Tool Briefing: Number 6 is AWOL

Diagnosing engine power loss with vacuum and compression testing, valve timing testing and more.


When diagnosing engine power loss or other driveability issues, mechanical problems are not the most common cause. But after ruling out ignition and fuel injection problems, checking the mechanical condition of the engine is the next logical step. Most techs would start by looking for a vacuum leak or compression leak, but the Tech Tip also points to a valve timing problem. Let’s take a look at some of the tools for these tests.

Smoke and Vacuum

Originally developed for finding Evaporative Emissions (EVAP) System leaks, diagnostic smoke is also the easiest way to find a vacuum leak. In fact, it’s so easy that many techs overlook one of the oldest and most basic diagnostic tools: the vacuum gauge.

Although this engine has no vacuum leak, a vacuum gauge will also indicate a cam timing problem. If overall cam timing is late, the intake manifold vacuum will be low. If there’s a valve timing or sealing issue with only one cylinder, the vacuum gauge needle will pulse at idle. If there are no mechanical problems big enough to cause a constant misfire, the needle will be stable and vacuum will be about 18 inches of mercury (in/Hg) or higher at idle.

Using a Degree Wheel and Dial Indicator

A degree wheel and a dial indicator are used along with engine mechanical specifications to check valve lift and timing. It’s not always easy to find valve opening specs, but here we only need to compare valve timing for cylinder number 6 with another cylinder that’s firing correctly. On this engine, the firing order is 1-2-3-4-5-6, so it’s easiest to compare cylinders 4 and 6 because only one valve cover need be removed.

 

With the degree wheel mounted on the crankshaft, rotate the crankshaft in the normal direction of rotation until number 4 piston begins moving up on the compression stroke. Stop at Top Dead Center (TDC) and note the degree wheel reading. Since you’re just checking valve timing against another cylinder, it’s OK to watch the rocker arms to determine TDC. To get an accurate valve timing measurement, you would find the exact TDC using a piston stop tool.

Set up the dial indicator on the exhaust valve rocker arm and zero the indicator. Rotate the crankshaft in the normal direction until the dial reaches its highest reading. Check the degree wheel and make a note of the reading. Move the indicator to the intake valve and repeat the process. Now you know the valve lift for cylinder 4 and, more importantly, how many degrees before or after TDC those maximum lifts occur. Repeat the process on cylinder 6. If the camshaft lobes have slipped, maximum lift will occur a different number of degrees before or after TDC. If there really is a slipped cam lobe, the difference will probably be significant.

Compression Test

A standard cranking compression test will tell you if a cylinder is leaking compression. A wet compression test (squirting oil in the cylinders) will tell you if compression is leaking past the rings due to fuel flooding. However, neither test will find a slipped cam lobe because there is no compression leak. But a running compression test will show how effectively a piston pumps air in and out of the cylinder: the cylinder’s volumetric efficiency. This would be affected by incorrect valve timing.

For this test you need a compression gauge that has a release valve. First remove all the spark plugs, do a standard cranking compression test and record the readings. All cylinders should be within 10 percent of each other.

If the car’s battery is low, use a jump pack or a battery charger that has an engine-start mode. Don’t forget to keep the throttle wide open. This not only improves accuracy, it also puts the fuel injection system into ‘clear-flood mode’ to prevent the injectors from operating.

 

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