A Few Engine Diagnostics

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Written by Don Stewart | 30 September 2004

This month Pony Tricks will cover some engine diagnostic procedures that you can perform with a vacuum gauge. The information is a compilation taken from Chilton, Haynes, and Acton. None of the information is engine-specific; rather, it is more in the vein of general diagnostic work. Where test values are given, it is important for you to check the specifications for your engine before reaching a decision on the engine condition. Also, it is important to note that much of the information gained from vacuum testing is preliminary and used to point you toward other tests that are more specific.

CRANKING VACUUM TESTS

Engine Condition Test

1. Warm the engine to operating temperature. Shut it off and disable the ignition so it won’t start.

2. Remove the air filter, remove the PCV valve from the valve cover and plug the valve on the bottom with tape, disconnect the idle stop solenoid if equipped, back out the idle adjusting screw (count the turns so you can put it back to spec. later), and lightly seat the idle air adjusting screw, if equipped (count the turns here too).

3. Connect the vacuum gage to a source of manifold vacuum.

4. Crank the engine and note the vacuum reading(s).

5. Compare the reading(s) with manufacturer’s specifications.

6. Remove the tape from the bottom of the PCV valve and crank the engine again. Note the reading.

Test Results

1. Vacuum with the PCV taped, should compare favorably with manufacturer’s specs, normally  4-inches of mercury or more on emission controlled engines and 10-inches or  more on non-emission engines. The vacuum reading should be steady or have a certain amount of even pulsing.

  • Low vacuum can be caused by vacuum leaks (examples: carburetor gasket, intake manifold gasket, worn throttle shaft, ruptured vacuum line or PCV hose), incorrect valve timing, or slow cranking (check battery, battery cables, starter)
  • Uneven pulsing can be caused by intake manifold gasket leak at one or more (but not all) cylinders, burned valve(s), stuck valve(s), worn valve guides, broken piston, or broken rings.

2. With the tape removed from the PCV, the reading should be about one half what it was with the tape in place. If the reading is higher or lower, replace the PCV valve because it’s stuck or plugged.

RUNNING VACUUM TESTS

Engine Condition Test

1. Reset the adjusting screws, connect the idle solenoid, install the PCV, and install the air cleaner. Enable the ignition system and connect the vacuum gauge to a source of manifold vacuum.

2. Start the engine and warm it to operating temperature.

Test Results

1. At idle, a healthy engine should have a reasonably steady reading of 17 to 22-inches of mercury. A steady reading below 17 down to 15 or lower may indicate late ignition timing, late valve timing, low compression, leaking intake manifold gasket, leaking vacuum line, or leaking carburetor-base gasket.

2. A healthy engine at 2000 RPM should read about 19 to 21.

3. A reading at idle that sweeps or wanders through several inches indicates a condition that affects all cylinders but unequally or inconsistently. Increase RPMs to about 2000:

  • If the sweep steadies down, look for an ignition problem or an ignition timing problem.
  • If the sweep gets larger, check for weak or broken valve springs.
  • If it becomes shorter and more rapid, check for carburetor leaks, intake manifold leaks, or sticky valves.

4. A “vibrating” reading at idle indicates worn valve guides.

5. A “vibrating” reading at increased speeds indicates a leaking intake manifold gasket, weak valve springs, burned valves, or ignition misfire.

6. A “vibrating” reading at all speeds indicates a leaky cylinder head gasket.

7. An irregular drop in vacuum may indicate a sticking valve or ignition misfire.

8. A healthy engine at wide open throttle should read close to zero. And when the throttle is snapped shut, vacuum should read above 21 to as high as 28.

9. Suspect worn rings when the same exercise of snapping the throttle shut results in very little peaking of the vacuum.

10. A large fluctuation on the order of 10 inches of mercury indicates a probable weak or dead cylinder or a blown head gasket.

11. With the RPM at 2000, an exhaust restriction is indicated if the vacuum slowly decreases.

DISTRIBUTER VACUUM ADVANCE TEST

1. By inserting a “tee” in the line, connect the vacuum gauge in the vacuum signal line that runs to the vacuum advance unit on the distributor..

2. Slowly increase engine speed from idle to about 3000 RPMs. Vacuum should increase to a maximum of about 18 to 22. If vacuum does not increase inspect for a ruptured vacuum line or a ruptured diaphragm in the vacuum advance motor.

If you also have a timing light, the vacuum advance curve can be verified against vacuum advance benchmarks specified in the repair manual. If the benchmark values are not achieved, look for a misadjusted vacuum motor, a rusty advance mechanism in the distributor, or a vacuum motor that is disconnected from the advance plate.