replacing the valve seals to try to buy some time before the next valve job.
9. If the needle holds steady while idling, but fluctuates wildly (10 -14 in/hg) while accelerating, then that means that the valve springs are worn. They are much easier to replace than a valve, and usually do not involve removing the cylinder head to change.
10. A low and steady reading on the gauge indicates a problem with late valve timing, or a vacuum leak. You can spray flammable fluid like carburetor cleaner along your vacuum hoses and gasket mating surfaces to see if the engine revs up, in order to find a vacuum leak very quickly. If there are no vacuum leaks on the engine then it means it is time to do something about the timing belt before it breaks!
11. A steady reading at 15 in/hg usually means late ignition timing. Just advance the distributor until it comes up to normal. If the needle also pulses, then check the gap on the spark plugs as at least one of them will be worn or too big, causing a pulsing fluctuation.
12. A low reading of 3-6 in/hg while the engine is idling is an indication of a major vacuum leak. This will usually be something like a throttle body or carburetor gasket. Look and listen for big vacuum leaks in the engine, they will be fairly obvious to hear while the engine is running, once you are aware that there is a problem in the system somewhere. Other than that, you will have to go through the whole vacuum system and make sure all the hoses are hooked up properly.
13. A needle that starts out normal, but drops as the engine runs more and more is a sign of a bad head gasket. Though vacuum gauges are not usually used to diagnose a head gasket type of issue, if this happens you can rest assured that there is definitely a blown head gasket in the engine.
14. If the reading on the needle starts out normal, but decreases to 0 as the engine runs and before dying, that is an indication of a clogged exhaust system. This can be verified by loosening the exhaust from the engine and runningn it and making sure that there are no other problems on the vacuum gauge.
15. If you have a carbureted engine and the needle fluctuates between 13-17 in/hg, then you need to adjust the air/fuel mixture screws for the idle circuit. Once you get it right, then the vacuum levels will get steady and level out to normal.
http://expertscolumn.com/content/how-use-vacuum-gauge
and thought it was fairly comprehensive. I am posting it here so our experts can add/edit their feedback.
james
=========================
How to Use a Vacuum Gauge
Tweet
Published by AaronCoates | August 24, 2011 - 45 weeks 5 days ago
As the piston in your engine pulls down, it draws air into the cylinder. This creates a vacuum in the intake manifold, and monitoring this pressure can give you a good look into the health of your engine. That is done with a vacuum gauge. It is a must have for any mechanic's tool box, and anybody who tells you different does not have a clue what they are talking about. A vacuum gauge is nothing but a gauge with a piece of vacuum hose on it, that is used to measure the amount of pressure in the intake manifold of your engine. This is very important to monitor when diagnosing an engine, because you need to know how the air is moving through the engine as it runs, in order to diagnose many different problems. The only thing you need to know about hooking a vacuum gauge to the engine is to make sure that everything is hooked up. This means you will have to tie it in to the lines that are there. If you 'bypass' one of the systems that run off the vacuum hose you are tied into, then that can affect the readings of the gauge and ultimately lead to a false diagnosis (e.g. waste of time and money).
There is no mystery to how to read a vacuum gauge, and they even come with excellent instructions. But if you have one in your toolbox, and just do not know how to read it, then this article will prove to be an invaluable resource to you. So now you have the vacuum gauge hooked to the engine, but what do you do now, right? Well, about all you can do is watch the needle, and play with the throttle. Depending on what the needle does and how it reacts, you can determine what is wrong with the engine. There are 15 different ways the vacuum gauge can read, and here is a list of them, and what they mean.
1. A normal engine will have a vacuum reading of 20in/hg at sea level, and that number will drop 1 in/hg per every 1,000 feet of elevation above sea level. The needle will be steady and not moving at all on an idling normal engine with good compression, no vacuum leaks, and no ignition system issues.
2. When you open the throttle to make the engine accelerate quickly, the needle should drop down close to 0, very quickly. Quicker is better. Then as you close the throttle the needle should go up past the normal 20 in/hg to something like 25-30, then return back to normal after about 1/2 a second.
3. Racing engines with big camshafts will have lower levels of vacuum, and the needle will shake while idling about 1 in/hg. This is from the valve overlap on the big race car camshafts causing turbulence in the intake manifold at idle, since they are designed to run at high RPM, instead of for maximum fuel economy.
4. A normal engine with worn rings, or in bad need of an oil change will have the same 'lower than normal' reading like the race car engine, but the needle will still remain steady while the engine is idling. Then when you rev it up, the needle will drop all the way to 0. Then when you let go of the throttle to let it close, the needle will rise up to only about 23in/hg instead of the 25-30 of a normal engine. This is because the rings are not sealing good enough to pull the proper amount of air into the engine. At this point, you would change the oil, and try again.
5. If the needle has the right amount of vacuum while the engine is idling, but fluctuates 1-2 in/hg down, then the problem is a sticky valve. Valve jobs are not cheap, either.
6. A bigger fluctuation than the plain sticky valve issue would be a downward fluctuation of about 6-8 in/hg. If you see a big fluctuation like this, then the engine has a burned valve that is leaking the whole time the engine is running. To find this valve, you would do a compression test of each cylinder, and remove the head to change the burned valve.
7. A slow fluctuation of 2-4 in/hg means that there are some valve seating problems, and the best way to fix that issue is (once again) a valve job.
8. A needle that fluctuates 4-6 in/hg while idling, and gets bigger as you accelerate it, is an indication of worn valve guides. If this is the case, you would look for which spark plug was fouled, and either do the valve job, or 'try' replacing the valve seals to try to buy some time before the next valve job.
9. If the needle holds steady while idling, but fluctuates wildly (10 -14 in/hg) while accelerating, then that means that the valve springs are worn. They are much easier to replace than a valve, and usually do not involve removing the cylinder head to change.
10. A low and steady reading on the gauge indicates a problem with late valve timing, or a vacuum leak. You can spray flammable fluid like carburetor cleaner along your vacuum hoses and gasket mating surfaces to see if the engine revs up, in order to find a vacuum leak very quickly. If there are no vacuum leaks on the engine then it means it is time to do something about the timing belt before it breaks!
11. A steady reading at 15 in/hg usually means late ignition timing. Just advance the distributor until it comes up to normal. If the needle also pulses, then check the gap on the spark plugs as at least one of them will be worn or too big, causing a pulsing fluctuation.
12. A low reading of 3-6 in/hg while the engine is idling is an indication of a major vacuum leak. This will usually be something like a throttle body or carburetor gasket. Look and listen for big vacuum leaks in the engine, they will be fairly obvious to hear while the engine is running, once you are aware that there is a problem in the system somewhere. Other than that, you will have to go through the whole vacuum system and make sure all the hoses are hooked up properly.
13. A needle that starts out normal, but drops as the engine runs more and more is a sign of a bad head gasket. Though vacuum gauges are not usually used to diagnose a head gasket type of issue, if this happens you can rest assured that there is definitely a blown head gasket in the engine.
14. If the reading on the needle starts out normal, but decreases to 0 as the engine runs and before dying, that is an indication of a clogged exhaust system. This can be verified by loosening the exhaust from the engine and runningn it and making sure that there are no other problems on the vacuum gauge.
15. If you have a carbureted engine and the needle fluctuates between 13-17 in/hg, then you need to adjust the air/fuel mixture screws for the idle circuit. Once you get it right, then the vacuum levels will get steady and level out to normal.
