Engine - Rough Idle Diagnostic Information

ENGINE TESTING -- BACK TO THE BASICS

The Technical Helpline continues to get calls concerning rough idles on Subaru vehicles. If you call the TechLine, the first thing we will ask you is if you have performed a COMPRESSION TEST or a CYLINDER LEAKDOWN TEST. These are two of the most basic tests you can do to determine the internal working condition of the engine.

Just for review purposes, each cylinder must be "turned over" the SAME number of times to see if they are capable of doing the same amount of work. The throttle plate should be fully open and the engine at operating temperature. If three of the four cylinders come up to pressure in three revolutions of the engine and another needs nine revolutions, obviously there is something wrong with that one cylinder. It doesn't matter that it got to the same pressure as the others; it took too long for it to get there. If the difference between the highest and lowest cylinder is greater than 25% then the engine has a problem internally.

NOTE:
ALL SPARK PLUGS MUST BE REMOVED PRIOR TO DOING THIS TEST.

The next step would be to follow this compression test with another called a "WET COMPRESSION TEST where a SLIGHT amount of oil is added to the cylinders to help the rings seal. If too much oil is added, there is the danger of seizing the engine by HYDROSTATIC LOCK. If compression readings increase, then this is usually an indication that the rings are bad, although it is normal to get slightly increased readings even on a good engine.

We recommend you perform a CYLINDER LEAKDOWN TEST where air is put into the cylinders and you look for leakage, regardless of the outcome of the compression tests. The throttle plate should be open, remove the engine oil dipstick/filler cap and radiator cap. Air pressure should be regulated to 100 PSI if possible. We recommend the use of a Leak Down Tester, if available, to perform this test. Maximum allowable leakage is 10% (as per SUN Corporation). The cylinder being tested should be at Top Dead Center to be certain the valves are closed. With the cylinder pressurized, check for leakage. Air out the throttle body indicates a bad intake valve; out the tailpipe, a bad exhaust valve; out the dipstick tube, oil filler tube, bad rings (some leakage here is normal due to ring design); bubbles in the radiator could indicate head gasket problems or cracked cylinder wall(s). Once the source of leakage has been determined, it is easy to determine your course of action.

If the engine passes these tests and EVERYTHING involved in the tests has been done correctly, then the cause of the rough idle is elsewhere. Perhaps it is electrical or fuel related.

If you have any questions concerning this matter or the tests involved, contact the Technical Helpline.