VACUUM GAUGE & TRANSDUCER TESTING

Motor,  Jul 2006  by Seyfert, Karl

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One of the earliest diagnostic tools, the vacuum gauge, retains its effectiveness in pinpointing engine mechanical faults. It can also be complemented by its electronic equivalent - the vacuum transducer.

Is there still a place for the vacuum gauge? We're living in a world that's filled with engines that may never need a valve job and that adjust their own ignition timing, monitor their own misfire conditions and correct fuel delivery to compensate for small vacuum leaks. And yet the typical computer-controlled fuel system still depends heavily on a sound engine and strong, reliable vacuum signals.

That's why vacuum readings are as valuable today as they've ever been. Vacuum testing also happens to be the fastest, easiest-to-perform test in the book. You don't need to hunt down special 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.

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 atmospheric pressure of about 14.7 psi, 101 kpa or 29.9 in./Hg. At higher elevations, atmospheric pressure is lower.

When pressure is lowered in an area, it causes the air to rush in from the surrounding high-pressure area. We often refer to the low-pressure area as a vacuum, when it's simply an area of relatively lower pressure.

When we measure the pressure in an intake manifold, we're really comparing the pressure inside the manifold to the pressure of the atmosphere outside the manifold. It's this pressure differential that causes the air and fuel to be pushed into the combustion chamber. Because it's a term we all use, we'll continue to refer to the lower pressure in an intake manifold as "vacuum."

The amount of manifold vacuum created is directly related to the engine speed and position of the throttle plates. If we take away fuel and ignition, then spin the engine, we'll still produce vacuum in the intake manifold. The faster the engine spins, the higher the vacuum will be-as long as the throttle plates create a restriction by remaining closed. However, if we move throttle plates open, the vacuum will decrease - as long as engine speed remains constant. It's important to understand this basic concept before moving futher.

You've probably heard of using a vacuum 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 ignition 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 running 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 running. We know a problem exists when the vacuum is low, but where do we start looking? Realistically, a low vacuum reading can be the result of absolutelv 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 lingerprint.

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 engine and intake design, where you connect the gauge can have a big effect on the accuracy anil sensitivity of the readings 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 stronger and steadier cranking vacuum tends to he.

When the engine cranks erratically, the cranking vacuum will also lluctuate erratically. Valve timing problems (a belt or chain) are the most common cause of abnormal or erratic cranking vacuum and cranking speed. However, the engine could also be so hot it's actually dieseling during cranking.

Compression problems can also produce abnormal cranking vacuum. If cranking vacuum is healthy but drops regularly and rhythmically, look for a compression problem. Every time the weak cylinder tries to fire, two things happen momentarily: The cranking speed increases and the vacuum decreases. A burned valve can make the needle drop regularly to zero vacuum.