Possible reasons for very poor leakdown test?

[jwakil]

My 81 930 runs very well I think, although must admit I haven't driven another one to compare power. It pulls strong, and idles steady. It smokes sometimes on startup, but that I regard as normal. The oil usage is a little high (probably a little more than a quart per 600 miles, but those miles are all mostly city miles so hard to judge). Anyway, I ran a leak down test and found all cylinders less than 10% leakage, except for #1 which was showing something like 80% leakage!. The #1 cylinder spark plug hole is a ***** to access and ensure proper tightening of the leak down tool, but I ran the test twice and got similar results so have to think the test was done properly. So my question is, how can I have such poor performance in one cylinder, and still have the car running well? If the valves are leaking and need adjustment for that cylinder, would the car still be running ok? What else can explain this?

[ClickClickBoom]

Originally Posted by jwakil

My 81 930 runs very well I think, although must admit I haven't driven another one to compare power. It pulls strong, and idles steady. It smokes sometimes on startup, but that I regard as normal. The oil usage is a little high (probably a little more than a quart per 600 miles, but those miles are all mostly city miles so hard to judge). Anyway, I ran a leak down test and found all cylinders less than 10% leakage, except for #1 which was showing something like 80% leakage!. The #1 cylinder spark plug hole is a ***** to access and ensure proper tightening of the leak down tool, but I ran the test twice and got similar results so have to think the test was done properly. So my question is, how can I have such poor performance in one cylinder, and still have the car running well? If the valves are leaking and need adjustment for that cylinder, would the car still be running ok? What else can explain this?

From the airplane world, listen to the intake while pressurized, then the exhaust, then the crankcase breather line. If it's the crankcase breather line hissing, drive for 10 hours and retest. If it's either of the valves, sorry, motor work is in your near future. The reason it's running well is due to the huffer(turbo), but it ain't right, find out why, or drive it till it blows.

[jwakil]

Just to add...I could not hear where the leak was coming from, there was too much noise of all the air flowing through the tool. The second test was also after I had rotated the engine through a full revolution.

[snbush67]

Sometimes pulling the plugs can drop some carbon. Carbon on the valve seat can cause leakage, as stated above run it for a bit and do the test again.

Never mind...I misread 80% as 80% good, 20% bad.

[jwasbury]

Assuming you did the test right (and you probably did), then it could very well be you have a "melty" piston. Overboost, ignition timing too advanced (or not retarded enough on boost) can lead to detonation, excessive heat and the pistons will deform. The piston rings also will shatter, but leak down can still be very good even with the rings in 25 pieces.

This is from my engine...44% leak down on this one. You can see lots of oil build up on the piston crown. Look closely and you also see that the top ring land is deformed and much larger than its supposed to be. The 2nd ring land was squished so the the ring would not rotate.



This is from a friends engine...same deal, but a bit more obvious.



Both of these cars (930s) ran very well (not smoking out the tailpipe.)

The below pic is from another friend's SC. Piston chunked by severe detonation. This car didn't run very well and was puking oil from the tailpipe

[j911brick]

You probably have a rebuild in your future, but I would do a compression test to see if the results match. In any case I probably wold not drive the car till the issue is resolved.

[jwakil]

I don't understand how a car could be running well with one cylinder at 80% leakage (not 80% good, but 80% bad). And my thinking right now is if I have a engine rebuild in my future anyway, why not keep driving it until it start running horribly. I will do another leak down test soon. Haven't had access to the tool.

[slow&rusty]

Jwakil - Where in Texas are you?

[j911brick]

Originally Posted by jwakil

I don't understand how a car could be running well with one cylinder at 80% leakage (not 80% good, but 80% bad). And my thinking right now is if I have a engine rebuild in my future anyway, why not keep driving it until it start running horribly. I will do another leak down test soon. Haven't had access to the tool.

You dont want to keep driving it because if you have a ring/piston problem you could grenade the piston sending bits all through the motor. Its also possible the piston seizes, and you break a rod or the crank.

[jwasbury]

Originally Posted by jwakil

I don't understand how a car could be running well with one cylinder at 80% leakage (not 80% good, but 80% bad). And my thinking right now is if I have a engine rebuild in my future anyway, why not keep driving it until it start running horribly. I will do another leak down test soon. Haven't had access to the tool.

80% leak down does seem high to have no noticeable running issues, but perhaps the engine is still developing enough compression on that cylinder. There could be an accuracy issue on the guage of the leakdown tester you used. You can do a compression test and see how #1 compares to the other cylinders. Likely it is down vs. the others (corroborating your leak down test), but still manages to develop some compression.

In the photo series I posted above, my cylinder with 44% leak down had lower compression than the other cylinders. I don't recall the exact numbers, but 5 were at 135-140, and the offending cylinder was at 125 or so. Plenty of compression to run, and run well. What is really surprising is that pistons with completely shattered rings showed excellent compression and leaks own numbers. The ring bits were all captive in the ring lands, of course.



My car ran very well, no smoke at start up or on boost. Ignorance is bliss, they say. Had I not done any tests, I might still be driving it with these issues. I was motivated to run tests when my car with aftermarket headers and a K27 turbo could not outrun a stock example

[jwakil]

Originally Posted by slow&rusty

Jwakil - Where in Texas are you?

I'm in Houston.

[slow&rusty]

If you need any help I am in Jersey Village and have a 930 also and lots of tools.

[jwakil]

Originally Posted by slow&rusty

If you need any help I am in Jersey Village and have a 930 also and lots of tools.

Cool...I'm just ~10 miles from you. sent you a private email (I think) . Mine is j_wakil@hotmail.com

[Bob Kontak]

Originally Posted by slow&rusty

If you need any help I am in Jersey Village and have a 930 also and lots of tools.

Capitalize on slow&rusty's offer. Seriously.

Also, not a crime to replace just one piston if all else ok.

He may find odd-ball issues that will help your car scream. Turbo's are weird but when they work right they are two cars in one. Regular and very scary.

[Chuck.H]

First thing I'd check is the valves are adjusted to have some free play... sorry I wasn't sure if you did that already; if so of course disregard.

And yes, in my experience cylinders with high leakdown numbers (lots of leak) can still generate ok compression numbers.

Chuck.H
'89 TurboLookTarga, 371k miles

[Jerry S]

Agreed....check your valve adjustment first....could be too tight.

Not to scare you..but when I rebuilt my engine, it had decent compression all around except slightly lower on one cylinder. I rebuilt mainly because of extensive leaks, but when a took it down I found that EVERY ring in the house was shattered into many, many little pieces. Prior to rebuild, it did run strong with minimal smoke.

Just my two bits

[Jerry S]

Also, smoke is usually only an indicator of toasted oil scraper rings, not compression rings