s there still a place for the vac- uum gauge? We’re living in a world that’s filled with engines that may never need a valve job and that adjust their own , monitor their VACVAC UUM UUM own misfire conditions and correct fuel Idelivery to compensate for small vacu- um leaks. And yet the typical computer- controlled fuel system still depends heavily on a sound engine and strong, GAUGEGAUGE & & reliable signals. That’s why vacuum readings are as valuable today as they’ve ever been. Vac- uum testing also happens to be the fastest, easiest-to-perform test in the book. You don’t need to hunt down spe- TRANSDUCERTRANSDUCER cial fuel gauge fittings as you do for many fuel pressure tests. You won’t need to remove spark plugs as you do for a compression test. Just find a good place to tee in a vacuum gauge and hook it up. TESTINGTESTING Let’s take a moment to discuss the concept of vacuum. “Vacuum” is really a misnomer. What we really mean when we talk about engine manifold vacuum is less pressure. Depending on weather conditions and the unit of measurement you use, the air at sea level has an at- mospheric pressure of about 14.7 psi, 101 kpa or 29.9 in./Hg. At higher eleva- tions, atmospheric pressure is lower. When pressure is lowered in an area, it causes the air to rush in from the sur- rounding high-pressure area. We often refer to the low-pressure area as a vacu- um, when it’s simply an area of relative- ly lower pressure. When we measure the pressure in an intake manifold, we’re really com- paring the pressure inside the manifold to the pressure of the atmosphere out- side the manifold. It’s this pressure dif- ferential that causes the air and fuel to be pushed into the combustion cham- One of the earliest diagnostic tools, the ber. Because it’s a term we all use, we’ll continue to refer to the lower pressure vacuum gauge, retains its effectiveness in in an intake manifold as “vacuum.” The amount of manifold vacuum cre- pinpointing engine mechanical faults. It can ated is directly related to the engine speed and position of the plates. also be complemented by its electronic If we take away fuel and ignition, then spin the engine, we’ll still produce vacu- equivalent – the vacuum transducer. um in the intake manifold. The faster the engine spins, the higher the vacuum BY KARL SEYFERT will be—as long as the throttle plates

create a restriction by remaining closed. Seyfert Karl Photos: However, if we move the throttle plates open, the vacuum will decrease—as long as engine speed remains constant. It’s important to understand this basic concept before moving further. You’ve probably heard of using a vac- uum gauge to check cranking vacuum. This is a useful test because spark and fuel are taken away so that all we see is the engine’s mechanical condition. Without fuel and ignition, vacuum is a simple concept to understand. It’s based strictly on mechanical integrity, as long as we know rpm and throttle position. Things get complicated when we add fuel and ignition because fuel and igni- tion have a direct effect on engine rpm. For example, if identical engines are running at the same throttle position, a lean condition will cause one engine to spin slower than the other engine run- ning at the correct mixture. This will cause a lower vacuum reading at the same rpm because the throttle plates must be opened further (reducing the restriction) to bring the rpm up. You can see why vacuum provides such an accurate indication of how well an engine is running. The higher the vacuum for a specific rpm and throttle opening, the better the engine is run- ning. We know a problem exists when the vacuum is low, but where do we start looking? Realistically, a low vacu- um reading can be the result of ab- solutely anything, including ignition, fuel delivery or mechanical problems. This is where vacuum interpretation can simplify the process. Each item that affects engine vacuum leaves a unique fingerprint. Vacuum Gauge Testing Whenever possible, connect the gauge to a large, centrally located vacuum port. Be sure the port isn’t obstructed by carbon deposits. Depending on en- gine and intake design, where you con- nect the gauge can have a big effect on the accuracy and sensitivity of the read- ings you get. In order to start, an engine usually has to pump at least 1 in. of vacuum. When it’s developing normal cranking vacuum, a healthy engine will pump a fairly steady 3 to 6 in. The more vacuum the engine pumps, the quicker it’ll start. The more cylinders the engine has, the VACUUM GAUGE & TRANSDUCER TESTING

Find a reliable temporarily steady the vacuum reading. vacuum source When the vacuum reading fluctu- located after the ates sharply between a normal and a throttle plate very low reading, there may be a com- (manifold pression leak between adjacent cylin- vacuum). This ders. If so, these two cylinders will both port is after the show up weak on a balance test. throttle plate, Weak valve springs will cause abnor- but it’s located mal idle and high-speed vacuum on a single intake readings. The gauge needle will flutter manifold runner. Readings may be rapidly—more rapidly when you rev the biased toward engine. Depending on rpm and spring the cylinder condition, the needle may flutter irreg- that’s closest to ularly. When weak/broken springs can the transducer no longer close a valve, the reading will or gauge. take on the pumping motion a burned valve creates. stronger and steadier cranking vacuum richening the mixture will improve idle Badly worn valve guides will cause tends to be. quality. If richening the mixture doesn’t abnormal idle and normal high- When the engine cranks erratically, help, look elsewhere and continue fol- speed vacuum. At idle, the gauge nee- the cranking vacuum will also fluctuate lowing your diagnostic procedure. dle will flutter rapidly over a wide area, erratically. problems (a belt Abnormal idle and high-speed but the reading steadies when you rev or chain) are the most common cause of vacuum causes the needle to drop reg- the engine. When the guides are this abnormal or erratic cranking vacu- ularly and predictably at idle, and is worn, the engine will have a serious oil um and cranking speed. However, the usually caused by one or more valves consumption problem. engine could also be so hot it’s actually that are leaking. During the compres- At 2500 rpm in Neutral, the normal dieseling during cranking. sion , a burned intake valve allows high-speed vacuum reading should Compression problems can also pro- positive pressure pulses to enter the in- at least equal idle vacuum. Usually, vac- duce abnormal cranking vacuum. If take manifold. When you rev the en- uum at 2500 will be greater than idle cranking vacuum is healthy but drops gine, the reading doesn’t stabilize. vacuum. If vacuum at 2500 rpm is less regularly and rhythmically, look for a When the vacuum reading drops than idle vacuum, disable the EGR sys- compression problem. Every time the erratically and unpredictably at tem and retest. If the reading is still low, weak cylinder tries to fire, two things idle, a valve or valves are sticking. The check for an exhaust restriction. happen momentarily: The cranking needle may not drop as much as it does You can monitor a vacuum device speed increases and the vacuum de- when the valve is burned. If the valves and its vacuum source simultaneously. creases. A burned valve can make the are sticking, cooling the engine down or For example, connect one gauge to needle drop regularly to zero vacuum. using a valve-freeing oil additive may manifold vacuum and tee the other one Do you have zero cranking vacuum? into the tranny’s vacuum modulator Before you look for large air leaks, see if hose. If both readings don’t react the the throttle blade(s) are sticking open. same way during a road test, inspect the If they are, close them and retest. Un- modulator hose and its connections. less the throttle blade(s) are closed or Use your vacuum gauge on as many nearly closed, some vacuum gauges road tests as time allows. The more you may not respond very well to a cranking use it, the quicker you’ll learn what vacuum test. “normal” readings are. With an exhaust On a normal engine, normal idle restriction, under-load readings will all vacuum should be a steady reading be- be lower than normal, and it will take tween 17 and 21 in./Hg. Larger dis- very little throttle movement to drop a placement engines tend to produce reading to zero. stronger idle vacuum than smaller en- gines do. The better the rings and Too Much to Remember? valves are sealing, the more vacuum the Unless you have a photographic memo- engine will pump. A healthy engine should be able to ry, remembering all the possible combi- Steady, lower-than-normal idle produce 3 to 6 in. of vacuum while nations of vacuum gauge readings and vacuum can be due to a vacuum leak, cranking. Multicylinder engines that their causes is darn near impossible. To an EGR leak or an ignition/valve timing are in good shape will show a stronger make things easier, we’ve boiled vacuum problem. If it’s a vacuum leak, artifically reading with less fluctuation. gauge testing down to its bare essentials. VACUUM GAUGE & TRANSDUCER TESTING

On a normal engine, idle vacuum should be steady between The MAP sensor can also be used to collect vacuum readings, 17 and 21 in. of vacuum. Once again, larger engines that are as long as you’ve figured out the conversion factor. Remember, in good shape will tend to produce higher vacuum readings vacuum is really pressure that’s less than atmospheric pres- at idle than smaller engines with fewer cylinders. sure. The MAP sensor measures manifold absolute pressure.

The following two simple tests will verify fall slowly as the engine speed falls. The if it had a broken valve spring or two. the presence of good vacuum before re- vacuum needle should come back to And the valve/ synchronization sorting to other test procedures: rest at the idle reading seen at the begin- must be correct or the engine couldn’t 1. Cranking vacuum. ning of this test, and hold steady there. keep pumping at higher engine speeds. 2. Vacuum readings on a warm, run- If the engine passes these tests, the Finally, internal engine parts (valves and ning engine at idle, at part throttle, with odds are high that it’s mechanically piston rings) must be sealing well no-load at 2000 and 3000 rpm and dur- sound—at least sound enough to pump enough to jack up the vacuum on ing closed-throttle decel from a WOT air at idle, part throttle and closed- closed-throttle decel. stab. throttle decel. If the vacuum readings are good but First, test cranking vacuum (usually Steady vacuum readings in the range the engine still doesn’t run right, look conducted on an engine that won’t start). of 17 to 21 in. at idle are good. Vacuum for your problem elsewhere, with Connect a vacuum gauge to a manifold readings should stabilize at the same checks of specifics like fuel pressure, vacuum source. Make sure the throttle levels or better at part-throttle cruise. secondary spark and emissions. If the is closed and that the engine cranks at The engine wouldn’t be able to do this vacuum readings aren’t good, here are the recommended speed. Cranking vac- your options: uum should be at least 3 to 6 in. •If the cranking vacuum is low, or Second, test the warm engine vacu- zero, look for a major problem—like a um at idle, part throttle and during that stopped dead in its tracks, closed-throttle decel. or a large vacuum leak. Measure engine manifold vacuum at •If idle vacuum is low but steady, idle first. Vacuum should be “high” and check valve timing first. steady at idle (17 to 21 in./Hg). •For all the in-between gauge read- Now raise the engine speed to about ings, refer to the information at the be- 2000 rpm. Hold it steady and watch the ginning of this article to help you match vacuum gauge. After an initial drop in up the results. the vacuum reading as the throttle The vacuum gauge’s biggest strength opens, it should return to the idle vacu- is its ability to pinpoint low-vacuum um level recorded in the previous step, problems. Other tests, such as power or close to it. Some EGR valves will balance, four gas, secondary ignition kick in no-load. If you see a small drop and fuel pressure tests, will also help in vacuum during a steady throttle test, you isolate specific problem areas. disable the EGR and repeat the test. This Fluke transducer can read vac- Vacuum Transducers Do the same test at 3000 rpm and uum or pressure and can be used look for the same results. with a scope or a DSO. DSO use re- Reading the needle on a vacuum gauge Allow the throttle to snap shut from a quires an adapter to convert the is almost a lost art. If you’d prefer to not snap WOT. Vacuum readings should rise transducer’s banana plugs to the be a gauge practitioner, there’s another above idle vacuum (20 to 25 in.), then scope’s BNC connector. way to check the engine’s vacuum sig- VACUUM GAUGE & TRANSDUCER TESTING

To stabilize and to the DSO with a couple of adapters. synchronize the The rpm input can be connected to the scope trace, we external trigger or Channel 2, so we’ll need an external know when the No. 1 cylinder fires. Ad- trigger. Usingan just the scope until you see the first in- inductive trigger ductive hump on the left of the screen attached to the and another at the right. No. 1 cylinder With the time base adjusted for one and plugged into the engine period, move the signal to the DSO’s trigger bottom of the screen. Set the trigger to terminals allows this channel. Use the Normal mode, us to identify which causes the scope to draw only the activity in when the trigger is present. This assures specific when that the pattern for cylinder No. 1 will analyzing a always appear first on the screen. vacuum Think of a vacuum waveform as an waveform. inverted signal. An increase in pressure drives the signal lower. Setting up the nal. An oscilloscope is a very useful tool rect if the vacuum line is connected to a trace for the vacuum signal can vary de- for testing electronic signals. A trans- single plane of the manifold. pending on the transducer and what ducer can take a certain condition and Once the transducer is installed, con- you want to see. If you want to see the convert it into an electronic signal. For nect and adjust your scope. If you’re us- signal amplified as much as possible, set example, the vacuum pulse from an en- ing the factory MAP sensor on the , your scope to AC coupling. Adjust the gine can be converted in this way. Using connect the scope lead to the signal voltage level until the signal looks good. a transducer to convert a vacuum pulse wire from the MAP. There should be A voltage setting of 200 to 500mV/div into an electrical signal, then synchro- three wires—ground, reference voltage works well. nizing it with the ignition produces a and signal wire. The signal voltage will If vacuum readings are necessary, very useful scope pattern. change with changes in vacuum. turn off AC coupling and measure actu- The first step is to acquire a transduc- The vacuum signal is of little use at al voltage levels. Then convert the volt- er that will convert vacuum into an elec- this time because we can’t determine age into vacuum measurements. These tronic signal. There are a few ways to do which signal is caused by a particular measurements will vary, depending on this. The first is to use a MAP sensor on cylinder. We must synchronize our the transducer you’re using. Use a vacu- the you’re testing (if one exists). transducer with the ignition. The easiest um pump and an accurate gauge to This works well because you can test way is to connect on inductive lead to convert your transducer readings before the MAP sensor and engine integrity the No. 1 cylinder. attaching the transducer to the engine. with one connection. The inductive lead can be connected Most factory MAP sensors use the Another way is to use the transducer same inches-of-vacuum-per-volt con- that came with your engine analyzer. version factor, so once you convert them Some of the older Allen and Sun ana- you’re set to read others. Check the cal- lyzers had this feature. When connect- ibration of the MAP sensor with a vacu- ing these transducers, be sure to find a um pump before assuming there is a central location on the intake manifold. problem. Don’t connect to one intake runner, as Start by learning what good vacuum this will greatly affect your waveform. waveforms collected from sound en- The third way to get a waveform is to gines look like. Then, if you notice any use a remote transducer that’s connect- irregularities, you’ll know you need to ed to your DSO. You can make one check further. As you become more yourself with a 9-volt battery and an old proficient at using vacuum waveforms MAP sensor. If you want to get fancy, for engine diagnosis, you’ll be able to you can even construct one that’s cali- determine whether cam timing is cor- brated correctly. Think of a vacuum waveform as an in- rect, whether the is verted signal. An increase in pressure After you’ve decided which transduc- drives the signal lower. A conversion plugged and even whether the engine er to use, connect it to a central vacuum factor is necessary to translate the has EGR problems. source with a short vacuum line. Make voltage levels shown in the vacuum sure the source is as large as possible waveform to actual vacuum readings. Visit www.motor.com to download with no restrictions. If you’re checking a This conversion factor is specific to a free copy of this article. V6 or V8, your readings will be incor- individual transducer types.