How to: Leakdown Test

A Leakdown Test is a very useful way to gauge the health of your engine and ensure no major problems, its a similar process on all engines but here is the process on Jesse's Reborn Miata!

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[GearHeadsGarageChannel]

One VERY Important thing I believe I forgot to mention in the video is making sure the piston is at Top Dead Center on the Compression stroke. If you arent sure how to tell, take your spark plugs out since you need them out anyway for the test, turn the crankshaft over with a big bar or ratchet with your finger in the spark plug hole lightly sealing it and when it is on the compression stroke you should feel the compression and air pushing your finger out of the way. Then make sure its at top dead center and you are good to test! If you are at TDC and have a large leak try turning the crankshaft exactly ONE more rotation as you may have been on the wrong stroke.

[johnnyjohn]

I have major smoke coming out of the exhaust on my 1990 Miata 1.6. The problem, if that alone wasn't enough of a problem, is that it's on a newly rebuilt engine. I never had problems in the past with my engine rebuilds. This time what's different, is that I resurfaced the cylinder head myself, lapped the valves & also installed the valve stem seals & springs myself. Usually I send all my cylinder heads for my rebuilds to the shop and the bare engine block also. Then I assemble the block & install the cylinder heads. I thought: A little Miata cylinder head, I can work it myself without a machine shop...what could go wrong? Boy, was I wrong. Based on some research, excessive smoke out the exhaust would indicate bad oil seals; oil seep through the valve seals then burns out as smoke through the exhaust pipe especially after the first start of the day (after it sat overnight or sat for half a day). Eventually the oil burns off and little or no smoke would be visible as it got burned off during a drive. At first I thought that maybe I installed 1 or more valve stem seals incorrectly, not fully seated. However, this crazy smoke is continuous, it does not decrease even after driving it for an hour. One time I even let it idle at my friend's shop for 2 hours after I drove it & no luck, the smoke did not decrease. I saw a video that explained the difference between when (1)the engine burns oil as smoke out the exhaust pipe, but eventually goes away VS. (2) the engine burns oil as smoke out the exhaust pipe, that is continuous & doesn't go away. When the problem is BAD VALVE SEALS: the oil that seeps through the seal or seals, eventually burns off while driving & the exhaust smoke goes away: smoke may reappear during quick acceleration or coasting down hill but goes away again. However, when the problem is BAD PISTON RINGS: the smoke is continuous, it doesn't go away. In my case, since I put new Hastings piston rings, I'm afraid that for the first time, I may have broken or chipped a ring during the installing of the piston down the piston bore on the engine block. Doing a dry & wet compression test will guide me to the problem. The leak down test will point to the problemed piston. I sure hope that it's some badly installed valve seals, but it all points out to damaged piston ring or rings. Hopefully I'll remember to update this comment once I figure out the problem & fix it.

[billwhite]

Also take off the radiator cap and lock for bubbles in the coolant to check for head gasket

[MikeBrown-iipt]

Ah, the good ol leak down test. So valuable for diagnosing many engine problems. I already knew how to do it, but watched the video to make sure that there was nothing new about them. Great, plain language video! However, you forgot to mention 1 VERY important thing that "newbies" might not realize. The pistons need to be at top dead center ON THE COMPRESSION STROKE. I'm not criticizing you at all for the oversight. I'd just hate to see someone perform this test and decide that they had to rebuild or replace a perfectly good engine because their intake and/or exhaust valves were open. Again, great video that showed me that I still know how to do a leak test.

[ian]

Some kind of coincidence because I was thinking of doing some sort of test to my engine today before I even think about boosting mine. Very useful!

[justinteal]

Did you forget to mention taking the radiator cap of to check for head gasket leak or did I miss that

[cWdogG]

I just found this channel today and I LOVE it. Great work guys, keep it up. 👍

[BecauseMiata]

Thanks for the master class, sir. I've been curious about this type of test for a while now. Very informative. 😎

[panamaliano]

Great videos from awesome man. informative.
I wish you all the best in your life.
keep up the great favors to your students,
Glad to have you as my teacher. God bless you.

[MrLevi]

Great video. Are you using Kastar CLT-4 Cylinder Leakage Tester 2 Gauge calibrated for 35 Psi not for 100 Psi?

[ASuperBee]

“I can hear leakage everywhere....it’s not bad” 😂🤣😂

[Garage]

I hadn't actually seen that screwdriver TDC trick. That's pretty awesome, hahaha.
Some old school mechanic listening here, like using a screwdriver as a stethoscope.
Nice job!

[HomebrewSubaru]

Hopefully it works out. A small head leak into the cooling system can be overlooked and not cause an overheat for weeks. If it starts running hot intermittently, gut the thermostat and run the car without it. If it never overheats, head gasket would be likely suspect.

[edwardfourie]

like your video learned me even more thanks guys

[FullySpooled]

Great vid Jesse! Super informative and easy to follow. And I also want to drive this car. Gotta get that track day on the books!

[Okie-Tom]

Very nice video. Right to the point. Oh, it's better not to leave the socket wrench on the crankshaft bolt while adding air. If not on TDC it can spin the motor and possible hurting you or breaking something. Tom

[TheOnlyToblin]

Might get myself one of these and test my engine. Well, once I get it back running of course. It broke down a few days ago.

[ironfistarrival]

Hi there, so how do you differentiate between a bad head gasket or head cylinder and a bad piston
rings or bad valves ? Could you please do a video to precisely PIN POINT these differences and
required symptoms and tests .... ? Thank you so much ..

[Brucelee-dwtw]

Very good video, only you give the number(below 30% is ok) I have watched many similar videos. 
Engine QR25de: compression test: 140 psi, ( in service book is 181.3 psi-153.7 psi.)
doing(cold) leakage down test: 25% lost in total. when you listening: sounds coming from each area: intake, exhaust, oil cap, even coolant cap still bubbling. the car is still powerful like normal except the milky oil cap. can you pls give a piece of advice that the car is worth fixing or not ? i dont want to touch the piston ring because it is very big job for me. I cannot make a decision still.

[billwilliams]

GEARHEADS, when you were listening to the hissing noise from the next cylinder using a hose placed in your ear. What would be the problem of the engine if there is Hissing noise from the next cylinder while you were leakdown testing cylinder#1 ?