Vacuum & Pressure Diagnostics

Engine vacuum and pressure can be very useful tools for diagnosing engine performance and driveability problems. As we know, vacuum is necessary to operate power brake systems and, in the past, used to shift automatic transmissions. Vacuum is also used to operate some emission control devices, such as the EGR and PCV valves. Proper oil pressure is required to adequately lubricate the engine. Engine compression is necessary for combustion. It follows then, that monitoring these two parameters can help find problems when the systems operated by vacuum and pressure fail.

DIAGNOSING WITH VACUUM

Engine vacuum leaks can present some driveability issues, especially if the leak is in the intake manifold. Such a leak can be very easy to find. Simply spray some water at the suspected leak site. If the engine idle speed changes, you’ve found the vacuum leak. Using vacuum to find a driveability problem, however, is not that easy. Let’s look at a couple of emission control devices and how vacuum can be used to diagnose a problem.

Let’s begin with the Positive Crankcase Ventilation (PCV) valve. This device is the original pollution control device. Its purpose is to purge blowby gases from the crankcase and direct these gases back into the intake stream to be burned. In the old days, the crankcase was just vented to the atmosphere. As you can imagine, these gases were a major source of air pollution. Directing the vent tube to the base of the carburetor mounting flange on the intake manifold was an easy fix.

Vacuum will be used to determine if the valve is operating properly as shown in Figure 1. Before starting the test, apply the parking brake and/or block the wheels. With the engine idling, remove the hose from the PCV valve. If the valve is not plugged, a hissing noise will be heard. A strong vacuum should be felt when a finger is placed over the valve inlet. Re-install the hose. Remove the make-up air hose from the plenum at the rear of the engine. Hold a piece of stiff paper (such as a parts tag) loosely over the end of the make-up air hose. Wait about a minute for the crankcase pressure to reduce. After that time, the paper should draw up against the hose with a noticeable force. If not, replace the PCV valve. If the engine does not draw the paper against the hose, replace the PCV valve hose or clean the valve cover baffle where the PCV valve is located.

The Manifold Absolute Pressure (MAP) sensor monitors atmospheric pressure—one of many inputs for determining the air/fuel mixture. After diagnosing and repairing any sensor supply or system voltage Diagnostic Trouble Codes (DTCs) and inspecting/correcting any wire problems, it’s time to use vacuum to look for problems.

Using a scan tool, such as the WiTECH,™ DRBIII® or equivalent, read the barometric pressure. The barometric pressure should be approximately equal to the actual barometric pressure. If necessary, compare the barometric pressure value of the tested vehicle to the value of a known good vehicle of a similar make and model.

Connect a vacuum gauge to any manifold vacuum source. Start the engine. If the engine will not idle, maintain a constant RPM above idle. Read the MAP sensor vacuum with the scan tool. Compare that reading with the reading on the vacuum gauge. The scan tool reading for MAP vacuum should be within 1" of the vacuum gauge reading. If not, continue with further electrical testing of the sensor.

DIAGNOSING WITH PRESSURE

Pressure can be used to diagnose a variety of engine problems. When we talk about vacuum, our discussion is limited to air. When pressure is the topic, it can be either air or a liquid. And in the automobile engine, the liquid can be oil, fuel or coolant. Let’s review the use of pressure to diagnose a couple of engine problems.

The first pressure test that we will review is one that is used to detect leaks in the cooling system. Before performing this test, be sure that the engine is not at operating temperature. With the engine off, remove the radiator pressure cap and wipe the radiator filler neck sealing area clean. Top off the radiator if it is not full.

Attach Cooling System Tester 7700 (Figure 2) or equivalent, to the radiator filler neck. Apply 15 psi pressure. If the pressure drops more than 2 psi in 2 minutes, inspect all hose connections and external cooling system points for leaks.

All radiator and heater hoses should be shaken at 15 psi to simulate engine movement that would occur if the vehicle were being driven. If there are no external leaks after the pressure drops, detach the tester. Start the engine and run it until the thermostat opens, allowing the coolant to expand. Re-attach the cooling system tester. If the needle on the dial fluctuates, it indicates a combustion leak, usually a head gasket leak.

Warning: Pressure will build up fast with the tool in place; excessive pressure must be released; never allow the pressure to exceed 20 psi

If the needle on the dial does not fluctuate, raise the engine speed a few times. If an abnormal amount of coolant or steam emits from the tailpipe, it might indicate a coolant leak. This leak might be caused by a faulty head gasket, cracked engine block or cracked cylinder head.

We just reviewed how a coolant pressure test can be used to detect a bad head gasket between the cylinder and coolant passage. Well, that’s not the only pressure test that can be used to find a blown head gasket. The Cylinder Compression Pressure Test is performed to determine if a head gasket is leaking between side-by-side cylinders. If such a gasket leak exists, there will be a reduction in compression pressure in those two cylinders of approximately 50-70 percent.

Before doing this test, be sure that the battery is fully charged and the starter motor is in good working condition. This test is performed in the following manner:
• Clean the spark plug recesses with compressed air, then remove the spark plugs
• Inspect the electrode of each spark plug; look for signs of abnormal combustion
• Disable the fuel system and remove the ASD relay
• Insert a compression gauge into the #1 cylinder spark plug opening
• Rotate the engine with the starter motor for three revolutions; record the compression pressure after the third revolution
• Pressure test the remaining cylinders
• Compare the compression pressures; pressures should not very by more than 25 percent from cylinder-to-cylinder
• If one or more cylinders have abnormally low compression pressures, repeat the compression test on those cylinders
• If these cylinders continue to have low compression pressures, perform the Cylinder Combustion Pressure Leakage Test
• The two adjacent cylinders with abnormally low compression pressures indicate that the head gasket is bad

Note: Keep in mind that abnormally low compression in a cylinder is not necessarily an indication that the head gasket is bad; burnt valves or worn/broken piston rings can also cause a drop in compression pressure

The Cylinder Combustion Pressure Leakage Test is similar to the compression test, but it is performed after the engine has been brought to operating temperature, then shut off. The oil filler cap, spark plugs and air cleaner are removed. Each cylinder, one at a time, is pressurized. Leakage should not exceed 25 percent. Signs of a problem include the sound of escaping air in the throttle body, tailpipe and oil filler cap opening, along with air bubbles in the coolant.