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Thread: Leakdown Results - What do they mean?

  1. #1

    Leakdown Results - What do they mean?

    Guys, I got the compression and leakdown test results on the Viper Green 73 911E I am pursuing. It has 98K miles on it. It has been sitting for about 6 months. The shop is just getting ready to build it up for me. The car is Numbers Matching, the price with a full bare metal repaint is $12 - $14K.

    Still need to have my PPI done, but the first step was a Compression Test and Leakdown Test. The shop gave me the following information after the test.

    I got the results of a compression and leakdown. I will list cylinder, then compression psi, then leakdown %

    1: 140psi 19%
    2: 140psi 18%
    3: 145psi 18%
    4: 145psi 9%
    5: 152psi 10%
    6: 140psi 10%

    I'm generally pretty happy with these. Compression is well within the range of what it should be. The first three leakdown numbers are a little high, but not that bad. I reinstalled the fuel injection pump yesterday. This was different than the one the prior owner had on the car. He had the injection pump adjusted to the intakes in a fashion not within specs. It was still in the right ballpark so it ran well. So with this different one on it, it was running really rich. What I do with these injection setups is start from scratch beginning with a dissassembly on the intakes and flushing of injectors (time consuming but I feel neccessary to know what's going on). Then I adjust throttle plates, linkages........That will all happen when I remove the engine shroud to detail the engine.

    Anyway.......with the motor running rich there is a high amount of carbon deposits which can hold a valve open a little and cause higher leakdown numbers.


    So now to the questions. The car has a bunch of mechanical upgrades, which I won't detail here, unless someone asks. There is light rust in three places that the shop will repair. I am confident in his coachwork. I want to make sure that the engine and MFI are in good shape, so I don't immediately need to pump money into it. Are these numbers OK? From my reading, it seems that the compression numbers are good. But the leakdown numbers on the first 3 cylinders bother me. But, they are inconsistent with the compression numbers.

    Since the engine has been sitting, could this cause the higher numbers on those three cylinders? Does the fact that they are on cylinders 1, 2, and 3 mean anything? Is it possible that a stiff run at high rpm would "blow out the carbon" that is sitting there and bring the numbers down?

    Do these numbers tell me that I should be prepared for some mechanical repairs? Is there some mechanical work that I should have the shop do NOW, as part of my purchase agreement?

    As you can tell, my newbie-ness is showing. I am excited about this car, just don't want my emotions to take over, or to be scared away by something that maybe is within range, or within expectations.

    larry
    Early 911S Registry Member #537

    73 - Viper Green 911E Targa - Kermit - Gone but not forgotten

    Kermit's Short Story and Pix on the 911E Website

    06 - Lexus IS250 MT6
    98 - Volvo 70V XC

  2. #2
    The following is a quote from Wayne Dempsey's "How to rebuild and modify Porsche 911 engines". A book I regard highly.

    "Good running engines will typically have leakage less than 10%. Engines with significant problems will exhibit leakage of 30% or more."
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  3. #3
    PWD72S

    I did read that chapter in Wayne's book, and it is good info. But it seems that these tests are not hard and fast, since the leakdown test is prone to error.

    If the engine has carbon in it, that could result in the variances the shop gave me. Should I have the shop run it for a while at high speed to try to burn the carbon off, and then do the leakdown test again to see if the numbers stabilize? The compression numbers are consistent, the leakdown numbers are not.

    Should that worry me? or not.

    larry
    Early 911S Registry Member #537

    73 - Viper Green 911E Targa - Kermit - Gone but not forgotten

    Kermit's Short Story and Pix on the 911E Website

    06 - Lexus IS250 MT6
    98 - Volvo 70V XC

  4. #4
    In all honesty, I dunno. I mean, is one shop's leakdown test gauge set the same as another? Too many variables to figure out???
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  5. #5
    pwd72S

    I don't think that the equipment used to do the leakdown test has different accuracy from shop to shop. I just think that a small amount of carbon, how warm the engine is, how rich the engine is running, can affect the results. Is the goal to just do it again and again till you get the results you want, or until they stabilize?

    I don't know how hard to push the shop to re-do the test, or if the leakdown test just gives a clearer picture of the trouble spots in the engine if the compression tests are weak. If the engine has good compression, then can leakdown still point out a problem?

    larry
    Early 911S Registry Member #537

    73 - Viper Green 911E Targa - Kermit - Gone but not forgotten

    Kermit's Short Story and Pix on the 911E Website

    06 - Lexus IS250 MT6
    98 - Volvo 70V XC

  6. #6
    I think your mechanic is on the right track stating that he's happy with the results. I don't think leakdown tests are the fairest test for a motor that's been sitting with carbon buildup that may be flaking.

    Some people, namely Bruce Anderson, don't put a lot of stock in leakdown numbers. He believes compression tests are a better judge of condition.

    The carbon issue your mechanic points out is caused by the carbon falling and blocking the exhaust valves from sealing properly. This occurs because they are on the bottom in a flat six motor.

    As far as carbon removal, I've heard of people spraying a mist of water in the intakes while the motor is running. Sort of a steam cleaning if you will. Then a good run.
    Bobby
    71' Olive 2.2E Targa / Early S #491

    I've always considered the glass to be half full...that is until I reached middle age and realized that it is actually half empty.

  7. #7
    If the car was yours, I'd suggest a few hundred miles of fairly hard running with a good deposit removing fuel additive, then a retest, see if the results have changed...
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

  8. #8

    In Mechanic I trust?

    I appreciate the advice. It seems that the carbon answer is the best one, but he is unable to take it on the street to burn it off. The car is no longer street legal, having been partially torn down for the restore, and not being registered. Based on what I have read, it seems logical that the leakdown results are OK, but unfortunately he can't confirm that for me by running it hard.

    He is pulling the engine to detail it this weekend, so that kind of puts the end to the story for now. We are now negotiating on the add-ons, oil cooler, fixing the a/c, radio, targa front seal, etc.

    Here's hoping I join the club of owners soon.

    larry
    Early 911S Registry Member #537

    73 - Viper Green 911E Targa - Kermit - Gone but not forgotten

    Kermit's Short Story and Pix on the 911E Website

    06 - Lexus IS250 MT6
    98 - Volvo 70V XC

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