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Leak Down Test

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Tim_Auckland, Sep 24, 2018.

  1. Tim_Auckland

    Tim_Auckland New Member

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    I tried to do a leak down test the other day and it left me with a few question.

    1. My crank shaft pulley (no belt attached of course) has both a yellow dot and an orange dot (which is about 30 degrees after the yellow one, I'm assuming that the engine turns clockwise). The yellow dot seems to be TDC (top dead centre) but what is the orange dot?

    2. It seemed to me that I could do the test on cylinders 1 and 4 at the same time and after cranking the crank shaft 180 degrees do 2 and 3 at the same time. I thought that if one cylinder had all the valves closed its sister/brother cylinder would have the exhaust ones open but this does not seem to be the case. Can someone confirm / explain ?

    3. Can some one confirm the gen 3 firing order please. I have a diagram,

    upload_2018-9-24_20-26-24.png
    Some people want to call cylinder 4 on the above diagram (the one closest to the transaxle) cylinder 1. Is the above the correct firing order but not the way to refer to the cylinders or is it correct full stop?
     
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  2. Tim_Auckland

    Tim_Auckland New Member

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  3. ChapmanF

    ChapmanF Senior Member

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    By ancient custom, cylinder 1 has always been the closest to the "front" of the engine. After they started turning engines sideways, it's still at the "front" of the engine, that is, the end with the crank pulley, not the end with the flywheel.

    I don't think you can count on your plan of testing two cylinders, turning the crank 180° and testing the other two. Yes, two pistons will be at TDC in each position, but only one will be there on the compression stroke. The other one will be transitioning from exhaust to intake and will probably not have both valve sets fully seated, so it won't hold pressure. I don't know if it adds anything to pursue the details further than that.

    -Chap
     
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  4. Tim_Auckland

    Tim_Auckland New Member

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    Thanks for the explanation, makes sense now. I guess if I was right on TDC it explains why I could do a leak down test on two cylinders at once. Obviously not the prescribed method but worked for me.
     
  5. ChapmanF

    ChapmanF Senior Member

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    I guess if you got a good result that way, it would tell you the valves did happen to be seated, unexpectedly, in that position, and nothing would stop you from trusting the result.

    It's if you got a bad result with that technique that you would have to spin it around to the correct position and test again to confirm.

    -Chap
     
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    I stumbled on a mention of "front of engine" when looking up the Knock Sensor:

    upload_2018-9-27_8-35-54.png

    The knock sensor's on the intake manifold side of engine, so that corroborate the passenger side end of the engine being the "front".

    @Tim_Auckland Have you watched this Eric the Car Guy episode:

     
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  7. Mendel Leisk

    Mendel Leisk Senior Member

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    If I recall correctly from the video: it has to be at Top Dead Center and on the compression stroke (there's another instance of TDC, on the exhaust stroke). So one cylinder at a time?

    The video has some suggestions on how to determine that I think.
     
  8. ChapmanF

    ChapmanF Senior Member

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    That's the only way it's guaranteed to work. The other top dead center could have valves incompletely closed, so you wouldn't get a good test.

    That said, it seems that Tim did it that way, and got good readings. That's partly luck, but I think if it happens, you can take it. Good is good.

    It would only be if you got a bad reading that way that you would have to spin the crank around 360° and test it right, before you could really be sure the reading was bad.

    -Chap
     
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  9. Tim_Auckland

    Tim_Auckland New Member

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    Thanks for the discussion guys. To give you a bit of background. I was doing the leak down test as a newbie, I had watched the videos but this is a Prius (some things don't always translate) and I couldn't find any good info on this site. I wasn't even sure I could or should turn the engine over by hand. The tester I was using is very low quality and the first time I did it I was using a bicycle pump (a big one with a gauge) as the compressed air supply. I didn't really think my equipment would work even if I did everything else right.

    Anyway I knew I had two cylinders at top dead centre and thought I would be able to work out which one was on the compression stroke by performing the test on both. The first cylinder I picked gave me a good result. Being unsure of my equipment and methodology I tested the other one without moving the crankshaft and assumed I would get leakage. When I also got a good result on that cylinder I was a little confused but my initial reaction was to blame my equipment. However a couple of days later I did it again at my brothers place with a good compressed air supply and got exactly the same result. That's when I started this thread. I thought if we could confirm firing order and a few other details it might make life a bit easier for the next newbie.

    Obviously now I can see that all I needed to do was move the crank 5 or 10 degrees off TDC and do the test on both cylinders again. Only one would have still given me a good result (although the compressed air would also probably start to turn the engine). From there it would have been easy for me to confirm firing order.

    It would be interesting to do the test on the exhaust stroke cylinder in small increments around TDC to work out how may degrees both valves are closed when it is transitioning between exhaust and intake strokes. Maybe next time.
     
  10. ChapmanF

    ChapmanF Senior Member

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    I typically stick a socket on a breaker bar so I can put it over the nut on the crankshaft pulley and the bar pointing down toward the ground where I wedge my foot against it, so the crank doesn't spin when I apply the air even if I was a little off from TDC.

    It is such a low-friction engine, not only is it ok to turn it by hand, it's downright easy; I just grab the crank pulley and turn it. :)

    -Chap
     
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  11. Mendel Leisk

    Mendel Leisk Senior Member

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    I'm out of my depth btw: if the piston is at Top Dead Centre on the exhaust stroke, there's a valve open? Is that what nixes things?

    Also, since the Prius doesn't "crank" in the traditional manner, does that have an impact on the test?

    Hey, I slept through some of the video... :oops:
     
  12. Tim_Auckland

    Tim_Auckland New Member

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    Slow this animation down to 0.25 speed.



    On the exhaust stroke (upwards) the exhaust valves are open before closing them and opening the intake valves for the intake stroke (downwards).

    Prius does mostly crank in the traditional manner it just leaves the intake valves open for a short time at the start of the compression stroke (upwards). This effectively shortens the compression stroke. An engine with a shorter compression relative to its power stroke (downwards) is apparently more efficient (but less powerful). I'm no expert but I have been doing a little bit of research lately...

    Four-stroke engine - Wikipedia
    Four-stroke cycle used in gasoline/petrol engines. 1 = Intake, 2 = Compression, 3 = Power, 4 = Exhaust.
     
    #12 Tim_Auckland, Sep 28, 2018
    Last edited: Sep 28, 2018
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  13. ChapmanF

    ChapmanF Senior Member

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    As it turned out, in Tim's case, nothing got nixed. He got a good reading that way. It's just that that's a somewhat contingent fact, hinging on precisely what the engine's valve timing happens to be when the engine isn't running and there is no oil pressure to the VVT-i adjuster. In another very similar universe, he might not have gotten a good reading that way, and would have had to spin around to TDC compression, where God intended the test to be done. But he got one, so it's all good.

    -Chap
     
  14. Tim_Auckland

    Tim_Auckland New Member

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    Yeah, I think there is a hydraulic piston that stops the cam from changing timing when there's no oil pressure. Its so that the cars timing is locked at startup and any time that the cam isn't under computer control.
    Re "Drews" comment at the bottom of the following video.

     
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