Leak down test

[mulrik]

I just did a leak down test. I was wondering if anybody could comment on the numbers. 1: 20% 2: 42% 3: 35% 4: 30% 5: 41% 6: 35% I'm pretty sure I set all cylindres at TDC but it's difficult for #2-5 to be absolutely sure.

All the air was streaming pass the rings, I could hear nothing at the exhaust, intake, neighboring cylinders, nor were there bubbles in the expansion tank.

In the BBB it says max 25% (20% pass the rings and 5% throgh the valves) I'm getting around 7 km/l and burning oil, even though only spark plug #4 is black, all the others look fine. Funny thing is the motor is doing very well, smooth idle, fast steady acceleration all thru the revs. I have nothing that indicates that the engine should be in bad condition.

Now my questions and doubts are mounting. Do I risk anything with running the engine like this? Should I do another leak down or perhaps an additional compression test? Or should I stop talking, pull the plug and start restoring the power unit?

Your input are appriciated as always !!
Ulrik


'67 250 SL Papyrus White 113043-10-000023

[DavidBrough]

Hi Ulrick, Sorry to hear of your woes. I have just finished rebuilding my 280 engine which had similar problems to yours. In the end it was consuming over 1 litre for every 200 miles but ran very well whilst doing so. I had a leak down test carried out but don’t have the specific results as the mechanic just told me the pistons were shot. In view of the expensive possibilities here it may be beneficial to complete the test again in conjunction with a compression test and perhaps an exhaust CO reading. On the face of it you have two cylinders over 100% off the best with the rest at least 50% off which is quite a wide variance. How much oil are you using and is the oil pressure good?

[Cees Klumper]

Definitely check the compression. 7 km/l is normal I think, depending on your driving (city or highway, speed on the highway). What is your oil consumption? If you do go the rebuild route, piston/ring sets are still a lot less expensive in the US for some reason.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[mdsalemi]

You can do a google search on leakdown testing and find a plethora of information on the concepts and how to do it.  An Astro Pneumatic leakdown test kit runs around USD $60; they are also pretty easy to build but as a buyer of a lot of these kinds of parts for my car wash, for $60, buy the kit or find someone who has one.  Of course they come with a guide for interpreting the results.  As a ballpark estimate, 5-10% leakdown is what you would expect on a good fresh runner; I think the BBB number is the bottom end or what they consider to be the action point.  Perhaps more importantly--like compression--is you want to see consistency from cylinder to cylinder.

If Mulrik's testing was done correctly, sad to say it looks like the engine is ready for a rebuild--when he's ready to have it done.

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[Benz Dr.]

Your results while not perfect really aren't that bad either. It shows a moderate amount of wear and MB always gives you specs well above a useable limit. I think they give 150 PSI of compression as rebuild time on 280SL's but the engine will run OK at 130, only it will be down on power.

The leak down test has to be done right to get the right results. I do a dry and then wet test. If there's little difference the valves are leaking. If there's a lot of difference then the rings are leaking. 15 to 25% leakage would be normal for a good engine.
I also do a vacuum test which is very accurate because of the low numbers used. 15 - 17 inches of vacuum would be a leak free cylinder.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061