Leakdown Test Variables and Variability

The following summary of leakdown test variables was came from a discussion on the E34 530i/540i Mailing List. Ken noted a list of six critical variables that can affect leakdown test results. Chad provided a strong second and added a short discussion of each variable based on extensive experience with leakdown testing of aircraft. Here's what they had to say:

"I'm sorry, I must chime in here, for what it's worth. The procedure of doing leak down tests on piston engines is very old. They have been doing it on aircraft forever and I have personally witnessed hundreds of them, performed by many different mechanics. Ken has some good points, that I would like to elaborate on:

  1. Temperature of the engine, this is very important, if BMW specs it to be at Normal temp, it needs to be at normal temp. If BMW specs cold, it needs to be cold. This will make a HUGE difference, for example, if they spec it to be cold and they test it hot, this will produce much better,(probably passing),results.
  2. Oil viscosity, In my opinion, not very important, most all of the oil will have drained out of the cylinders by the time the test is performed. Unless done HOT, then thick oil may make a marginal difference.
  3. Cylinder at top dead center, This will bring ENORMOUS differences in the results. In aircraft, they turn the engine back and forth to get the best results, while the test is being performed. I have seen a failing cylinder suddenly "pop" into a passing cylinder, just by rotating the engine back and forth to seat the rings and/or valves. I think this is a good & acceptable thing to do, otherwise you are just looking at one spot, that may not even be perfectly TDC.
  4. Calibration of the leakdown tester, This is also very important. BMW should have a "Master orifice tool" that the mechanics should use to verify that their pressure regulator and gauges are reading correctly, prior to the test.
  5. Seal of leakdown tester hose to the cylinder, Although this is important, I can't imagine a mechanic allowing that connection to be anything but perfectly air-tight.
  6. Cylinder ring alignment at the time of the test, This is the #1 biggest reason that you could pass one day with flying colors, and fail miserably two weeks later, or vice versa. For that reason alone, I can't believe that BMW didn't require at least two tests. I suppose that is why they created the idle quality screening first. I have my aircraft tested at every oil change, because once a year during the annual inspection does not tell you very much.

Also, regarding this type of test, in the aircraft world, they do not allow ANY leakage past the valves. not sure if anyone has found any valve leakage, but just thought you may want to know.

As for the "human error" concept, if BMW clearly specifies the procedure to be followed and the dealer employs competent mechanics, there should be no human error. Only laziness or a mechanic that wants to guarantee a certain answer. (the mechanic can easily skew the test to get any result he wants)

You may also be interested to know, that in the Aviation industry, warranty work is far preferred over charging the customer. Nobody likes to hand the customer a $50,000 invoice and say thanks for the patronage! You thought BMW maintenance is expensive, I've had oil changes turn into $11,000 bills.

Hope this gives everyone a little insight to the test that seems to be a mystery to most people.

PS. If you really want to know the condition of the block,(cyl walls), take a look with a bore scope. You can easily see corrosion with that, and it's as easy as changing the spark plugs."