Considering a leak down test
 

My 1979 216k+ mile SC is going in for a tune up this week as part of my scheduled maintenance. The car is running perfectly, idles steady at 900 rpm, and engine sounds good. No valve ticking to be heard. I’m having some oil leaks fixed, getting valve adjustment, and changing plugs. However I’m not naive to think an engine at 216k miles is factory fresh. It’s never been rebuilt however I would like to know if it could benefit significantly from one or if a rebuild is in my near future. The leak down test will be an additional $150. Would you have this test done while it’s in, or just continue to drive until there is some indication a rebuild is needed. Also oil usage is negligible.

Thanks.

I think leakdown numbers are an important trend to follow, and if done correctly can provide a lot of info about where problems might be arising (rings, valves, loose/broken head studs).

If your mechanically inclined, for less than a hundred bucks you can own your own tester -

Same as Snap-on I believe
https://www.amazon.com/Lang-Tools-CLT-2-Cylinder-Leakage/dp/B008PV86UW/ref=sr_1_4?dchild=1&keywords=leak+down&qid=1619033 945&sr=8-4

Every valve adjust I'll do a leakdown test, listening at the crankcase vent, exhaust and intake and document my results. Worthwhile piece of mind and probably valuable info if you were ever to sell.

The information you list is exactly what I’d like to have at this point. If something is bad, I could start setting money aside each month and plan for a top end job. I am mechanically inclined but think I may just pay for the test since the car will be in for a tuneup.

Thanks for the info. I didn’t realize it could tell me about broken head studs as well.

Regarding head stud leakage. Doing a leak down is how I discovered my head stud issue. Air was leaking out at the head to cylinder mate at #'s 1 and 4. Helped that I had the engine tin off to verify where the air was leaking -

http://forums.pelicanparts.com/uploa...1619040537.jpg

After a top end job I'm good to go :D

http://forums.pelicanparts.com/uploa...1619040828.jpg

no

if the fuel consumption is normal, it doesn't smoke and it passes smog then there isn't anything to doubt the condition of the motor

if you are just curious then maybe buy or make a leak down tester and do it yourself

you can also buy a compression tester and do it yourself and that will give you a comparison from cylinder to cylinder


http://forums.pelicanparts.com/uploa...1476761526.jpg

here is my home made leak down tester, cost me around $15 in parts

ask the shop how much they want for a compression test

if all the tests are within a couple psi of each other then you got a good engine

I had the LD test done as part of the tune up. The best reading was 10% leak down. The worst was 15%. The other readings fell in between. Needless to say I’m happy with these readings on an engine with this many miles. I knew the car ran strong and this is one reason why.

If I do decide to drop the engine to replace fuel lines and fix some minor leaks (I’ve been waiting 35 years to fix some), would you go ahead and do a top end rebuild as well with these low leak down figures? I haven’t decided at all to drop the engine, but there are just some things easier with the engine out.

It’s ok to have a worn in Porsche engine. I wouldn’t rebuild unless you have money burning a hole in your pocket. If it goes, keep driving. Leak fixes are good but leave the heads on.

Thank you for your comment. I agree, no need to spend money on the heads. I’ll just fix everything I can on the outside. I’ll use the opportunity to fix leaks, change fuel hoses, replace sound mat, and put new A/C barrier hoses in.

I recently built my own leak-down tester after researching these a bit. Something I found is that there is no real "standard" in the automotive world for the offered leak down testers. I found a document published by Lycoming which apparently is the standard in the aviation world (and corroborated by a few others posts on this forum):

https://www.lycoming.com/sites/default/files/Cylinder%20Compression.pdf

Based on that, I built a tester similar to the one pictured above in post #6; I made an orifice as described in the Lycoming document. I perform the test at 80psig as described in the document. I used two identical pressure gauges to make my tester and calculate percentage drop.

Something that bothers me about the automotive leak-down testers with the percentage gauge is that you don't get to see the math. Incidentally, I have always understood that Bourdon-tube type pressure gauges don't like to repeatedly be run to full scale (affects accuracy over time) and all the commercial automotive leak down testers seem to make them do that.

I do think that leak down test data is valuable if you do it periodically and watch for trend changes. Just be sure to always use the same tester and test parameters. Also be careful to not remove spark plugs until just before you test each cylinder (get that cylinder to TDC first) to prevent dislodging a piece of carbon into the exhaust valve seat.

You know what they say about opening a can of worms :-)