Is This True about A Compression/Leak Down Test?
 

I'm looking at a 91 964 Turbo and called a well known shop with 40 yrs experience to get a PPI done. When I asked about the compression/ leak down I was told on these 964/Turbo cars it wont be accurate, the reading will be all over the place. He also told me Bruce Anderson had a write up on this in Excellent magazine.

So where does this leave me? How do I know the engine is in good shape?

that's ridiculous. In three words, find another shop.

Isn't the 965 motor pretty much the same 3.3 liter as in the earlier cars? I'd like to hear the wrench's explanation why compression and leakdown in a 965 motor would be any less accurate than in any other Porsche motor. (Maybe it'll teach me something.)

I think the bigger question is what numbers are we supposed to be getting for compression? I've never seen anyone that from any official source.

Interesting, I would like to know why or what his reason for the statement was.
Just curiously. Now, I am not accusing the man of anything but I will say that some of the most unpleasent job's to be carried out via such testing are on Rear or Mid engined cars.
But I can't imagine someone stating such facts.

1. Do you have a history with this shop, I mean are you a long time client.
2. If so and you are a long term client then you should trust the shops word so long
as they have not wronged you in the past.
3. And not to be a jerk, but make sure you did not miss qoute or that you did not have a miss understanding or take a statement out of context. Sometimes customers asking something and they hear a portion of a statement and run with it as soul fact and cause themself a panic attack. ^_^

Hello there, As a shop owner , I have performed many PPI's , And 90 percent of the time a leakdown test is requested, It can be used to verify the amount of air lost (expressed as apercentage) and of course it can tell us which components are suspect, (rings, valves etc.) ...On a healthy engine the numbers will be close to each other, That being said , in evaluating a porsche for my own purchase I can remember only doing a leak-down once , after driving that particular car it was ovious that there was a problem and this test was used to verify my suspicion. In six other ocasions I purchased without doing a leakdown test and had no regrets.

Of course I will provide the customer with all the relevant info and help them select the procedures to be used,

Two things I always insist in verifying: 1- pull rocker covers ,inspect studs
2-Drain oil, Insp. magnets, cut oil filter
inspect element
Going slightly off topic, Turn the ignition switch on and verify that all the warning lights are in working order, (specifically : ABS, AIR BAG, CH ENGINE)

Over the years we have found cars with missing bulbs,
Also if the car still has the dual mass flywheel check the clutch operation,

Hope everything goes well, 965's are a great car!

Lou / AERO DYNAMICS

I can offer up a few opinions about leakdown/compression tests and well known/respected p-car shops with years of experience based on my personal experiences with both of them.

I think the shop's statement that the "numbers will be all over the place" is BS. This shop simply doesn't want to do the job. This shop sounds a bit lazy to me, maybe they don't need the billable labor hours. These tests are not a cakewalk on a turbo porsche, but they aren't very difficult either. I've performed both compression and leakdown tests myself on my 930. The leakdown probably took me 2-3 hours in total, and that was the first time I've ever done a leakdown test on any vehicle. A shop with 40 years experience should be able to do it in half the time.

These tests are certainly one of the best ways to assess the health of the engine, but I think they will only tell you so much. Good numbers are certainly no guarantee that the engine does not have some real issues. Perhaps the shop's BS answer is really a way to dodge this issue: if they give you a test with "decent" leakdown numbers and you buy the car but later find a problem, you may come after them.

Bad numbers, or more importantly significant variances between cylinders are probably a sign of something very serious and obvious. You should have them done to rule out the obvious, but realize that they are not a guarantee. I think that many people have elevated "the car passes a PPI with leakdown with flying colors" to be the equivalent of a certified pre-owned vehicle.

My personal experience with leakdown/compression testing on a 3.3 liter turbo:

44% leakdown on #6. That's a number that would have most people run away screaming Look closely and you can see how large the top ring land is...this piston got very hot and melted a bit, deforming the crown and enlarging the top ring land. The 2nd ring land was compressed and the ring would not rotate properly.
http://asbury.smugmug.com/By-Land/Po...9_YcnNX-XL.jpg


10% leakdown on #5. This was the 2nd worst leakdown number of the bunch, but its perhaps not a number that everyone would say "run away"
http://asbury.smugmug.com/By-Land/Po...0_ZUfTH-XL.jpg

%6 leakdown on #2...3rd worst of the bunch but the most obviously damaged ring. %6 is not a such bad number and was actually very close to the "good cylinders"
http://asbury.smugmug.com/By-Land/Po...0_pgjnf-XL.jpg

All other cylinders had intact rings and had 2%-4% leakdown numbers which I believe would be "flying colors" territory.

Incidentally, a compression test I performed about 1 year prior to the leakdown showed 135-140 psi on cylinders 1-5 while 6 showed 120psi. So I knew something was up with #6, but the problems with #2 and #5 were not obvious to me with either of these tests.

I should also mention that this car had no visible smoking and ran very strong prior to the teardown. I can't imagine that anyone who test drove it would conclude there was anything wrong. Oil consumption was maybe the only telltale, but wasn't too bad for long trips on the highway (when mostly off boost) and well within "normal" limits.

Good luck with the search

Great post, Jacob. Thanks for scaring all of us even more. ;)

What Lou said....... He is right.

Cheers

Probably the case...
 

Prior to buying Das Babe, one of the top instructor, racer and machine shop owner (guess who is paying his racing addiction :D) at Mont-Tremblant did mentioned the same thing... A lot of shop don't want to do them or down know how to do them properly.

I witness the compression test (161-169 Hi/Low; 5% Variation) and had the plugs replaced (my first while in there :D;):D and the car was not even mine yet)

So it is true a leakdown/compression test is not the final word on an engines health. In fact it could give false security to the potential owner.

I believe the shop that made that statement was conveying exactly that.

for the record i'm not saying don't have it done, just don't take the results as gospel

Thanks for all the info guys. I've never used the shop, I called them because the owner would be in that area for business and he offered to take it to them. He too has never used them, the shop was recommend by someone he knew.

I really like the car, overall its very nice, only thing I noticed was some oil under the engine and it had a small pull to the right.

It has 73K miles on it but looking at it you would never know.

hahaha, you just descibed every 930 on the planet ;)

If the car pulls to the right be careful.. most do not pull to either side.
Good chance it's not uneven tire wear or internal belt seperation but may have been in an accident and the chassis or suspension is tweaked or bent, even if you see no cosmetic signs of it.
If there is uneven tire tread wear ask why.

Most of them leak or sweat oil in more than one place after a bunch of years and miles but not all of them.

This will not be my 1st Turbo Porsche. My 930 had a small oil leak and it never bothered me. This one runs aftermarket wheels & tires and I really didn't drive it long enough but I think it was the crown in the road. All the tires looked great, even tread all around.

The car gets dropped off Monday night for the PPI on Tuesday and I've told them to do the compression and leak down, look into the oil leak and alignment.

Here's my old 930

http://i414.photobucket.com/albums/p...g?t=1281562872

BMAN, good luck with the PPI. I remember when you were selling your old 930 above... loved the look of it.

Thanks Garrison... I sure hope it goes well, I really want this car :D

It seems that so many people sell these cars only to turn right around and buy another one. What is the story this time?

No story just stupidity for selling it. But its ok, if I get this one all is good :)

Since this thread will be here a while and others will be buying these cars...
 

Just a comment. No one addressed BA's comments about leak down tests. He wrote about this more than once. Mainly, it is easy to have something very small screw up a leak down test and make something think an engine has big problems. He experienced more than one where an engine ended up being pulled and taken apart... for no good reason. Small example, you remove the plugs and in the process of turning over a very small bit of carbon came off the plug. It gets somewhere....and.... Do you see where this is going?

You will also notice that the engine shown above showed a problem with a simple and much less expensive compression test.. a year prior to all this.

I will suggest the following to everyone. A lot of people are going overboard. Do you really want to pay the addition shop fees for a leak down? Frankly, I have seen a great compression and leakdown on a 930 that had several broken rings! In fact, a great many of these good running, non oil comsuming, non smoking.. .fine running 930s have broken rings! Why? People like ... boost. These engines take the beatings enthusiasts give them and keep on going. They only go bad when they are really hammered or the owner is absent-minded while on boost. I have seen a bunch of these engines ruined when the owners ignored the overboost had rusted shut! They sure enjoyed the rocket ride while it lasted... but those were awfully expensive rides.

I suggest common sense. A good inspection with a compression test and throughly inspecting for signs of neglect and abuse. If it passes these with flying colors, you are wasting money including leak down. You want leak down? Fine. It's your money.

911s since 1968.
930s since 1985.
JR

I took the OP to mean that the shop was not interested in doing either test--not just the leakdown.

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

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

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
http://forums.pelicanparts.com/uploa...1281753182.jpg

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

Words fail me.

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

Leakdown tester, $15 in parts:
http://forums.pelicanparts.com/uploa...1281826785.jpg

#6 piston ring failure from PO overboost, blow-by diagnosed by leakdown tester:
http://forums.pelicanparts.com/uploa...1281826874.jpg

Broken rings on #5 cylinder and #6 cylinder:
http://forums.pelicanparts.com/uploa...1281826937.jpg

New rings, gaskets, seals - priceless:
http://forums.pelicanparts.com/uploa...1281826984.jpg

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

Way to go...
 

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.