My Toyota Vitz 2007 burns engine oil. I replaced the PCV valve but it didn't help.

As per the answer to my previous question, this could be either due to bad piston rings or bad valve seals.

What can I do to differentiate between these two causes?

  • 2
    look for smoke: seals when you start, rings when you rev.
    – Jasen
    May 23 '16 at 6:11
  • @Jasen I get a white smoke when I rev. There is no smoke in idle. So what could be the problem?
    – Softcode
    May 23 '16 at 9:18

You can perform a Leak Down Test to pinpoint what's going wrong. You'll obviously need a leakdown tester kit, you'll need an air compressor, and some hand tools.

  1. Pull out spark plugs and either by hand or using a bar, rotate the engine so that the first cylinder you're testing is at top dead centre. You can put a screwdriver in the hole and watch it move up and down as you rotate the crank. DONT turn the engine backwards if you miss TDC.

  2. Put the car into gear and set the hand brake to prevent the engine from turning when you pump air into the cylinder. Start with the regulator turned counterclockwise so you zero the pressure coming in. Connect your air compressor, and turn the regulator clockwise to pressurise the cylinder.

  3. Write down what the leakage percentage is on the gauge.

  4. Then, open the radiator cap, the oil cap, pull the dipstick, and take off the air cleaner. You can now listen from all these points and see if you can hear air escaping.

Air whistling out of the intake probably means a leak around the intake valve.

Air coming out of the exhaust or exhaust mani indicates an exhaust valve leak.

If you hear it whistle out of the PCV valve, oil pour hole or dipstick is a sign of that air getting past the piston rings - i.e. cooked rings.

And if any air bubbles out through the coolant, it indicates a head gasket leak.

  • As I can see, mostly you have analyzed where the air is escaping. So, would it be possible to do this without a keakdown tester kit, just with the help of compressed air?
    – Softcode
    May 23 '16 at 9:36
  • 1
    The leak down tester helps with getting the air in there, unless you have a way of totally sealing the air input into the spark plug hole so nothing escapes from there. I suppose you can, but then you won't have a comparison between cylinders to see which (if any) are worse than others May 23 '16 at 10:09
  • 1
    @LahiruChandima, I don't know how common this is where you live, but major auto parts stores in the US allow you to borrow tools like this to diagnose issues. They typically do this for free (with a returnable deposit) in the hopes that you will buy all the parts you need from them once you figure out the issue.
    – JPhi1618
    May 23 '16 at 15:39
  • 1
    The purpose of a leak-down test is the determine the extent of a compression problem. Finding the cause is iffy- this test might find it, might not. The Wet/Dry Compression test narrows it down to the rings or not-the-rings (valves or head gasket).
    – kmarsh
    May 25 '16 at 2:33

A compression test will show whether its the seals or the rings.

Make sure the engine is warm and perform a compression test (dry) and take a note of the readings. Then pour a couple of teaspoons of engine oil and pour it inside the chamber through spark plug hole and re do the test again. If the readings are higher, then the rings are bad as the oil will cover the rings and will lead to a higher reading.



  • 2
    this will always lead to higher readings as oil is not compressible,
    – Jasen
    May 23 '16 at 6:19
  • Oil rings can be bad and still have excellent compression....
    – Moab
    May 23 '16 at 22:04
  • Wet/dry tests have been done since the beginning of the auto engine, and properly done, are reliable.
    – kmarsh
    May 25 '16 at 2:28

One sure way to test if it is the piston oil control rings is to put the vehicle in low gear and accelerate to 4,000 rpm, let off the gas quick and look in the rear view, when you decelerate with bad oil rings it will smoke much worse than on acceleration.

  • This is not a "sure way".
    – kmarsh
    May 25 '16 at 2:27
  • It is for me...
    – Moab
    May 25 '16 at 14:28

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