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Guest Message by DevFuse

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Leakdown testing explained in depth

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7 replies to this topic

#1 Nug


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Posted 11 March 2004 - 10:23 AM

Leakdown testing is extremely useful. Unlike a compression test, it can isolate the source of cylinder leakage. This can be done on any engine, running or not, without the need for a battery or starter. It can also be done at a junkyard if you are lloking at a used engine.

First you need a leakdown tester. There are 2 types: One and Two gauge. Either will work, they are both equal. The two gauge one simply shows line pressure (from your compressor or air tank, and it isn't vital. Two gauge testers abound on ebay, they are about $70, i bought a used Snap-on one for about $40.

First, You need to get whatever cylinder you are going to test on Top dead center, both valves closed. It's fairly important to get the piston near the top of the bore, as this is where most of the wear is, and it's more accurate. It might be easier to take the valve cover(s) off so you can see the valves working. Pop the radiator cap (if it's a H/g, you don't wanna blow up your heater core/radiator if pressure gets into the cooling system.. Plug the tester into a source of compressed air, and zero the gauge. A line screws into the spark plug hole, with a quick connect hooking to the gauge. Connect the two. Hold the crankshaft so it doesn't turn. Read the gauge.

2% or less-race engine quality ring seal
5-10%-good engine
under 20% Reasonable. You shouldn't see any problems
over 20%- um, there is some wear, or leakage, or something.

With compressed air in the cylinder, if there is a leak, then you can pinpoint it. If it's a h/g leak, bubbles should form in the coolant. If your rings are worn the hell out, you can hear air coming out of the dipstick tube or open valve cover. Intake valve leak- You can hear a change inpitch if you open and shut the throttle. You can put your ear over the intake.
Exhaust valve leak- you can hear air coming out of the tailpipe.

You can do this test in a junkyard if you bring a full air tank and work quickly. Only 30psi (approx) enters the cylinder, and if it doesn't leak too much, you should have more than enough air, maybe enough to do a V8.

Mods- i'd like to submit this to the Repair Manual. And If someone want's to link it to the new Gen Forum, or the Alliance Board, Or wherever else, fell free.

#2 iluvdrt


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Posted 11 March 2004 - 11:43 AM

i linked it for you. good to tool to have, i might be investing.

#3 Nug


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Posted 11 March 2004 - 04:09 PM

Bump for the 4:20 crew...

#4 baccaruda



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Posted 11 March 2004 - 04:45 PM

great post, thanks
USRM it!

#5 xforxlifex


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Posted 11 March 2004 - 04:48 PM

A cheap way to do it but doesnt have any guages is to get a hose from a compression tester (the part that goes in the plug hole) screw it in and hook a line from a compressor up and just listen. If the air sounds to be alot more then should then you know where the problem area is. Obviously if you've never done this before you wouldn't know what a lot more then normal air noise sounds like which is why the guage is good. But if u dont feel like blow 40-70 bucks then do it the shade tree way.


#6 GeneralDisorder


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Posted 11 March 2004 - 04:53 PM

Actaully you can get a lot from just listening - take the valve covers off and listen to the tail pipe with the exhaust valve open, then rotate till both are closed, and listen again. Do the same with the intake, and you can get a feel for the difference between open and closed. This is all without a gauge by the way....


#7 Nug


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Posted 11 March 2004 - 05:26 PM

Yes, you can do it like that, but with lower levels of leakdown, it's hard to tell the difference.

Besides, the thing I wrote was how to use the leakdown tester, not how to avoid using one.

That wasn't meant to sound harsh.

I've done it with just compressed air.

It IS nice to have quantifiable numbers, though.

#8 xforxlifex


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Posted 11 March 2004 - 07:04 PM

Not to sound harsh or anything but i wasnt necassarily directing that to you. more towards people who do not know about this test in general. But none the less good write up.

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