Leak down #...hmm what to do know?
 

Was planning on totally rebuilding my engine this winter but know I'm rethinking ('82 930 race car). Just did cold leak down numbers and came up with the following:

#3 18% exhaust.......#6 7% slight exhaust
#2 5%.......................#5 4%
#1 19% exhuast.......#4 4%

Could hear crank vent on all, and others, location noted. Going to burn myself this weekend and do hot, but assuming it comes out same ballpark. Few questions:
1. besides 3 and 1 these are really pretty good, right?
2. should I just take left side of engine off and send those heads off for fixing?
3. can you change head studs without completely taking heads off?
4. if i do decide to take heads off and leave piston's in cyls, can you change rod bolts without cracking case?

Joe:

#1 needs attention
#2 is fine
#3 needs attention
#4 is fine
#5 is fine
#6 is likely OK (carbon on the seat)

Pull all the heads and fix the 1-3 bank.

With the heads off, replace all the head studs with something better. :)

While you are doing the above, pull the P&C's (you can leave them in the cylinders) and replace the rod bolts and rod bearings. You do not have to split the case.

Steve,
I'd heard recently that Porsche engines don't always give useful either comp or leakdown tests when (and I can't remember) cold or hot. Can you discuss the usefulness of both tests briefly? You da man!

LOL,..Everyone who works in this business long enough has favorites for certain things and in the case of general engine diagnostics, leakdown tests are mine.

For me, these are FAR more informational & useful than compression tests since they help pinpoint the SOURCE of the cylinder pressure losses (if any). Compression testing tells one very little about the general health of the engine since camshaft profile plays a large role in cranking compression pressures.

All of these tests are most effective when performed on a warmed up engine. :)

Leakdown testing will allow one to evaluate the rough condition of valves and rings by permitting you to take note of the leakage. If you hear air escaping from the air cleaners, look for leaking intake valves; if you hear air escaping from the exhaust pipe(s), look for leaking exhaust valves. If you hear air escaping from the oil filler tube, you have a ring problem.

The figures on the leakdown test guage tell you whether everything is OK or not.

1% to 5%; Very healthy engine

6% to 12%; I'd be looking for the source of leakage

12% to 20%: You have a problem that needs followup

20% or more; You have a dead hole and should be repaired.

I do hope this helps,

What are the more common sources of leaks?

Great info, thanks Steve! You are such a huge asset to this board. I owe you a cold one, somewhere somehow.

Hi Paul:

Cylinder pressure losses are caused by normal wear, leaking and/or bent valves, and catastrophic failures,....:)

Mileage helps determine what to expect as well as one's maintenance regimen. Frequent oil changes certainly extend the life of the rings and valve guide wear is somewhat commonplace with engines that still have the original guides. Heavy guide wear allows the valve to move laterally on the seats and that can show up in leakdown testing.

Did that answer your question?

That was helpful, Steve.

What difference would you expect between a cold or warm test?

So would you just do a top end rebuild on a low mileage motor and a full rebuild on a high mileage engine?

When would you notice driveability issues? If the car doesn't smoke, consume oil, or leak in weird places, how would you know to even run this test?

Thanks for the help. Incredibly interesting.

P.S. What tools do you need to run a leakdown?

Paul:

1) Thats going to vary due to engine wear & conditions. I strongly recommend performing the test on a warmed-up motor.

2) Yessir, depending on inspection,...:)

3) I perform these diagnostics as a part of preventative maintenance so I can track each engine's condition. That enables one to spot trouble BEFORE it gets really expensive. Its best to be proactive, rather than reactive.

4) One uses a Leakdown Tester; Snap-on (and others) make a good one.

Thanks a ton, Steve.

Read this before continuing:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

Wayne seems to be a little more forgiving. This is from his "Time to Rebuild" article linked above.

"When you are running the test, it is a wise idea to make sure that the crank doesn’t turn at all. Have an assistant hold the crank steady or place a flywheel lock on the engine if it’s out of the car. Connect the leakage tester to the engine in the same manner that you would with the compression tester. Pump up the cylinder and let the leakage tester measure the amount of air lost. The gauge on the tester should give readings in percentage numbers. A newly rebuilt engine should have leak-down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine, but are still adequate for good engine operation. Leakage numbers of around 30% indicate that there are problems brewing, and that a rebuild may be necessary. Needless to say a large leakage amount like 90% indicates that there is a hole in the combustion chamber, and the engine is probably not firing on this cylinder at all. Rotate the engine crankshaft clockwise 180° when you’re done, and check the next cylinder. Repeat the process for each of the six cylinders."

Figured I'd share the warm numbers

#3 22% ........ #6 5%
#2 5% ......... #5 5%
#1 27% ........ #4 5%

basically worst cyl's got worst, and good ones stayed approx the same.

also went thought and made sure valve rocker clearances were correct on #1 and 3. some also mentioned to me to tap valves with plastic hammer to make sure they're seated if get high numbers. you know you've opened them because you can hear them pop.

Here's another interesting point. ...do a leakdown on the checker itself (crank it up and press your thumb hard over hole at end). after I did all these readings, put car on jack stands, started disconnecting everything, etc, this dawned on me. Turns out leak checker has about a 3% leakdown on itself -- discovered very small leak at a threaded connection so above numbers are really about 3% high.

Again, it's important to know that this is only one test. When you get a bad x-ray at the doctor's office, they don't just cut you up (usually), they run more tests to see if they get similar results. You have to go through the steps - measure oil consumption, perform a compression check, try to burn out extra carbon, etc...

-Wayne

See if you can get access to a borescope to look inside the #1 and #3 jugs-- if there are burns on the exhaust valves they might be visible.

joemag,
Did you hear air rushing out of any of the aforementioned areas....exhaust manifold, air intake, sounds from oil tank?

...Forgot to mention, but air pouring out of the exhaust on 1 and 3. Other's were hard to tell where, little from but a little on exh. Didn't mention in first post but this car ('82 930) has only been raced/de'd for last 4 years so the engine's coming apart to see what's up. ...I know I should do compression test, but don't have setup (no excuse though).