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Home Emissions Diagnostic Dilemmas: The Pressures of Intake Manifold Vacuum Tests

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Several years ago, a retired school teacher brought in a 1994 Chevrolet S-10 Blazer that had developed an intermittent rough idle condition. Although a rebuilt engine had been installed a year before, and all of the wiring and vacuum hoses looked as if they were in factory condition, I took nothing for granted.

An ignition scope and compression test yielded no result and neither did spraying the manifold gaskets with aerosol carburetor cleaner. When I connected a vacuum gauge to the intake manifold, the needle would jiggle ever so slightly when the engine began misfiring. A few moments later, the vacuum reading would stabilize and the engine would idle very smoothly. I suspected a broken valve spring, but removing the valve covers on this particular engine is difficult and time-consuming due to accessory interference. With these thoughts in mind, I began devising a diagnostic strategy that would tell me which bank was misfiring and, with a little luck, which cylinder was misfiring.

Vacuum Terminology
Intake manifold vacuum analysis can be a little tricky because the conventional term “intake manifold vacuum” is a technical misnomer. Technically speaking, the intake manifold must contain enough liquid fuel and air to support combustion, so what we have is not a complete vacuum, but an atmospheric “pressure differential” between the inside and outside of the intake manifold. A more current term refers to the pressure inside the intake manifold as Manifold Absolute Pressure or “MAP.” As currently used, the terms “pressure differential,” “MAP” and “intake manifold vacuum” refer to the difference between atmospheric and intake manifold pressures.

Atmospheric Pressure
Atmospheric pressure is about 14.7 pounds per square inch of pressure at sea level. Atmospheric pressure at sea level will also support a column of liquid mercury (Hg) 29.92” in height. Since local weather conditions may cause atmospheric pressure to vary from standard, the current reading is usually referred to as “barometric pressure” or “baro.”

When testing manifold vacuum, it’s important to remember that if an engine idles at 22” Hg at sea level, it will idle at about 17” Hg at 5,000’, 14” Hg at 8,000’ and 12” Hg at 10,000’ altitude. Variations from the calculated standard, of course, are the weather conditions, the engine design, and how well the engine management system adjusts spark advance and air/fuel mixture to correspond to a change in barometric pressure.

Operating Vacuum
Because atmospheric and intake manifold pressures begin to equalize as the driver opens the throttle plate, intake manifold vacuum is normally measured at idle speed. At wide-open throttle (WOT), atmospheric and manifold pressures become nearly equal as air rushes in to fill the engine’s cylinders.

At its most basic level, intake manifold vacuum testing consists of connecting a vacuum gauge to a port tapped into the intake manifold. From that point, testing intake manifold vacuum becomes a little more problematic because a number of engine design issues like variable camshaft timing, tuned intake systems, high valve overlap, and, of course, turbocharging or supercharging may affect intake manifold vacuum readings.

In other cases, a variation from normal values may indicate the problem itself, which might be a stuck-open EGR valve, a bent or burned intake or exhaust valve, worn camshaft lobe, broken or weak valve spring, a broken cam follower or rocker arm, incorrect camshaft timing, incorrect spark timing, clogged catalytic converter, burned piston, leaking intake gasket or vacuum hose, or rich or lean air/fuel mixtures.

Engine Management Strategies
With the exception of electronically controlled valve train systems, the idle speed of all other fuel-injected spark ignition engines is managed by a precision-machined throttle plate mounted in a precision bore. When adjusted correctly, the throttle plate will allow the engine to idle at a base speed of about 500 rpm. Due to the volume of air flowing around the throttle plate to maintain idle speed, the sea-level pressure differential or “vacuum” is normally reduced from 29.5” Hg to about 18” to 22” Hg at idle on a well-tuned engine.

At the most basic level, peak cylinder pumping efficiency, idle speed power output and combustion efficiency go hand-in-hand with peak intake manifold vacuum. Complete combustion of the air/fuel mixture is achieved when the fuel mixture is ignited a few crankshaft degrees before the piston reaches top dead center (TDC) and before maximum cylinder compression pressure is reached. The base spark timing is normally advanced to allow time for a flame front to propagate from the spark plug into the combustion chamber. As the cylinder reaches maximum compression, the rate of combustion increases because the compressed air/fuel molecules ignite very rapidly.

If the spark occurs too early in the combustion cycle, the combustion rate slows down because the cylinder hasn’t reached maximum compression. Consequently, a misfire develops because the loosely packed air and fuel molecules fail to support combustion. If the spark occurs too late in the combustion cycle, the slowly burning fuel mixture fails to exert maximum pressure against the piston.

Most mechanically managed engines ignite the fuel between zero and 12 degrees of base spark timing before top dead center (BTC) at idle speed. In general, maximum intake manifold vacuum is achieved just as the spark timing is advanced to the point of misfire. As the spark is retarded, idle quality improves because ignition is occurring as maximum compression pressure is achieved in the combustion chamber. Further retarding spark timing reduces intake manifold vacuum because less fuel is efficiently burned, which reduces the pumping efficiency of the engine’s pistons and cylinders.

Similarly, the air/fuel mixture reduces intake manifold vacuum when it varies from a stoichiometric mixture to a rich or lean mixture. In the heyday of the mechanically managed engine, skilled mechanics carefully adjusted spark timing and air/fuel ratios to balance the highest vacuum reading against the smoothest idle quality. In many cases, a skilled technician armed with an accurate vacuum gauge achieved optimum spark timing by advancing the spark timing into misfire and then retarding several inches of mercury to achieve a smooth idle.

To adjust the idle air/fuel mixture, the technician would adjust the mixture screw to achieve maximum intake manifold vacuum and then open the adjustment screw one-quarter to one-half turn more to compensate for temperature and barometric pressure changes. In most cases, fuel economy would improve because the carburetor’s power enrichment system, which normally begins to open at 6.4 to 8.5” Hg, remains closed with the higher vacuum achieved by precision tuning.

Electronic engine management systems essentially follow the same strategies, but with much tighter parameters. Most, for example, advance the spark timing and lean the fuel mixture to the point of misfiring at idle speeds. In most cases, highly efficient, electronically controlled engines produce a slightly higher vacuum reading than do mechanically controlled engines.

Vacuum Instrumentation
Intake manifold vacuum can be measured with a mechanical gauge, electronic pressure transducer, or through the engine management system via a diagnostic scan tool. Compared to consumer-grade instruments, a professional-quality mechanical vacuum is accurate, responsive and produces repeatable results. Responsiveness is a particularly important feature because, to produce relevant data, the gauge must detect minor vacuum fluctuations caused by leaking valves and other reciprocating engine parts.

Similarly, electronic vacuum transducers that connect to lab scopes and graphing or digital multimeters must have enough sensitivity to display minor variations in manifold vacuum. A digital multimeter, for example, can be set to display minimum and maximum values. Although lab scopes and graphing multimeters can display variations in manifold vacuum as a voltage trace or a graph, While scan tools usually display barometric, MAP and/or vacuum in numerical values, the update rates are usually too slow to be useful in diagnosing vacuum irregularities caused by reciprocating parts. In addition, the software strategies and hardware differ widely among vehicle applications. Speed density systems, for example, use a MAP sensor to measure both the baro and MAP. With the key on and engine off, the baro input informs the PCM how weather and altitude pressure changes will affect the MAP input. After the engine starts, the baro input becomes a value upon which the MAP reading is based. This strategy prevents the PCM from seeing a 14” Hg MAP input as an out-of-parameter reading at an altitude of 8,000’. Some mass air flow (MAF) sensor-equipped vehicles may use a separate baro sensor to adjust for weather and altitude changes. In other cases, vehicles equipped only with a MAF sensor will calculate barometric pressure by measuring the air flow into the engine at a specific engine speed, throttle opening and intake air temperature reading and storing that value in the PCM’s keep-alive memory as a barometric pressure reading.

Scan Tool Vacuum Diagnosis
Electronically managed engines react to vacuum leaks differently than mechanically managed carbureted engines. Assuming a carbureted engine is adjusted to a stoichiometric idle mixture, a vacuum leak will reduce intake manifold vacuum because the airflow is no longer controlled by the throttle plate and also because the excess airflow leans out a stoichiometric fuel mixture.

On the other hand, electronically managed engines add an idle speed control (ISC) or idle air control (IAC) device to the throttle assembly to provide a more accurate control of idle speed. While the so-called “base” or “minimum” idle speed is controlled by the throttle plate, the curb idle speed is controlled by a motor or solenoid that allows additional intake air to bypass the throttle plate. Since minor vacuum leaks increase idle speed on closed-loop fuel systems, the PCM commands the ISC to reduce air flow into the intake manifold. To illustrate, the ISC or IAC “count” for a typical General Motors vehicle ranges between 20 and 30. A major vacuum leak would force the PCM to force the IAC count to zero.

In addition, air from a vacuum leak will increase fuel trim readings because the PCM will add fuel by increasing fuel injector pulse width (IPW) to maintain a stoichiometric air/fuel ratio. Last, if a manifold gasket leak causes a lean misfire to occur on one cylinder, most OBD II systems will record a misfire for that cylinder.

The single exception to the PCM increasing fuel trim readings is when a sticking EGR valve causes a vacuum leak by allowing exhaust gases to dilute the air/fuel mixture present in the intake manifold. The effects of a stuck-open EGR are problematical because, on the one hand, chemically inert exhaust gas dilutes the air and fuel in the manifold and reduces the power output and pumping efficiency of the engine. Speed density systems often react to the reduced intake manifold vacuum by increasing IPW because the MAP input falsely indicates an increased engine load.

In contrast, MAF sensor-equipped engines sense a decreased intake air flow through the MAF sensor and may react by reducing IPW. The most important issue with EGR contaminating the air/fuel mixture of speed density and MAF-equipped engines is that vehicles may have different operating strategies programmed into their engine management systems to deal with stuck-open EGR valves.

To illustrate, I’ve had one Chrysler in which an EGR valve with a missing pintle caused only a minor rough idle condition. In contrast, a small piece of carbon trapped under the EGR pintle of a General Motors 4.3 engine can cause an extremely rough idle or engine stall. The logic revolves around how the PCM reacts to a stuck EGR valve. In the case of the Chrysler, the PCM may let the authority of the oxygen sensor override the authority of the MAP sensor. With the GM product, the exact opposite might be true.

Vacuum Gauge Analysis
Any vacuum reading, whether measured mechanically or electronically, should remain steady at idle and represent a typical value for the engine configuration and operating altitude. During cranking at closed throttle, an engine should generate at least 3 to 5” Hg of manifold vacuum. If cranking vacuum isn’t present, the engine might have a broken timing belt or chain.

If the vacuum reading is low, the base ignition timing may be retarded, one or more camshafts might be retarded, or the EGR valve might be sticking open. If the intake manifold vacuum is higher than normal, the base ignition timing or intake camshaft timing might be too far advanced. If the gauge fluctuates, one or more cylinders are leaking vacuum through a reciprocating part like a leaking intake or exhaust valve or burned piston.

Manifold vacuum should increase slightly as the engine is held at 2,500 rpm at steady throttle. If the vacuum is the same or decreases, the exhaust might be restricted. At snap throttle, the vacuum gauge should plunge to zero and then increase at least 25% above idle values as the throttle is snapped closed. If an increase isn’t noted, the engine may have worn piston rings or valves.

The Blazer Wrap-Up
I almost forgot about the Chevy S-10 Blazer with the intermittent engine miss! The diagnosis was simple: by connecting a tool called a “vacuum analyzer” to a lab scope and triggering the signal from the #1 cylinder, I noticed a slight variation in the waveform from the #5 cylinder. Although I could lift the valve cover only a few inches due to interference from the air conditioner compressor, I discovered that the #5 exhaust valve guide was completely worn out, which allowed exhaust gases to pass through the guide and overheat the valve spring. The over-heated, carbon-covered spring had weakened to the point of barely closing the exhaust valve. Although the spring would close the valve well enough to a cranking compression test, the weak spring allowed exhaust gas to be drawn into the cylinder on the intake stroke, which diluted the air/fuel mixture and caused the intermittent rough idle condition.

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Gary Goms

Gary Goms is a former educator and shop owner who remains active in the aftermarket service industry. Gary is an ASE-certified Master Automobile Technician (CMAT) and has earned the L1 advanced engine performance certification. He also belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).
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