leak down test procedure

[tcslr]

Should the test be conducted 'hot' or 'cold'?  'Hot' definition being fairly close to operating temperature?

 

Is it advisable to adjust valves before test?

should one add oil ( thought is to 'seal' rings) to cylinder before test? if so, how much?

Should a compression test be done immediately before test? or after?

[joe_padavano]

Metal parts expand and clearances change as temps increase, so obviously a hot or warm test is closer to actual operating conditions. The problem is one of practicality - it can be difficult to remove plugs and conduct the test on a hot engine, and it starts cooling off as soon as you then it off.

 

Adding oil defeats the purpose of a leakdown test, as it can seal one of the very leak paths you are trying to test.

 

Compression test is independent of leak test, but again if you do a wet compression test you need to run the engine to burn off the oil before trying the leakdown test.

Edited September 6, 2017 by joe_padavano
F'n autocorrect (see edit history)

[JACK M]

I do them cold.

Listen thru the breather, exhaust and carb. I use an old doctors stethoscope.

[Spinneyhill]

Would one do this at a pressure approximating the expected compression pressure? i.e. does the test pressure affect the result much?

[Bloo]

21 hours ago, tcslr said:

Should the test be conducted 'hot' or 'cold'?  'Hot' definition being fairly close to operating temperature

Generally it wouldn't matter. It would affect the test (less leakage hot), but usually a leakdown test tells you the difference between bad rings (who cares, just keep it full of clean oil and don't forget to change it) and other serious mechanical defects that mean you need to stop driving and fix it right now (burned valves, blown head gasket, hole in piston, etc.).

 

21 hours ago, tcslr said:

Is it advisable to adjust valves before test?

If you think there is a possibility there are any that are too tight, then yes.

 

21 hours ago, tcslr said:

should one add oil ( thought is to 'seal' rings) to cylinder before test? if so, how much?

If you are really interested in nitpicking how worn the rings are, then yes. About one squirt from an oil can. I usually don't.

 

21 hours ago, tcslr said:

Should a compression test be done immediately before test? or after?

Leakdown tells you more but requires every cylinder to be put on TDC. Compression is easier. If you are doing leakdown, the car probably runs horrible, and already failed a compression test. Now you want to find out why.

 

6 hours ago, JACK M said:

 

Listen thru the breather, exhaust and carb. I use an old doctors stethoscope.

^^This!

 

Also look for bubbles in the coolant if you suspect a blown headgasket or cracks. Where the air comes from tells you what is bad. Rings always leak some (listen at breather), even when new. Valves (listen at carb, exhaust) shouldn't at all.

 

A car that just sat for years might need to be driven some before you get accurate results.

 

 

Edited September 6, 2017 by Bloo
.. (see edit history)

[tcslr]

Mr Bloo, 

Thanks!  I had done a compression test and got 55/60/50/60/60/60.  I was told I should do a leak-down test.  Clearly I hear leakage past the rings and on almost all cylinders through the intake.   Exhaust are tight as is head gasket.

 The car seems to run fine except that - and here is the tough part - it SEEMS to not have as much power as say two-three years ago.  This engine ( 249ci Chrysler) was rebuilt before I got the car - and by a fellow who was a very good mechanic - but that was maybe 12 years past.  In the time I have owned the car ( seven years), I have driven it maybe 10K.  before that, it had maybe 500.

The car reportedly has a compression ration of 5.1 then 'perfect' is 75lbs?

My problem is 'what is the baseline'?

 

How does one determine if valves are burned?  Is there a test?

 

Tom

 

[Bloo]

Yes! The leakdown test.

 

When you have a burned valve, one cylinder will have extremely low compression compared to the rest. With those numbers its fine. The overall compression numbers ARE low, but that doesn't really mean anything. They are EVEN, and that is good!

 

When you do a compression test, just to pull numbers out of the air, on an engine where about 120 pounds is normal, an old engine with a burned valve is gonna do something like 120 110 105 30 120 100. The 30 is a problem. Nothing else there is. When you do a leakdown test on this hypothetical engine, it is gonna hiss out the exhaust on that 30 pound cylinder (probably). Most burned valves are exhaust valves. Burned intake valves are EXTREMELY unusual. On the other hand if it hisses real loud out the breather instead, there is probably a hole in the piston. If it blows bubbles in the radiator it has a blown head gasket (or a really big crack).

 

Has your car been sitting a while? Hissing out the intake indicates the intake valves leak. I would make sure they arent adjusted too tight and just drive it. Cars that sat for a while usually have some rust or carbon or something on the valve faces that screw up compression and leakdown readings.

 

Now to be sure, 55 is low. The trouble is, if you take 2 compression gauges you will get 2 different results. If the engine cranks slower because the battery is a little weak, the numbers will be lower. If you don't open the throttle (some people do, some don't). the numbers will be lower. If you use oil, the numbers will be higher.

 

Jumped valve timing will cause low compression on all cylinders. It should be running horrible if this is what happened. Low compression across the whole engine can also be worn rings. Engines with worn rings that have no other problems usually run fine. The power will be a little down, and they will dirty up the oil fast.

Edited September 6, 2017 by Bloo
.. (see edit history)

[Bloo]

3 hours ago, Spinneyhill said:

Would one do this at a pressure approximating the expected compression pressure? i.e. does the test pressure affect the result much?

 

I don't think expected compression is a factor. It has been a while I must admit. Basically the tester has a regulator and two (or maybe one) gauge(s). The readings are in percent. There is a measured (sort of) amount of air coming out the hose that represents 100% leakage. A plugged hose would be 0% leakage. The gauge reads what percentage of air is allowed to leak out of the cylinder.

 

If you are some guy who hones cylinders on a CK-10 with a torque plate while hot water runs through the block, and then file-fits his rings, you might get 3 percent. On a new engine normally... maybe 10 percent, to the best of my recollection. Many cars drive around with 35-40 percent or more.

 

Like compression, you can't expect any 2 gauges (or any two mechanics) to give exactly the same numbers. Racers may care about absolute numbers. For the rest of us, leakdown is about figuring out what is broken before taking the engine apart.

Edited September 6, 2017 by Bloo
.. (see edit history)

[tcslr]

Thanks! I haven't driven the car a lot this year ( unfortunately).  I do observe that it runs, feels, gets better with use.

I deeply appreciate your insight, explanation, patience.  It's easy to want to jump to bad conclusions.

 

Thanks again Mr Bloo.  Knowledge is only knowledge if it is shared.