How to tell if piston rings are bad or valve seals?

I have Grand Marquis 2005 with 200,000 miles, well maintained and regular oil changes. Yesterday I noticed that when I revved engine hard at redline, exhaust emitted white smoke that probably smells like oil. I opened the hood to see if I can find something unusual and unfortunately I found that the plastic valve cover on driver side of engine underneath the air intake pipe has a small crack and I'm sure engine is blowing smoke because of it. I got computer test of engine done and 2 or 3 (not exactly sure and unfortunately I accidentally left result report at my friend's home last night) cylinders had low compression 78% while other had around 90% or + . I'm not really a technical guy but this is what that guy said that some cylinders has low compression. I've parked the car and planning to get it fixed but I'm wondering if piston rings are shot or valve seals. Is there a way to differentiate between two? As I said engine doesn't smoke all the time or on normal driving or overtaking or even pulling hard but only on revving 5000+ rpm.

13 Answers

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  • 1 month ago

    Hey dicky...a cracked valve cover isn't going to cause smoke out of the tailpipe. It will only let oil leak onto the exhaust manifold and cause smoke under the hood. With 200,000 miles on the car you need a rebuilt crate motor.

  • fuzzy
    Lv 6
    1 month ago

    I hope the engine was thoroughly warmed before you redlined it. Good news for you - if the previous owner rarely if ever used the engines full rev range the odds are you have a lip worn at the top of the bore; redlining the engine is VERY likely to break your top rings - stupid thing to do to an old engine.

    Simple check of valves or rings. Engine warm, remove plugs, hold throttle wide open & perform compression test on each cylinder. now put a teaspoon of oil in each cylinder & repeat test. Large difference in result shows rings are worn. Little or no change, valves

  • Anonymous
    1 month ago

    They will do a wet check.  Oil in each cylinder and crank it over. If compression increases it is rings.  It is not enough of a difference to bother doing it.  You can drive with that difference for another 5 years if you want to...probably longer.  You will probably sell the car long before then.

  • 1 month ago

    Yes there is. After the car has been sitting all night, have someone start the motor while you watch the exhaust. Any trace of blue smoke means the valve seals and or valve guides are worn. Engines do not blow smoke out the exhaust because of a crack in a valve cover. We need pounds of compression not percentages. We need a wet and dry compression test by a trusted private mechanic to know if you have a ring or valve problem.

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  • Anonymous
    1 month ago

    Just burn your wallet, it's easier and faster than beating up an old car and then throwing money into repairs it doesn't deserve.

  • 1 month ago

    Do a compression test and a leak-down test.

  • 1 month ago

    White smoke is often a warning of a blown head gasket.  This would also show low compression.  The leak down compression test will show you exactly where the problem is.

    Of course, if it is not bothering you any other way, you could try to keeps your revs at less than 4000 (when not at the drag strip) and see how long you can make the engine last

  • 1 month ago

    That was clever, wasn't it, with an elderly engine? 

    Do a compression test with the plugs out and the throttle wide open. Then do it again after inserting some oil into the cylinders through the plug holes. If the results the second time are better it is the rings. 

  • Anonymous
    1 month ago

    Oh yeah - everyone should take a tired 15 year old car with 200,000 miles and "rev it hard at redline".  Some sort of diagnostic test you invented??????

  • Anonymous
    1 month ago

    You need to get the actual compression values in psi.  Compression percentage is not usually used by mechanics and is misleading.  The values as indicated in the pressure gauge is used.  Do a leak down test to check if there is a piston ring or a valve problem.  Engine might need a valve job.  Good luck.

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