Leakdown test - what is its purpose

Gentleman, I understand the concept of a compression test, but I do not know what a leakdown is. What is the purpose of this? I did read somewhere that if a compression test produced positive results then there would be no need for a leakdown - is that true?

thanks in advance

 

The leakdown test not only tells you if there is a problem, it can be used to isolate the problem.

If enough air is leaking in any one cylinder to measure on the test gauge, then there will be enough air that you can hear where the leak is. These locations are: a: in the pan->bad rings or cylinder walls, b: out the intake-> intake valves leak, c: out the exhaust-> exhaust valves leaking, d: bubbles in the water->head gasket.

 

On this subject, this is totally my opinion and i dont care if no one else shares it either.
But in the 9-10 years of me owning my F355 and with near 100K miles on it, 70k of them are my miles. I have never had a leakdown test ever. Why? because IMO if you go looking for trouble and worry you will find figures that are not ideal as per text book. I am not a Ferrari hypercondriac
My car pulls as strong as it ever did, does not smoke, does not loose hardly any oil in between oi changes and sounds as beautiful as the day i bought it.
The way i think about it is that many MAY have done leak down tests and unnecessarily have taken the engine apart to fix a problem that MAY not even be there in the real world of running the car on the road.
I am in no doubt that if my car had a leak down test, chances are it would not meet the text book figures. But i have put over 70K miles on it my self without a days serious problem, and i plan on another 70. I am not a ferrari hypercondriac
That should upset a few people no doubt

 

A leak down test is the ultimate test of the cylinders condition. Its ability to seal and hold air pressure is directly related to how much power it will make. As pointed out it is also an exellent diagnostic tool.

 

I agree that unless a car is symptomatic there is little reason to do a compression or leak down test on a car you already own and know the history of. That obviously does not applty to a car you might be considering for purchase. I disagree that there is a significant number of Ferrari hypocondriacs out there. In my 30+ years of experience in the Ferrari business, with few exceptions, leak down and compression tests are rarely done on cars that are not being considered for purchase or have some nature of running problem. In either of those two instances anyone who did not conduct those tests is just plain stupid.

 

You could not possibly disagree that there is significant number of Ferrari Hypochondriacs out there, from what i quoted. Because i did not say anyone else was a Hypochondriac, i said "I was not a Ferrari hypochondriac".

 

OMG!

How many miles do you have on your F355...?

 

Thanks Mitch and Brian.

 

I'll guess and say nearly 100k

 

How the hell did you know that???

 

Nice...

 

Bought mine just with a test drive and look over.
never bought a car any other way. reckon that puts me in the stupid camp, but
Its as strong as the other 355s i compete against. and i love it!

come on you can tell if a car engine is on way out or not!

 

Stupid? Then thats you and me both in that case. I bought mine from a Ferrari dealer and only took a test drive my self. I dont see many other 355s with the same reliable mileage i have on mine.

 

I've heard of cases where the leak was 3% or less on 7 cylinders, but as high as 8-10% on only one. What would cause this scenario, versus say 8% leak across the board on all 8?

 

What does the percentage mean when leakdown results are discussed? I know how to do a leakdown, and how to figure out where the compressed air is escaping, but I don't understand the percentage. Can someone enlighten me?

 

100,000km's is exactly 62,500 Miles.

Theres a guy here in Toronto with a yellow thats got something like 100,000km's on his as well.

 

I am very happy for the guy in Toronto,good for him
BUT i will say this again slowly just for you.
I.....have....100,000....MILES....on...my....F355, not kilometers 100,000 miles = 160.934KM
Does that help clear up the misunderstanding?

 

+1

 

Does anyone have answers to these questions?...

 

If you have higher leakdown on one cylinder it could be due to many things. Possibly valves leaking on that cylinder or just not as good of a ring seal on that cylinder. I've also seen times where it initially looked like one cylinder had poor leakdown but quickly disconnecting/reconnecting the tester a few times shocked the cylinder enough to square up the rings in the cylinder and bring them in line with the rest. (testing with 100psi is a tiny amount of pressure compared to what the cylinder sees in a dynamic state so sometimes it isn't enough pressure to square up the piston/rings in the bore if it's slightly canted to start with).

As far as percentage it is a comparative measure of the test pressure vs how much pressure the cylinder is holding. 1st gauge is test PSI, usually 100psi second gauge is connected to the cylinder and is usually read in percentage. If the second gauge was also scaled in PSI than 100psi would equal zero percent leakdown, 90psi would equal 10% leakdown and so forth.

 

Assuming that all the cylinders are within a couple of percent of each other:
less than 1%-2% the engine is too tight
between 3%-5% the engine is as good as it can be
between 5%-9% the engine is in the prime of life
between 9%-18% the engine is on the down hill leg
more than 20% you should consider gettting the engine refreshed
more than 25% and you will want to get the engine refreshed
more than 30% and you will have to get the engine refreshed

 

1-2 % ? Then all Ferrari engines when they are fresh are too tight?

 

Thanks for the interpretation of the results, but the nature of my question is how the percentage is determined. 3%-5% of what?

OK, but how is the 90psi determined in this example? Is that the psi the cylinder holds before it starts losing pressure? And if this ratio is always determined by a comparison with cylinder #1, then why are leakdown percentages never expressed in the negative; such as when #1 is 90, and number #2 is 100, in which case cylinder two would be -10%?

 

No, it's gauge 1 vs gauge 2, not always compared to cyl 1. Before the tester is connected to any cylinder you connect it to an air source and verify 100psi shows zero percent leakdown on the second gauge (which can be done with the attached regulator).

Now when you attach it to any cylinder you can see the leakdown. If it is connected to the cylinder and the gauge previously set to 100psi is now only showing 90psi than the cylinder is leaking down 10%.

 

That's helpful. So the percentage is the net difference of each cylinder when compared to the best cylinder? ... with the highest psi being 0% leakdown and the lowest psi having the highest percentage of change? Is the psi, as measured at the gauge, the most that cylinder can hold before it starts leaking (ie: 90psi)?