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A Leak Down Test Virgin

Discussion in 'Technical Q&A' started by staatsof, Jan 12, 2006.

  1. staatsof

    staatsof Seven Time F1 World Champ
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    After a couple of track event seasons I thought I should just take stock of the condition of my engine. Everything is fine but I kind of wish I'd done this when I first got the car for a benchmark.

    I'm going to perform this test tomorrow with my newly aquired dual gauge leak down tester seen here:

    http://www.etoolcart.com/browseproducts/Cylinder-Leak-Down-Tester-AST7751.HTML

    So basically my understanding is :

    warm the engine up well
    remove the plugs

    then for each cylinder

    bring the piston to TDC compression stroke
    lock the engine, 4th gear enough?
    perform the test with 100psi

    The second gauge is to supposed to give a percentage reading or range of good to bad but it's not entirely clear to me what I should be looking for.

    I know about listening for escaping air through the exhaust pipe, intake manifold and possibly the coolant tank as well as other cylinders.

    Does the 1st gauge stay at 100 psi and the second fluctuate according to the amount of leakage?

    How long should I test each cylinder?

    Should I rotate the engine completely for a second test of each cylinder?

    What else?

    Thanks Bob S.
     
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  3. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
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    Bob~

    I suppose there will be others here who will comment, but this was how I was taught to do it on aircraft as part of 100 hour and annual inspections, and its how the FAA wants it done.

    The engine should be at operating temperture, just shut off. remove all spark plugs. Turn the engine to bring whatever cylinder your starting at to TDC on compression stroke. With your leak down tester in the spark plug hole, apply pressure, while at the same time holding the engine with the propellor. Well you dont have a propellor, so use a breaker bar on the crank bolt. Dont use the transmission and brakes.

    With pressure applied, rock the breaker bar back and forth past TDC a few degrees. This does two things. First, it was quite common to feel the compression rings snap with a resulting increase in holding pressure. Additionally, on engines with a lot of wear in the very top of the cylinder, you could see a loss of pressure at very TDC that would increase as the piston came down the bore a little ways.

    You are correct, in that the second guage reads a percentage of your first guage. If you adjust your first guage to 100 psi, whatever the second guage reads is a direct percentage. So if your holding 90, your holding 90%.

    On aircraft, because the pistons are so large, we drop the first guage to 80 psi. a good reading would be 78psi holding, 76 psi was OK, and anything under 74 was just about time to ground the plane. So while you could still drive your car with 90%, its a sign your engine is dying. What you would really want to see is like 98, or 97 psi, not 90.

    Last, you can hear air escaping from the cylinder, and where. Exhaust valves leaking can be heard in the tail pipe, intakes can be heard in the aircleaner, and rings can be heard through the oil cap. Good luck, I hope this helps.
     
  4. staatsof

    staatsof Seven Time F1 World Champ
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    Paul,

    Thanks for the reply and yes you have provided some great hands on stuff for me. I wondered about moving the engine slightly before TDC to see just what you indicated might be the case.

    I have to admit this device has me a little confused and as is typical with Chinese tools the instructions are sparse.

    The leakage indicator meter has a set point of zero which indicate no leakage. The directions say that WITHOUT the tester connected to the engine and the pressure regulator set to zero you are supposed to connect the shop air and adjust the pressure regulator so that the loss gauge hits the "SET" mark of zero. At this point there is no air leaking because of the quick disconnect check valve. Then you're supposed to connect it to the cylinder and observe the percentage of loss.

    The trouble is when I do that the pressure gauge only reads 15 psi. I have read other descriptions of this procedure that specify using 100 PSI. If I do that the leakage gauge will be destroyed.

    So I'm wondering if the 15 PSI is going to yield a valid test or does this type of tester work differently?

    I also have no idea how the internals of this thing work but I'm always a lot more comfortable when I do.

    How does this thing work (technically)?

    Here's a link I found that does a pretty good job of explaining the procedure and they use a gauge almost identical to mine with the same exact instructions.

    http://proformparts.com/PDF/66839story.pdf

    I called the manufacturer here in the USA and they're mystified as well. They agree it doesn't appear to make any sense testing at 15 psi.

    Help?????
     
  5. ajsva

    ajsva Rookie

    Dec 21, 2005
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    A J Szumski
    If the tool has a large diameter orifice between the high pressure and leakdown gauge, then you would seem to get good readings since the tool would make up much of the air lost in the engine. Another tool may have a small orifice and even a good engine might look like it was leaking excessively. Is there any standard for these tools? I imagine the Snap-On vs the Chinese tool could vary in design.

    AJ
     
  6. staatsof

    staatsof Seven Time F1 World Champ
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    Actually, I think the reverse is the case. If the orifice is too large then the pressure differential between the gauges will be very small and the tool would not be of much help.

    I'm wondering if this tool is just made incorrectly. I think both gauges HAVE to be calibrated on the same scale or it will never work. Looks to me like the Chinese used a bunch of wrong meters on this tool.

    Or maybe I'm just not understanding how this thing works.
     
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  8. Artvonne

    Artvonne F1 Veteran

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    The two tools we used were about the same. A block of metal, with two guages screwed into a side, and an air regulator that adjusted the air pressure. Ours also had an air shut off valve on the air compressor side, so we could turn the air flow on and off. There is a air bleed passage between the two guages, then its full size straight through to the motor. On this chinese one, I am thinking they are simply getting you to turn the pressure down before you attach the hose to the spark plug line. You dont want to attach it with full pressure if your not ready. I would hit a hardware store and find a gas valve or some kind of ball valve to put on the input line. Then you can adjust your pressure up to 100, or whatever pressure you want to use, and simply shut it off with the valve. Also, make sure your always checking and adjusting your regulator to keep your input pressure where you want it. As your compressor drops in pressure, or as you hit a cylinder with more or less air loss, your regulated pressure will change. I made my own leakage tester from a few guages I had lying around, and mini air regulator. Not as sharp looking as a fancy new one, but it cost next to nothing.
     
  9. staatsof

    staatsof Seven Time F1 World Champ
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    Well here's what bothers me about this. You're supposed
    to calibrate the pressure level on the leakage gauge before you connect it to the cylinder. This is done by adjusting the pressure regulator. When that's done on my tester the left hand gauge is reading 15 PSI. So now if I connect it to the cylinder it would seem to me that 15 PSI is not enough of a pressure leak-down test. Everything I read says about 100 PSI. Even the instructions with this tester warn about the pressure causing the engine to rotate. That's not going to happen with 15 PSI. So I think they want 100 PSI as well. If I adjust it to 100 PSI after it is connected then the calibration is lost and it will damage the leak-down gauge on the right side.

    The other testers I've seen have you calibrate to 100 PSI on the leak-down gauge on the right and both right and left hand gauges are rated for 100 PSI. Then you connect it to the cylinder and the pressure drop between the two gauges is your % of leak down. So if the right hang gauge reads 90 and the left reads 100 you have 10% leakage.

    On this unit I think the one on the right, the leak down gauge, maxes out at 15 psi whereas the one of the left maxes out at 100 PSI.

    How is that ever going to work?

    I believe that the manufacturer may have inadvertently put a 15 PSI gauge in for the leak-down measurement and stuck a new percentage face on the meter. It's probably a subcontractor supplier problem.

    Or, am I missing something?

    Bob S.
     
  10. Artvonne

    Artvonne F1 Veteran

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    Can you show us a pic of this thing?? Something sounds goofy. Both guages need to be like 125, or 150 psi, so you dont hit the stops with em over 100. Also, you should cut your line regulator at the compressor down to 120 or so, so you dont have so much pressure hitting your regulator on the tester. I have a two stage compressor, so I could have 175 hitting it if I didnt cut it back.
     
  11. staatsof

    staatsof Seven Time F1 World Champ
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    Check both of the links I posted. The leak-down gauge (on the right only has colors, percentages and text from high => low. There are no PSI readings on that gauge. It's hard to see completely in these examples. Suspicious no?

    All of the testers with the direct percentage/color/text on the gauges are like this. It's for dufuses or dufi that can't calculate.

    It does correspond directly to the 100, 90, 80, 60, 40 and 20 PSI marks of the other gauge so that would yield a 0%, 10%, 20% 40%, 60% and 80% leak down if both gauges were rated at 100 PSI max and you kept the input air at 100 PSI.

    Bob S.
     
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  13. Ed_Long

    Ed_Long Formula Junior

    Nov 11, 2003
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    Paul:
    I raced a 200 hp, Datsun 510 GT4 car for about 20 years and did my leak down testing with a set of Craftsman guages from Sears and one of their big compressors. I remember placing the gearbox in neutral, blocking the car with big chunks of firewood, and also sticking a prybar into the ring gear on the flywheel through the starter motor hole to keep the engine from rotating. I usually attained about 90 psi on the compressor guage, but as Bob said regulated that pressure with a big old knob. It was important on that engine to take the valve cover off so I could see the rocker arms to determine that I was on the compression stroke for each cylinder and the valves were closed. More than once I would not be quite at TDC and the engine would rotate a bit. Start over. I also learned to use a mechanics stethyscope to listen to the exhaust header, breather, and intake manifold for escaping air. In my experience everything Bob has told you is correct. You might try looking for a better set of guages, this Chinese thing seems a bit wacko to me. I set a personal criterion of no more than 10% loss as recommended by my engine builder, but ususally saw about 7-9%. I ran my tests on occasion with the engine stone cold and got about the same result as when hot, the only difference being that the rings had expanded some in their grooves after being warmed up.
    Ed
     
  14. staatsof

    staatsof Seven Time F1 World Champ
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    Thanks for the input Ed. I have read one place that the percentage of loss can be higher for turbo and supercharged engines due to the different configuration of the rings. Mine happens to be turbocharged. I'm thinking this tool I got is just defective.

    Bob S.
     
  15. mikeyr

    mikeyr Formula 3

    Jun 17, 2004
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    good stuff here since I am planning to do this on my Dino.

    My question is about the motor being hot, I know it should be done with the motor at operating temperature but by the time I get the valve covers off and all the junk, the motor will have cooled some and then by the time I get to cylinder 6 it will be cold. If I run the motor between each cylinder, it will make quite a mess.

    So someone posed that when they tested cold, it did not seem to make much of a difference, does it really have to be done at hot ? I did test compression hot and cold and it was not very different, it was barely lower cold.

    Mike
     
  16. Artvonne

    Artvonne F1 Veteran

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    You do not need to pull the cam covers to do the test. Simply pull the plugs, and slowly turn the motor by hand until air comes out the plug hole. You can use a small rod or a screwdriver and if your careful, with it resting on the piston, slowly turn the engine until its at the top. You can remove a distributor cap and watch the rotor point at the cylinder your testing to know your close. Now apply your air pressure while holding the crank with a long breaker bar, and there you are. As I posted before, you do NOT need to use 100 psi, you can use 80 if the pressure is to heavy to hold the breaker bar steady or safely. Your only looking for a percentage of loss so its not a big deal. 78 over 80 is 98% and would be a decent number. Others may argue, but I recommend swinging the engine back and forth over TDC with pressure applied for the reasons posted earlier.

    You could do the test cold, but it may not read as high as the piston will not be fully expanded out to the cylinder walls as when it is hot. But as long as all your readings were relatively equal it should be ok. Hot is better,
     
  17. mikeyr

    mikeyr Formula 3

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    I was thinking I needed to pull the valve covers just for the purpose of checking TDC but "duh" a soft rubber rod in the plug hole to check for TDC and I can do it with the motor hot, re-fire the motor in the middle of the processs if I am too slow at it.

    Thanks...sorry to the original poster, I can't help with your guage problems, I still have not used the one I got for Christmas.
     
  18. Artvonne

    Artvonne F1 Veteran

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    Bob~

    I cant make out the picture clearly. Regardless, 98 percent of 100 psi is going to be 98 psi on your second guage. If it only reads 15 psi, take it back and go buy a good one at NAPA, or just about any parts store. Like I say, I made mine, they are pretty simple. I drilled a .040" passage between the two guages. This is what makes it work. If there is no loss of air at the end (your cylinder) the pressure will equalise and the two guages will read the same. If there is a loss, the small orifice will cause a delay to compensate, and the pressure on the engine side will drop. The larger the hole, the faster the air will come through, and you wont have a accuate readings. To small a hole and a slight loss might be to much for it to keep up, which would also cause inaccuracy. Everything I read before making this one called for .040", and thats what I did.
     
  19. staatsof

    staatsof Seven Time F1 World Champ
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    Thanks Paul. That's about what I've figured out by now. It looks like I'll have to cancel my test plans for today. No one has these things in stock and I hust happened to get one with one incorrect gauge on it yesterday.

    I also will probably end up doing it at 80 PSI since my small portable compressor maxes out at 100 PSI and by the time everything is hooked up and running sustaining 100 PSI will probably not be realistic. Better to just have 2
    simple identical gauges and do a little of that "hard" math.

    Thanks again.

    Bob S.
     
  20. staatsof

    staatsof Seven Time F1 World Champ
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    You might want to check yours out first before using it.
    I called the importer and they admitted they had not been able to figure out to to make it work properly. They had the same exact problem I identified so I think it's a big problem originating with the original supplier. There are a number of these out there under different brand names that all look alike. Mine is going back today.
     

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