Vacuum gauges are among the mechanic's most useful but lesser known tools. Intake manifold vacuum has long been known as an indicator of load on the engine; more throttle and more load equals less vacuum, and vice versa for more vacuum. A fluctuating vacuum reading can mean a number of things, many of which will cause a noticeable power loss or misfire. A needle that fluctuates fairly slowly and over a wide range usually indicates a problem with either the carburetor adjustment or the engine's air and fuel management system.
As vacuum rises, it sucks more fuel through the carburetor, throwing off the air-to-fuel ratio and causing a drop in manifold vacuum.
What Causes Cars to Idle Low?
The same thing might happen if you've got a bad throttle position, manifold air pressure or mass airflow sensor. A vacuum reading that very quickly drops and rises a couple inches of mercury or psi generally indicates some sort of ignition system malfunction.
You might have a misfire as the result of a bad coil or crossfiring in the distributor, or you may have a spark plug with too small or large a gap. If the needle is truly spastic, you could have a multicylinder ignition misfire.
On a computer-controlled car, a bad cam or crank position sensor could cause this, but these will manifest in other and more noticeable ways. A wide fluctuation at idle generally means that something's gone awry between two of the engine's cylinders; one is pulling when it should be pushing, the other may be doing nothing.
The only way this is going to happen is if you've either blown a head gasket or cranked the head, in which case you'll see other symptoms.
You're likely to see excessive exhaust smoke that smells like fuel, antifreeze, oil or all three. Low and periodically fluctuating vacuum readings under all conditions is a classic indicator of worn piston rings or an engine in need of a rebuild.
The worn rings or excessively large bore will allow combustion gases to escape into the crankcase and won't seal well enough to pull the standard vacuum. Additional symptoms include excessive crankcase pressure as a result of ring blow-by; you'll see this as a hot wind puffing out of your valve cover oil filler hole.
A vacuum gauge needle that vibrates rapidly at idle and then seems to smooth out with rpm can indicate bad valve guides or a bad cam lobe. The bad guides allow the valve to move around a bit while the valves are at their lowest opening point, and the bad lobe will effectively kill that cylinder at low lift.
Intermittent drops in vacuum at idle and under cruise could indicate a sticking valve, and needle drop of vibration under acceleration can indicate a bad valve or valveseat. Hydraulic lifter bleed-off will manifest as an intermittent vacuum drop at idle, and incorrect valve lash will mimic the effects of a bad valve.
Engine idle isn't a steady thing; it actually fluctuates regularly as the pistons suck in regular gulps of air. This is especially true on large displacement engines with a relatively low cylinder count, like a cubic inch V-8 or a cubic inch six cylinder. Vacuum gauges have dampeners inside to keep the needle from constantly vibrating, and those dampeners do wear out over time.
Additionally, aftermarket vacuum gauges, particularly those designed for diagnosis as opposed to installation, may require a separate dampener to keep from giving a false reading under normal conditions. This article was written by the It Still Runs team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Runs, contact us.
Wide Fluctuation at Idle A wide fluctuation at idle generally means that something's gone awry between two of the engine's cylinders; one is pulling when it should be pushing, the other may be doing nothing. Low Reading Low and periodically fluctuating vacuum readings under all conditions is a classic indicator of worn piston rings or an engine in need of a rebuild.
Valvetrain Problems A vacuum gauge needle that vibrates rapidly at idle and then seems to smooth out with rpm can indicate bad valve guides or a bad cam lobe.
Gauge Dampener Engine idle isn't a steady thing; it actually fluctuates regularly as the pistons suck in regular gulps of air. Graham Bell. About the Author This article was written by the It Still Runs team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information.Welcome to Club Hot Rod!
I want to register an account for free right now! Forum Rules. When you removed the EGR valve, did you clean the surface and ensure it was flat before installing the delete gasket and block off plate? Thank you all for the compliments. Apply your base color and let it dry completely. Turn on the acetylene to a very low flame Hey gents, So I'm helping my buddy work on this 37 Chevy pickup fender. We've been learning lots about metal bumping, etc but there's one spot we can't get to respond. There was previous damage in I believe it runs to a ported vacuum switch which I did end up capping off.
Did you plug the vacuum hose that went to the EGR?What Causes Very Low Vacuum In An Engine?
If not, you will have a vacuum leak that may be the culprit. Remember Me? Results 1 to 7 of 7. Register now for free! Worn rings? Here's a link to a site I found not long ago - at the bottom there's a set of situations with visual and word descriptions of how the vacuum gauge reacts to problems - How to Use and Interpret a Vacuum Gauge.
I'm sorry. I should have stated this is a freshly rebuilt engine.
August 2001 Issue
I've got around or so miles on it. Maybe rings aren't seated yet? Take a look at the link - helped me get a handle on reading the vacuum gauge.
Sorry if it wastes your time. No, it's not a waste of time. I've actually read that before. I built this engine over the winter and got it put in and running back in the spring. It's a small journal, bored. I've got a closed L79 PCV system with the valve screwed into the oil filler tube.
The motor idles and runs good and the plugs are a nice tan color although they do have a little carbon build up on them. Every compression calculator I've used comes out with compression ratio. I wish I would have used a dome top piston instead of flats.
I would rather have had Anyway, I've got two problems with it right now. I've adjusted the air mixture screws for max.According to About. Common reasons include vacuum leaks, carburetor problems, ignition issues, damaged power circuits and dirty fuel injectors. When a vehicle idles low, the vehicle stalls or shakes excessively when the brakes are activated, and this is an indication of a serious engine problem.
Diagnosing low idle issues requires checking the carburetor, fuel pressure regulator and fuel injectors. Underhood Service states that low idle issues are only a symptom of a more serious problem. A diagnosis of a low idle issue is achieved by checking various parts of the engine. A common issue associated with the carburetor is air problems. This issue occurs when the carburetor does not open properly to allow air to enter.
Underhood Service explains that another issue that causes low idle is vacuum leaks. This type of leak occurs when the vacuum line is not securely attached or there are bends in the line. Additionally, dirty fuel injectors can cause vehicles to idle low.
The fuel injectors are normally set to equal specifications; however, there are factors that directly affect the injectors. The site recommends repairing low idle symptoms immediately before the issues cause irreparable damage. Home World View. Why Won't My Dishwasher Drain?Years ago, a good friend introduced me to using a vacuum gauge to diagnose engine problems, in-fact it was often the first tool he would reach for when confronted with a poorly running engine.
Of course, high-performance engines with lopey camshafts often make little vacuum but even then the vacuum gauge can give you insights to the inner workings of your engine. Simply put, the vacuum gauge has proven itself time and time again to be an invaluable tool in troubleshooting engine problems.
The key to using a vacuum gauge to diagnose engine problems is to understand exactly what the gauge is telling you. Good quality vacuum testing gauges are available at most auto parts stores and online resources, and are relatively inexpensive. Before beginning any vacuum testing, a visual inspection should be made of the entire vacuum system. To get started, hook the vacuum gauge to an intake manifold vacuum source.
You simply need to find one small enough for the vacuum gauge line to slide onto firmly. This is also done with a tee on an existing line or pulling a line and connecting it direct for example, the vacuum line to the transmission can be used.
Start your engine and allow it to come up to operating temperature before testing. Normal Engine: On most engines, accelerate to around rpm and then quickly release the throttle. Check the timing belt or chain depending on the application.
Advance the timing on the distributor to correct this problem, and recheck vacuum. This is typically due to a clogged muffler or exhaust system. Click here to cancel reply. I put the vac gauge on and the trouble was in the intake manifold would you believe it was the carb mounting gasket that I installed, it had a flaw in it from the manufacture the mounting bolts were torqued correctly. Thanks again! Let me give you more info. You can spray some wateraround carburator, if it has a leak gauge will detect it.
You can do a lot of things. Being Chief Tech, I use vac gauge often on standard equipment and race cars in their tune up. You did excellent write up. How can I print off the chart? My readings are very low. Al Stokes, Huntsville, AL, The sleeve valve engines are notorious for low vacuum pressure, but there are things to check that can affect vacuum like proper engine timing.
Freshly rebuilt engines tend to have lower pressure than those that have a few thousand miles on them. We are convinced you will enjoy the benefits of becoming a member and having access to the best instructional how to videos and professional tips. We would like to offer you a special promotion for your first year membership. My 1. About 35k on engine rebuild, new cam included.Many customers who come to us learn that, despite a previous diagnosis, they don't need a new engine, only certain repairs or adjustments.
But if after testing, it is determined that you need a engine, We can take care of you at our facility. Frequent Questions. Technical Articles. Even with modern computer controlled engines a vacuum gauge is still a valuable tool for diagnosing engine and transmission problems. If the engine performance is incorrect, the transmission will receive the wrong information.
Many times what is perceived as a transmission problem is in actuality an engine problem. The engine sends signals to the transmission through a vacuum line, throttle cable or both. These signals basically synchronize torque with transmission line pressure, shift feel and shift timing.
Malfunctions in items like the air filter; spark plugs, EGR valves and other parts of the fuel, electrical and emission systems could result in improper transmission performance. Vacuum Gauge Engine Performance Testing A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold.
The pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of: Piston rings Valves Ignition system Fuel control system Other parts affecting the combustion process emission devices, etc. Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading.
Gauge readings that may be found are as follows: Normal Engine Operation At idling speed, an engine at sea level should show a steady vacuum reading between 17" and 21" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 21" or more. General Ignition Troubles Or Sticking Valves With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil.
Fluctuations of 3 to 4 inches may be sticking valves. Intake System Leakage, Valve Timing, Or Low Compression Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator.
Low readings could also be very late valve timing or worn piston rings. Exhaust Back Pressure Starting with the engine at idle, slowly increase engine speed to 3, RPM, engine vacuum should be equal to or higher than idle vacuum at 3, RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present. Cylinder Head Gasket Leakage With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs.
The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater.
You can determine the location of the leak by compression tests. Fuel Control System Troubles All other systems in an engine must be functioning properly before you check the fuel control system as a cause for poor engine performance.
Forgot Password? Mark Forums Read. Vendor Directory. What can cause lack of manifold vacuum? Thread Tools. I am about ready to just sink my boat! Can anyone give me some good advice? The engine was rebuilt using stock specs compression, cam specs etc and bored. Ever since getting my engine back from the engine builder my boat falls flat on its face once I get up on plane.
The issue is manifold vacuum which is reading 6 inches of vacuum at idle should be 15 to The computer puts the engine into power reduction mode MEFI 1 drops the engine to 4 cylinders and registers a map error. Any ideas of what could cause this? Compression is perfect. Thanks to anyone who has any ideas. What else should be checked? View Public Profile.There's nothing more basic than the fact that an engine is just a big air pump. It draws in air by creating a low-pressure area in the intake manifold and cylinders, compresses the air, mixes in a little gasoline, lights a fire, generates heat and pressure and finally pumps out the spent exhaust.
Our preoccupation today with things electronic sometimes makes us overlook old-fashioned mechanical symptoms of problems and the mechanical test equipment used to troubleshoot them.
Vacuum gauges are often in this category, but the insight that a vacuum gauge can provide is as valuable today as it was 30, 40 or 50 years ago. Remember that engine vacuum is just air pressure lower than atmospheric pressure.
The starting point to evaluate engine vacuum is the intake manifold. When you connect a gauge to a tap on the intake, you're measuring manifold vacuum. Note that vacuum will vary in different areas of the engine, such as above or below the throttle valve and right at the intake and exhaust ports.
Vacuum drawn from an opening ahead of the throttle is called ported vacuum. Throttle opening affects ported vacuum opposite to the way it affects manifold vacuum. For example, at closed throttle, manifold vacuum is at its peak. But there is no significant vacuum at a port ahead of the throttle plate when the throttle is closed. Vacuum appears at such a port only when the throttle opens.
It's important to remember that manifold vacuum is used to power vehicle systems that need a steady supply of low-pressure air under all engine operating conditions.
Ported vacuum is used to control vehicle systems in relation to engine load. These include old-fashioned distributor vacuum advance diaphragms and carburetor assist devices. They also include many emissions control devices and transmission shift points. Under some engine load conditions, ported vacuum may equal manifold vacuum, but it can never exceed it.
Most vacuum gauges are graduated in inches of mercury in. Some also show the modern metric scale of kilopascals kPa. For comparison, 1 in. For this review, we'll stick to in. Because engine vacuum is based on comparison with atmospheric pressure, it varies with altitude just as atmospheric barometric pressure does. The following table shows that as altitude increases, vacuum decreases about 1 inch for every feet above sea level. Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.
Manufacturers used to publish vacuum specs in service manuals, but this isn't as common as it was years ago. Still, the physics of internal combustion haven't changed in a hundred years, so the guidelines given here are a good starting point for vacuum gauge troubleshooting.