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Manassas, VA
 
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I built my own leakdown tester, the instructions were on the web. I had margninal compression on two cylinders on the same side (passenger). I performed a leakdown test and it confirmed I had broken rings because I could hear air escaping inside the engine and not into the exhaust or the intake like bad valves would cause.

If you shoot some oil into the spark plug hole and crank the engine over enough, you will almost always get some pretty good compression. The key is that the numbers don't need to be high, just consistent. My turbo with 7.5:1 will not generate as much compression as the 11.5:1 RSR (just guessing at the numbers).

So when you get readings like this 1-120#, 2-125#, 3-120#, 4-110#, 5-120#, 6-90# you know you have a problem, so you break out the leakdown tester. Do you test all six cylinders, no! Why test something that is good. I tested cylinder 4 and it only went to 70psi, then I tested cylinder 6 and it would not hold more than 10psi (from a 100psi source). When I heard the air flowing in the engine, I knew it was bad rings. I slapped in some new rings and the compression test results were even numbers across the board.

Mark
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1991 964 Polar Silver Metallic Turbo Coupe
Old 08-12-2010, 04:41 PM
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Location: S.F. North Bay
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Hi Compression like a 2.8 RSR engine gets you up around 200psi. Not for street gas!

And I agree that using a leak down to better define a problem is valid and a LOT better thinking than just blindly doing a test. Tho, if you have low compression on 2 adjacent cylinders, you already KNOW you got a leak between the 2 [911/930 owners can ignore all this and should understand why I am saying this].

Remember, these devices were designed for pros to help them estimate how much and for exactly what.. they were gonna lighten customer's wallets. Prior to this it was a matter of holding a length of hose in the intake or exhaust, or inside the oil filler and listening for the hiss.. Not as trick, and you didn't get a nice percentage [to inform a breathless customer], but you KNEW what was going on.

It wasn't until the 90s that commoners started discussing this subject because almost no one did it and few people understood it. It was very handy for racers to determine how long before the big boom might be expected. Either way, if your engine is proper adjusted and you have significantly low compression on one or more cylinder... you got..a compression leak and that means something is old, broken, worn and gonna make you take your toy apart. Is it important to do a leak down to say.. OH it's a valve vs OH it's rings?

I hope you guys know I am having some fun with this..tho I am entirely serious at the same time. I say, if you don't have a leak down kit, don't waste your money. Someone with a functioning brain and a compression gauge is just as well off. My leak down kit is still lovey because it just..sits.. in it's nice box in the tool chest. Some day I will probably sell it to someone who wants to dazzle someone with facts.. or thinks he needs it!

JR
Old 08-12-2010, 10:37 PM
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Define "functioning brain"...

The fact that you have a leakdown tester is comical after reading your post. Your post speaks to qualitative results "a compression leak..." Some of us with a functioning brain may want quantitative results, for example HOW MUCH is leaking past the rings or the valves. Your compression test will not tell you that.

Why? because if you have a compression leak and leakdown shows it is 10% then you can continue to drive the car with the knowledge that something is worn and not failed. Some people will tap the valves with a small hammer after a 10% leakdown shows which valve is leaking because you can hear the air flow upward (intake) or downward (exhaust) and the leakdown drops to 6-8%. That is some good quantitative results, right there.

My car was running fine, just using quite a lot of oil. The excess oil in the cylinder (see the picture below) kept the compression up. The leakdown indicated 90% of my supply air was going past the broken rings. The big difference is that the compression test is a dynamic test (the engine is turning over) but the leakdown is a static test (the engine is at TDC to with the valves closed or at some point along the piston travel to check cylinder wall integrity).

Mark
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Old 08-13-2010, 06:33 PM
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If you need a number to tell that thing has internal problems and needs to be taken apart... well, great. If you have the time to blow and need those figures, so be it. To ME, if it's broke.. it's broke and I have no need for useless-to-ME-numbers explaining precisely how badly ruined it is.

I mean, let's be serious for a moment. Look at the images of that engine! You can actually suggest that you NEED or even want to do a leak down test? Simply pulling the spark plug would provide all the clarity I would need to know that cylinder is oiling like a stuck pig! If you were driving it like that and you aren't running it out of oil,.. you really don't notice that you are having to add a ..lot.. of oil? Really? We old guys would call this common sense. In terms of determining exactly what is wrong, me thinks that pulling it apart down there and observing which parts are ... falling off the engine rather than requiring disassembly... would be far more specific than any leak down. Oh, you want percentage with all that burned up sticky stuff too? Ok ok ok.. I know.. This is a... technical site..and this is a very technical engine.. ok. So we get the numbers to satisfy a deep inner need to verify precisely what extend the poor thing has been run down to.

Sorry, I don't buy that as a reasonable excuse to need a leak down. As for why I bought one of these silly tools in the first place, it came with a bunch of other...useful.. tools I was buying so there it is. The only genuine useful need for these or using them is when you are racing and trying to get that absolute last hp out of a given engine and you want to know when to replace slightly worn components just as their optimum ability to produce maximum power. I no longer race. It sits. Just for fun. When you get it all back together and correct, break it in.. then run compression and another leak down. It may surprise you.
JR
Old 08-13-2010, 11:50 PM
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Words fail me.
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Old 08-14-2010, 12:17 PM
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Errrrrr, so did that engine.

Sorry, but I do believe that the images make my point that a Leakdown Test was a waste of time.

Best words fail us both at this point as you obviously love the tests.Start pouring some love, instead of oil, into that poor 911 engine and this will have a happy result.

;->

J
Old 08-14-2010, 12:37 PM
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Leakdown tester, $15 in parts:


#6 piston ring failure from PO overboost, blow-by diagnosed by leakdown tester:


Broken rings on #5 cylinder and #6 cylinder:


New rings, gaskets, seals - priceless:


The car has been running great since this I did this rebuild back in July, 2008. The engine went from using a quart every 500 miles to maybe a cup in 1000 miles. The plugs (Bosch WR6 DP0) looked good then because the synthetic oil doesn't burn like regular dino oil. The plugs are a perfect tan color now. Never thought to crank the engine over with the plugs out, it would have spit oil, but I had no idea there was any oil in the cylinder anyway.

In hindsight, the oil in the cylinder confirmed the compression and leakdown tests I had done the week before I opened up the engine. It was amazing how well the car ran with about 2 tablespoons of oil in the #6 cylinder.

Other than rings, nothing fell off the engine or suffered any damage. It is easy to look at pictures and say, "Of course, look at what is wrong there." The higher skill is to diagnose what is wrong before the engine is disassembled, buy the parts ahead of time to have on hand, and get the car back on the road in the shortest possible time. We old guys would call this intelligence. BTW, this car is a daily driver, not a garage queen. Today, compression test on all cylinders within 10psi, leakdown on all cylinders 5% or less.

One question, have you ever actually used a leakdown tester?

Mark
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Last edited by lucittm; 08-14-2010 at 03:36 PM..
Old 08-14-2010, 03:32 PM
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83 911 Production Cab #10
 
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Thumbs up Way to go...

Quote:
Originally Posted by lucittm View Post
... The car has been running great since this I did this rebuild back in July, 2008...
Good for you Mark. Nothing like doing it yourself and getting great result.

I'm rather new to the Porsche world (45 years for the dream coming true), just came back from our PCA chapter monthly run which took us to Mont-Tremblant race track (use to be the F1 Canadian Grand Prix Track) and Das Babe perform very well when you consider her age (1983).

I hope (maybe wishful thinking) that she will never require a rebuild but if she does, between Wayne book and you guys, I will do it myself and will not cut corners...

J.J.
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83 911 Production Cab #10, Slightly Modified: Unslanted, 3.2, PMO EFI, TECgt, CE 911 CAM Sync / Pulley / Wires, SSI, Dansk Sport 2/2, 17" Euromeister, CKO GT3 Seats, Going SOK Super Charger

Last edited by JJ 911SC; 08-14-2010 at 05:10 PM..
Old 08-14-2010, 05:06 PM
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