How do you perform a leak-down test?

pbs911 · pbs911's Garage

Ok, I am armed with my newly acquired compression guage and want to do a leak down test along with my compression test. I searched the archives but can't figure out how to do it. It is probably pretty simple and I'll feel pretty dumb, but how do you do a leak down test? Is it best to do it while the engine is warm or cold? Step by step instruction are appreciated.

john walker's workshop · 08:02 AM

a leakdown tester is needed. starting on #1 cylinder, at TDC, engine warm, the tester is connected to the short hose that screws into the plug hole and what reads on the gauge is the percentage of leakage from 0 to 100.( the tester is pressurized at about 90# from the shop compressor). then turn the engine clockwise 1/3rd turn to TDC for the next cylinder in the firing order (6) and repeat, etc. any high readings should be taken with a grain of salt until the engine is run again and that cylinder retested. (potential for a bit of carbon to cause the high leakage).

pbs911 · pbs911's Garage

Thanks JW, now if I could find an inexpensive tester.

boyo · 01-29-2002, 04:20 PM

A trick from the Old Chevy Truck guys I know for figuring out if a low compression cylinder is due to rings or valves...

Do a compression test on a cylinder, then remove the spark plug and squirt some oil in there. Then run the compression test again.

If the compression goes up, your rings are shot. If it's about the same, your valves are shot.

Of course, how do you squirt oil into a 911 cylinder?

-Boyo

pbs911 · pbs911's Garage

In case anyone else is wondering, a DIY leak down tester. Ask the internet and you shall receive.

http://www.xs11.com/tips/misc/misc3.shtml

Superman · 01-29-2002, 06:15 PM

Leakdown test numbers are not particularly reliable, so I believe half the value is in listening to where the air is going. Just pressurize the cylinder and listen. Hissing in the crankcase means the air is getting past the rings. I am told all cylinders always hiss, so expect that. You can hear them by taking the oil tank cap off.

Valves on the other hand, are NOT supposed to leak. Take some exhaust pieces off to listen to exhaust vaalves, and take you air filter off and open the throttle plate to hear the intake valves.

This helps isolate problems suggested by the compression test.

Roland Kunz · 01-29-2002, 10:17 PM

Hello

Mildon had a very good priced unit in the 90´s ( just pick a US V8 magazine like; CarCraft, HotChevy, HotRod, AutoBuff and go trough there adds )

Grüsse

rob911 · rob911's Garage

Quote:

Originally posted by pbs911
In case anyone else is wondering, a DIY leak down tester. Ask the internet and you shall receive.

http://www.xs11.com/tips/misc/misc3.shtml

Hell you can even make your own compressor - top DIY guy

http://facstaff.uww.edu/fieldsj/mcycle/Comp/

T Bird · 01-30-2002, 03:57 AM

PBS911
If you're not up to making your own, you can buy a leakdown tester from Eastwood for under $50. Check out this link.
http://www.eastwoodco.com/cgi-bin/sgsh0101.exe?AID=43625&FNM=20&PID=12778&SKW=leakdown&UID=%21%2BUSID%21

Good luck.

Roland Kunz · 01-30-2002, 01:56 PM

Hello

Good leak down testers show the actuall flowing volume and you have charts from new rebuild engines to compare.
In fact every engine has a other flow level depending on the diameter from the piston, compression ring design and for special cases ( backside messuring if the heads are off or the valves leak or just to go sure if the spark plug hole isn´t thight ) how much pistons are in the engine. Technical seen you can add up all the borderlines and look at the engine as one big piston.

However the %-tage scale on most versions is accurate enough.

The interessting thing on leak down tests is how will it show up when you cycle the engine. F.E: if you have a flow rise during the stroke up to ITDC* the liners are worn. If they are at LDC your piston skirt is worn out and the pistion will flip over while duration witch leads to not proper seatet rings. ( However if the engine is runing and the masses are oszilating that doesn´t affect that much ).

If you are used with the stuff you don´t need a instrument. You just need a hose and your finger ( thump ) on the hose. You run it up into compressionstroke and you will feel the pressure and hear if & where it blows by. Thats my short check on used car lots

.
I never trust compression diagrams even not my own ones ;D
However most people like to see numbers and like to sort things into tidy charts. Most will dump good race engines due the low compression. ( Einstein; Everything is relativ )

Grüsse

*( ITDC = Ignition top dead cycle LDC lower dead cycle sorry miss the correct shortterms )

Wayne 962 · Wayne 962's Garage

Someone mentioned earlier doing the test when the engine is warm. If you remove the spark plugs when the engine is *very* warm, then you can gall the heads and damage your spark plug holes. BE VERY CAREFULL. Use a 1/4" ratchet...

-Wayne

911pcars · 01-30-2002, 02:27 PM

...... and a small dab of antiseize on the spark plug threads. This helps prevent future galling and metal-to-metal fusion.

Sherwood Lee
http://members.rennlist.org/911pcars

jkeyzer · jkeyzer's Garage

At the risk of straying OT with this, I have heard that antisieze should NOT be used on plugs. It might have even been on this BBS. I'm not sure which to believe. Sherwood?

john walker's workshop · 01-30-2002, 04:52 PM

to each his own on the antiseize thing. years ago, porsche put out a bulletin suggesting it not be used. i'll try and find the bulletin.

Doug Zielke · 01-30-2002, 05:13 PM

So John, what do you think their reason might be?
Are they concerned a well lubed plug may back out of the hole?

I have always used Anti-Seize on plug threads, especially m/c engines which seem to have very soft alloy in the cyl head.

Roland Kunz · 01-30-2002, 10:16 PM

Hello

JW statet that. There is a difference between warm and hot.
When you finished the test the engines are mostly "cold" again.
Also the leak down is one step to find problems and sometimes you have the valve covers removed or other parts are off.

However a bit sensitivness is always asked if you work on aluminium engeins. They just can´t "handle" the same care a ols iron will need to keep runing. I know mechanics who use power tools to change the plugs.

Grüsse

john walker's workshop · 01-31-2002, 07:12 AM

personally, i think the bulletin was to warn against too much compound on the plugs. people use things other than antiseize, and i've pulled plugs out that were very difficult to remove because of copious quantities of baked on goop. if you're going to use it, just a minute amount is all you need. nothing that would collect under the seating surface as the plug was screwed in.

RoninLB · 01-31-2002, 07:20 AM

Loctite makes an anti-sieze Graphite-50 that is good to 900deg. F. Any info on this?? Has 2X the conductivity as copper anti-sieze. Wonder if 900deg F is good enough for spark plugs??

KTL · 01-31-2002, 09:48 AM

Since we're off topic.... and Roland and John covered the leakdown issue......

RoninLB,

Here's another source for the nickel-based anti-seize you posted about before (if you're still interested).

Permatex Brand

Anybody got specific uses for the different types of anti-seize?

RoninLB · 01:28 PM

KTL---I found the nickel anti-sieze. Great with allum/mag metals, good for 2400 deg F [I think,can in garage] Iwant to use the nickel on exhaust system. But the nickel doesn't have the conductivity of Loctite Graphite-50. I want to use it on ground straps, etc.