Leak down v. compression test - PeachParts Mercedes-Benz Forum

Potomac Pat · #1 05-10-2002, 11:57 AM

I am basically trying to learn as much about these two different procedures to better my knowledge. What are the pro's & con's of performing a leak-down test v. a compression test. Thanks

#2 05-10-2002, 01:41 PM

If you perform a leak down test, you can more easily and accuartely determine if the problem is rings, valves, head gasket or crack related. That's because you can listen and tell where the leak is located. This is provided that you have a leak somewhere.

I never did a leakdown test because I had a compression guage and nothing else. Now that I have started doing the leakdown I find it more useful.

Good luck,

mbtjc · #3 05-10-2002, 01:58 PM

Can only a dealer do a leakdown test with special equipment? Also, if you put (I'm guessing compressed air) into the cylinders, couldn't that theoretically make a little problem worse by exerting all that pressure on let's say the rings or whatnot? I have no idea...that's why I'm asking!

Potomac Pat · #4 05-10-2002, 03:23 PM

We (PGA) have the equipment for both tests. I am just looking for a more descriptive summation of what the pro's & con's are. I know what specs are for a compression test, but what are good specs for a leakdown test? What are the advantages / disadvantages of each?

Cap'n Carageous · #5 05-10-2002, 03:56 PM

Compression is how much the cylinder can pump up; a good test of rings. Leakdown , which requires a special tool, measures the cylinder's ability to hold the pressure. A good test of all the components. As Larry said, if you hear air coming from the intake, then the intake valve is leaking. If it comes through the crankcase, then the rings are probably worn. You get the picture. From what I have learned on leakdown, 25% "total" is considered high, with 35% being maximum. Just my dime's worth and probably subject to "expert" rebuttal.

Billybob · #6 05-10-2002, 04:09 PM

A compression test is a measurement of the momentary peak pressure that comes to exist during the compression stroke of any given cylinder. It is simply an absolute measurment of how high the pressure gets, it does not indicate much else.

The leakdown test is a procedure that is used to determine how well/long the conditions, i.e. valves, rings, cylinder walls, pistons,head of a given cylinder can maintain a given pressure. A known pressure is applied to a cylinder at TDC and to loss of that pressure it measured over time - The faster the applied pressure is lost the worse the condition of the cylinder! Some diagnosis of potential cylinder component issues is possible when you listen for or observe the leakage of air pressure from the cylinder.

Leaking air heard at;

Intake manifold = intake valves

Exaust manifold/tail pipe = exhaust valves

Valve cover oil filler hole = valves, valveseats, valve stem guides, or valve stem seals

Oil dip stick = worn piston rings, cylinder walls

Coolant resivior (bubbling or hissing)= head gasket, cracked cylinder liner

Leakage at the valve cover alone usually indicates top end /valve problems.

Leakage at the valve cover and dip stick usually means piston ring/cylinder wall problems.

Head gasket/cylinder liner/cracked head problems can leak air pressure into either or both the oil circuit or coolant circuit.

Both tests used together is the best way to go, a compression test will determine if the minimum threshold conditions are met, i.e. is there enough compression meeting the spec for each and between all cylinders? And a leakdown (loss of pressure over time) test of the cylinders will determine how well the compression values are maintained by the components of each cylinder.

If you have a diesel compression tester you can use the injector/glowplug adapter, a check valve, open/close ball valve, and a couple of gages and a tire inflator fitting to make your own leakdown tester.

assemble the parts in this order:

Tire inflator fitting - T - ball valve - check valve - T - adapter to the cylinder, at each T attatch a presure gage (use gages that have a full scale twice the pressure you will be using for better accuracy).

To use the compression tester adapter you only need to remove the tire stem valve from the adapter that is usually functions as a check valve allowing pressure from the cylinder to the gage because in the leakdown test you will be appling pressure from the gage array to the cylinder.

Attach the adapter to the TDC cylinder - open the ball valve - apply shop air to the tester - when both gages read the same close the ball valve (this will hold the first gage at the applied pressure reading) - observe the rate at which the second gage drops over time (this second gage reads the pressure in the cylinder)

P. S. It may be wise to secure the crank in position to prevent engine rotation when air pressure is applied to a cylinder, in theory the applied pressure could force the engine to turn. Of course the higher the pressure the more likely that this could happen.

On another note, one could assume that using the highest level of air pressure up to the compression pressure measurement level would be safe and would give the most realistic/accurate measure of the engine condition.

sixto · #7 05-10-2002, 04:23 PM

I've seen leakdown testers for under $100 so it's not that much of an investment if you already have an air compressor.

I think leakdown tests are done at 100psi or so. Nowhere near the 150-180psi a healthy gasoline engine imposes on itself or the 400psi or so a healthy Diesel engine imposes on itself.

My amateur opinion is that you perform a compression test to get a general feel for the engine. If compression checks out (all cylinders are strong and little variation among cylinders) then there's little reason to do a leakdown test. If compression is below spec or on the low side of spec on any cylinder, a leakdown test will help you figure out what's going on (rings, valves, head gasket).

I don't know if a formal leakdown test tells you more than an informal leakdown test in which you pump air into the cylinder and use your eyes and ears to figure out where it's leaking. AFAIK, a formal leakdown test adds only a number such as 5% leakdown. I don't know what to do with that number. No suggestions, please

Sixto
91 300SE
87 300SDL
81 300SD

Potomac Pat · #8 05-10-2002, 04:34 PM

Thanks guys for your input in regarding this post. I am always trying to better my knowledge about Mercedes in general from inside & out. This information will help me gain a better perspective about how each test can be beneficial to someone in my trade. Thanks again.
Pat
Potomac German

Speedtek · #9 05-11-2002, 03:09 AM

We leak down every mortor before adding more performance goodies to it. It can save a lot of headaches. I found a Rabbit GTi engine that was going to be turbocharged missing the TOP compression ring from the factory! The leakdown was 20% with only 10k miles! all other cylinders were 5%. Leakdown tests have saved me a lot of time more than I can remember.

When I was working with the AMG race team they performed a boroscope test and a leakdown between the sprints and after the race to keep tabs on the engine. This was to ensure the motor was able to run at optimum levels before replacement 6% was the limit.

Compression Rings Performance Guide.
BASED ON 100 PSI:
3% or less (Excellent, USUALLY Only found with motors with TOTAL SEAL Rings)
3-6% Excellent
5-9% Average engines (OK for TURBO, SUPERCHARGER)
10-12% (Borderline for TURBO, SUPERCHARGER. Don't expect FULL potential)

When a leakdown test is used in conjunction with a compression test this can tell the engineer how the engine is when it is running. If a leakdown in all cylinders is the same and the compression test in all cylinders is the same then it is a pretty good chance that all cylinders are running evenly. If the compression is weak in a cylinder than it may be time to adjust the valves. So to me using both test go hand in hand.