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The TTR250 FAQ web pages -> TTR250 -> Repairs and maintenance -> Leak down tester
Post Info TOPIC: Leak down tester
TTRfan


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Leak down tester
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Anyone have personal experience of using a leak down tester and, if so, does this one look as if it will do the business please? 

http://www.ebay.co.uk/itm/281699757535

Brian

This is what my book says:

 

Leak Down Testing Methods

Leak down testing is a more definitive way to assess the health of an engine compared to compression testing because a leak down test allows the mechanic to pinpoint the problematic area within the engine. Whether the valves are no longer sealing, rings are worn, or the head gasket is leaking - a leak down test can be used to find and diagnose all these potential issues. A leak down test works as follows. With the piston just shy of TDC and the valves closed (compression stroke), air pressurizes the cylinder to a defined pressure which is recorded by a pressure gauge. A second pressure gauge is used to monitor the amount of air escaping combustion chamber. A comparison is made between the air going into the cylinder and the air escaping. The percentage of air escaping is used to determine the overall health of the engine.

The amount of air escaping can roughly be quantified to assess the overall health of the engine. When race engines are built, the accuracy and precision that goes into the build results in the lowest leakage values. Most race engines will have a pressure loss of between 0% and 5%. Standard builds resulting in good running engines typically lose up to 15%. Any engine that is close to or past being ready for service will leak from 16% to 30%. These engines will most likely be running poorly if at all. Engines beyond 30% leakage are usually broken and will not run. The more the engine leaks, the worse the engine’s health. Keep in mind these values are provided as a reference point, however each engine can be a little different.

It is possible to pinpoint where leaks are coming from once the cylinder is pressurized. When performing the test, the throttle should be fully opened and the radiator cap should be removed. Air can exit the combustion chamber at four points: past the intake valves, past the exhaust valves, past the rings, or past the head gasket. Each of these four points will exhibit a unique tell-tale sign if air is leaking.

Intake valve leaks can be diagnosed by listening for air escaping out the carburettor or throttle body. Exhaust valve leaks can be found by listening for air escaping out the exhaust system. A leaking head gasket will result in air bubbles showing up at the radiator fill cap neck. Excessive leakage past the piston rings will result in pressurizing the crankcase and the resulting air can be traced out the crankcase or cylinder head breather hose. Air escaping past the rings may also be heard or felt passing through the access hole in the engine side cover where the wrench has been inserted to position the crankshaft. The location at which the air exits when it leaks past the piston rings will depend if the engine has a separate crankcase cavity or a joint cavity. Usually on engines with a separate cavity, the air will be routed through a one way valve which then directs the air up into the cylinder head and out the cylinder head breather. 



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alanwebbo


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I havent used that particular one, but leak down testing is a great diagnostic tool to give you some clues before you strip an engine. A simple way is to turn the regulator on your compressor right down (much more than 10 psi and you wont be able to hold the piston at tdc.) and use the hose from your compression tester to pressurise the cylinder. You will hear the blow past leakage by removing the oil filler plug . By Slowly rotating the engine you will hear when the valves open.

My only comment on the tool is that nothing is calibrated and you are working right at the bottom of the pressure gauge. The accuracy and repeatability of the results is debateable, but if you are just using it as a guide i dont think you will go far wrong for the money.



-- Edited by alanwebbo on Friday 30th of October 2015 10:27:10 AM



-- Edited by alanwebbo on Friday 30th of October 2015 10:28:11 AM

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heffh084


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Hi all!

I recently bought the same tester to diagnose my engine's oil burning condition.

It's a new 325 rebuild, it's cover about 2200 miles since rebuild.

 

I did a compression test cold (190psi) and then hot (207psi).

 

Cylinder leakage test was done cold and hot with very little difference between them.

What I noticed is that when the engine was at TDC on the compression stroke, I could hear air pressure leaking out of the oil filler, the LHS crank shaft access cap, AND the air intake snorkle. I suspected I'd hear the first two as I suspect my piston rings are damaged, but I was surprised by the intake snorkel noise as I had the valve guides, stem seals and valves lapped in when I rebuilt the engine.

 

Is there any other path for the air leak/noise coming out of the intake snorkel other than getting past a bad intake valve seat?

 

I've never done this test before on a known good engine - is a slight air leaking noise normal from the intake port, or should it be that when the intake valves are closed there should be no noise present at all from the intake?

 

Oddly, twisting the throttle or pulling out the choke effects the pitch of the noise but it's still there - i guess these operations just change the available airflow passages.

 

A few things to note A)

one of my intake valve clearances is very tight. Maybe it's too tight and I've burnt the seat and/or it's not sealing 100% due to carbon buildup from burning oil.

 

B) whilst the cylinder is pressurized, with the cam shaft cover removed, oil slowly gathers and overflows the cylinder head (pictured). I know oil collects here usually, and leaks out when you first remove the cover, but whilst pressurised, oil continually flows from here, and removing the air pressure in the cylinder stops this flow. Is this the air from the cylinder getting into the oil pump passage ways and slowly flowing from the cam shaft bearings?

 

Enough to say the test wasn't as conclusive as I hope it had been, and I've got results I wasn't expecting and can't explain.

 

I guess the next step is to remove the head and cylinder and visually inspect for damage/causes!

 

Thanks for any understanding anyone can share!



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