leak down result diagnosis
 

I ran a leak down on the motor in my '83 track car. All cylinders were in the range of 4-6% except for #1 at 37%. No leakage through intake or exhaust, all at the oil tank. I'm guessing a broken ring. What do you think?

I'm planning to take the car out to warm it up, circulate the oil and do a couple of hard pulls, then repeating the leak down test. Unless the number improves, I foresee a teardown to examine the #1 piston and rings.

Anyone have a better suggestion?

Background:

• Test performed on cold engine (has not been run since Memorial Day track event).
  
• I'm running the test with supply at 80 PSIG. Observed cylinder pressure was 50 PSIG.
  
• Engine is stock '83 3.0 CIS (except for open track exhaust)
  
• At the last event we did have a fan belt break. We saw the alternator light and limped in early. The engine was hotter than usual afterward, but the oil temp was not excessive.

Thanks in advance,
Steve

might check the valves for a tight lash and then redo the test and listen to see if the leak is coming out of the intake, exhaust or crankcase

Definitely from the crankcase. I can hear (and even feel) the air flow at the oil filler.

Couldn't hear any flow from exhaust or intake.

If you have a borescope you can pull #1 plug and look for damage on the bore. If you're lucky its a broken ring and all you need to do is drop the motor and get that head off and put a new ring in.

Still a lot of work but it won't be a huge bill if you do it yourself.

Sorry to hear, please, for my knowledge, what was the compression across cylinders?

Phil

I didn't run a compression test this time. I occasionally run one, but I think the leak down test provides more information.

That's an idea. I do have a borescope. A scored cylinder would strongly suggest a broken ring and would be enough reason by itself to tear down the engine.

I do my own work, so labor is cheap and parts would be minimal (assuming piston and cylinder are ok). Besides, it's been 6 months since I had the engine out. ;)

I dug out the borescope and took a look inside #1. It's hard to get a good look at the cylinder walls, but I could see the piston top clearly. Cylinder wall appears ok, but I see a couple of things that are suspicious.

This is actually a pretty good look at the cylinder wall. The cross-hatching is still visible, but no scoring.
http://forums.pelicanparts.com/uploa...1658517997.jpg

That shiny area is suspicious. Isn't that clean place where the carbon deposits are blown away an indication of detonation? At least that's what I think I read here somewhere. Any experts out there who can identify signs of detonation?
http://forums.pelicanparts.com/uploa...1658517997.jpg

What's that imprint in the piston top (midway out from center at 3:00)? Did an errant piece of a broken ring get driven into the piston top?
http://forums.pelicanparts.com/uploa...1658517997.jpg

Close up of the suspicious area.
http://forums.pelicanparts.com/uploa...1658517997.jpg

Steve, I don’t believe you old find pieces of a broken ring in combustion area
Broken rings because ring lands are worn
If you need a piston, let me know.
Bruce

As you suggest, can't hurt to warm it up and do another leakdown before deciding the course of action. But sounds like a new piston or cylinder may be in your future.

Of course a ring fragment couldn't get past an intact piston. What was I thinking? I'll probably need that piston. If the ring lands weren't already out of spec, they surely are now. Thanks for responding.

Yep, I think you're right. :(


So after I do the the perfunctory warm up and recheck, I'll drop the engine and pull #1. Here's the game plan:

• drop engine only (I've left the transmission in place a couple of times and I have a car lift and engine lift that give me good control of the engine angle when re-installing)
• release the left side cam drive (I'll measure cam timing on both sides first, just for grins)
• pull the left side heads and cam tower intact (take the manifolds off at the air box?)
• remove #1 P&C
• confirm broken ring, measure piston lands
• install new rings, piston, cylinder gaskets; reinstall head assembly
• install cam drive and set left cam (maybe right, too)
• check valve timing (why not?)
• check for leaks (oil, vacuum, fuel, electrons) while it's out
• put it all back

What have I missed?

per·func·to·ry
/pərˈfəNG(k)t(ə)rē/
adjective
(of an action or gesture) carried out with a minimum of effort or reflection.
"he gave a perfunctory nod"


So this time the warm up and retest worked. I drove a few miles to get the oil warm (pretty easy when you're in Houston) and repeated the leak down test. This time it was 2.5% :D

I knew the test could read low on a cold engine, but this went from a massive leak to a very good number. Confused but relieved.

Thanks for the hand holding, guys. Now I'm ready for the next track event.

I have seen, before a sale inspection, using a 13 socket, air chisel and modified chisel point
To drive and shake carbon from the exhaust valves.
Most of the time it improves readings.
Bruce

I wonder if the rings had rotated to the point that they all lined up?

My leakdown tester is a little fiddly; the rubber o ring doesn't seal to the head well sometimes.. not sure if I need to get a new tester... or a stiffer o ring that doesnt deform as easily

A good result!

Yes, I've heard that bits of carbon can hold a valve open and give a false high leak down. I think John Walker even recommends not removing the plug until the piston is at TDC to avoid problems with carbon dislodged when removing the plug.

If the high leakage on #1 was audible at the intake or exhaust, I would have never posted a question without first doing a test drive and retesting. That it was flowing from the oil tank had me convinced there was a problem with the rings.


In general, I wouldn't rule out operator error, but in this case I could clearly hear (feel, even) the air flowing from the oil filler neck. I checked all cylinders in their firing order and came back to #1 to confirm the high number.

I normally use the hose from my compression tester to connect to the spark plug hole. When I did the first retest after driving the car I used another adapter I had made from some fittings and the bottom half of spark plug. That did not seal well at first and it was clear the leakage was from the plug hole. I went back to using the compression tester hose and got a good test. (maybe put the Stomski adapter on my Christmas list?)

Would lining up all the ring gaps give a high leak down? Probably? High enough? Don't know. What the test was saying is the quantity of air pushed through a 1/4" by 0.040" round orifice by a 30 PSI pressure drop (80 PSIG - 50 PSIG) is the same as the quantity of air pushed through the mystery gap by a 50 PSI pressure drop (50 PSIG - 0 PSIG). So it wouldn't take much of an opening. But like I said, I could feel the flow of air out the oil filler, so there was substantial flow.

Indeed, a great result. I suppose the moral is to not place too much stock in a single test on a cold motor that has been sitting.

Now I can work on my new motor instead of tearing down the current one. :)

Thanks again for the helpful comments.

Steve

The rings are constantly rotating. Setting the rings 120 degrees apart is just to ease the mind. After running, even shortly, the rings will move to where they are comfortable.
Next time you open a motor, notice where the rings are, they certainly are never 120 degrees spaced.
Bruce

ring stagger
 

Bruce is 100% correct.SmileWavy

This is good news and bad news, isn't it? Good that you got a good reading this time. But baffling that it was ring air loss and not from one of the valves. Once you've leaked an engine (with or, even simpler, without, the gauges) you get pretty confident in deducing where the leak is coming from. Especially a large leak. There is always some air leak noise coming from the sump via the oil fill tube, but you get to know that level of noise and factor that in. Intake valve noise could come out there too, though, couldn't it, on engines where the oil tank is vented to the middle of the rubber boot connecting the two sides of the intake system?

However, if you have the air filter off, you could compare just below the actual intake to the tank? Or block off the hose to the tank?

Walt, that's a good point about crankcase blowby being routed ultimately to the intake. I have to remember that when testing my street car, but on the track car I have the oil tank venting to a catch can rather than to the intake. As you well know, this is common on track cars to keep oil vapors and combustion products (and occasionally grenaded engine debris, DAMHIK) out of the intake air stream. I checked for intake valve leakage by listening at the pop-off valve.

Is there such a thing as a "stuck ring" that is compressed into the piston lands enough to lose contact with the cylinder wall? I'm in the process of running a bottle of Techron Concentrate through it (added to the fuel) to hopefully clean up the combustion chamber a bit. I'll get some borescope photos when it's had a chance to work.

Thanks for responding.

Steve