Leakdown Test Tips

Leakdown Test Tips

In the aviation world, we run the engines to get them hot (only accurate way for a leakdown) and put nothing in the cylinders to alter the test (does your 928 normally run with a tablespoon of oil in the cylinders?).

At TDC, apply air pressure via a differential pressure tester and note the reading. I'm not sure what "normal" leakage is for a 928, but in my experiences you should be maintaining somewhere in the area of 90% of what you put in for a nice "tight" cylinder. More importantly, all cylinders should be within 5% of each other.

When rotating a cylinder to TDC, don't go past TDC and then back. If the rings are not seated against the bottom of the lands, your readings will be low. It was common during aircraft tests to rotate the piston slightly back from TDC (with air pressure applied) and then back to TDC to ensure proper seating. Often times this would "fix" what initially appeared to be a leaking cylinder.

If you have excessive leakage, use your aural indicator (ear) to locate the cause. Pop the oil cap off and listen for a hiss (blowby from rings or worse), next listen at the exhaust pipe (exhaust valve) and finally at the intake with the butterfly open (intake valve). This will isolate your problem and direct you to the proper corrective action.

Best of luck,
86.5 Indischrot 5-speed

The information I have regarding leak down for automotive engines indicates the following:

3% to 5% - good racing engine
5% to 7% - good street engine
10% - weak
20% - shot

Michael Howard

>> I would also perform a leak-down test to see if any valves
>> are not closing. An open valve is either burnt or bent/broken.
>> The leak down is like a compression test but more detailed
>> by forcing compressed air into each cylinder and seeing if
>> the cylinder holds pressure. You do not need to crank oven
>> the engine to perform the test.
> Just wanted to make sure that readers are clear on some of
> the procedure. Each cylinder that will be tested needs to be
> brought just to its own TDC on the compression stroke. To do
> that, the timing belt needs to be installed and tensioned
> correctly before the test. So, in the "checking for disaster"
> scenario this post was referring to, it could be done before
> the covers and accessory drives were put back on. Ed's comment
> that you don't need to crank over the engine is true but might
> be misleading-- You won't need to "crank" the engine with the
> starter, but you will need to be able to roll the engine over
> to get each piston in the proper position for testing.
> dr bob

> can you explain to me why you have to have the pistons at TDC for a
> leakdown test????
> I understood such a test was to test pressure loss on a per cylinder basis.
> That the factor was how well any cylinder held the pressure not how much
> pressure in what volume.
> I also understand that in order to perform a leak down test it is a
> requirement to ensure there is sufficient oil on the cylinder walls prior
> to testing, the recomendation as I understand is to actually put engine oil
> in through the spark plug hole and crank prior to undertaking the test
> Am I misinformed?
> Thanks
> Robin


The simplest reason for doing the test at TDC on that cylinder's compression stroke is that both valves are closed. Keep in mind also that you are applying 75-100psi of air pressure to the cylinder, so having the engine off of TDC means that the crank will be turned by the air. That crank holder is handy when you are doing the timing belt, so this may not be quite so important.

Adding oil to the test is not normally recommended-- that's a step in the compression test, where you can sometimes isolate piston ring wear by letting the oil seal the rings somewhat during a second compression test.

Another respondent reminds us correctly that the leakdown test is normally done on a warm engine. This is particularly important on air-cooled engines where normal cylinder clearances are large when the engine is cold. In the situation we started with, this is a macro test used to identify damage to the valves from the piston, so temperature is not really that important.

After writing the first response, I've looked around to see what's really included in a leakdown test. My early understanding was that you measure the time it takes for a certain amount of pressure drop, with the leakage percentage calculated from that. Drawbacks are that you need to know thevolume of the chamber and the deck clearances to get anything close to a valid number. How do you relate time to a leakdown percentage?

Another description says that the test is done by measuring airflow through a precision orifice, watching the pressure upstream and downstream and calculating the flow from that. Any leakage causes flow, so a lower downstream pressure equals more leakage. But-- How do you relate flow toleakage on a percentage basis? So, engine diagnostic guru's, what is the true method?

Also, I found a leakdown tester for about $300 at a website. with no real description given.

dr bob

My leakdown tester was made by Tavia. I bought it from Powerhouse Products of Memphis, TN
www.powerhouseproducts.com. The price was $76. You can probably buy one from Summit Racing or Jegs as well. It consists of two gauges, a pressure regulator, manifold, & hose. One gauge shows the pressure being applied, the other shows how much pressure the cylinder is holding. The difference between the two gauges is your leakdown.

Michael Howard

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