BMW X5: Compression Testing

In order for an engine to run smoothly and efficiently, the combustion chamber must be free of leakage. An engine with low compression in one or more cylinders is inefficient and will run rough or lack in performance.

Low compression may or may not cause the MIL to illuminate.

Low compression can be caused by the following:

• Leaking valves caused by burned valves or seats. The valve guide can also be worn causing the valve not to seat properly. Valves can also be bent from piston contact (from over-rev).
• Piston Rings which can be worn from high mileage or poor maintenance. Also, the rings can be damaged from foreign material or improper installation.
• Cracks in cylinder head or engine block. Cracks can be caused by overheating resulting in misfires or rough running.
• Defective cylinder head gasket. The cylinder head gasket can fail due to overheating which can cause cylinder leakage resulting in misfire, low compression and rough running.
• Bent connecting rod. A connecting rod can be bent from a defective fuel injector or water ingress into the combustion chamber causing hydrostatic lock.

Compression testing can be performed using a conventional compression gauge. There are some preliminary tasks and safety precautions that must be carried out before starting the compression test:

• Remove the fuel pump fuse and or relay, start the vehicle and allow vehicle to stall out on residual fuel
• Disable ignition by unplugging all ignition coils and remove ALL sparkplugs.
• Connect battery charger to vehicle
• Ensure that the throttle is wide open during cranking
• Crank engine until compression gauge stops increasing. Be sure to crank engine equally between cylinders.
• Continue compression test on ALL cylinders so comparisons can be done. Record readings.
• If necessary, re-check cylinders with suspect readings.
• If some cylinder readings come up low, add a few drops of oil and re-check. This can differentiate between valves/rings.

Cylinder Leakage Testing

Once a problem cylinder is detected via a compression test or by other means, a cylinder leakage test is used to pinpoint the problem area.

The leakage test uses a gauge and compressed air to indicate the percentage of air loss. By listening and observing at key points, the problem can be narrowed down before the engine needs to be disassembled.

The piston (one or more) should be brought to TDC, compressed air should be introduced into the cylinder using the cylinder leakage tester. Be sure the engine does NOT rotate, if the engine rotates, the engine was not at true TDC.

Check the gauge on the tester, it should read in percentage of leakage. Check the engine specification for permissible leakdown. A general rule of thumb is 15 % or less for a good cylinder. However, some engine have a tighter tolerance. Most BMW engines should be at 8 % or less.

Fig. 3: Testing Cylinder Leakage

If any cylinder shows excessive leakdown, check for leakage by listening or observing the following points:

• Listen for air (hissing) at the tailpipe. This would indicate leakage at the exhaust valves on that cylinder.
• Listen for air (hissing) at the throttle. This would indicate leakage at the intake valves on that cylinder.

  (Be sure throttle is wide open and listen at throttle opening)

• Open the oil cap and listen for air. This would indicate air leakage into the crankcase. This would be piston rings or cylinder bore concerns.
• Observe the coolant reservoir and or remove the radiator cap. Bubbles in the coolant would most likely indicate head gasket leakage or cracked block/head.

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