Looking for a Leakdown tester [Archive] (2024)

Gilligan

23rd March 2004, 17:28

I found THIS ONE (http://store.yahoo.com/toolsplus/mils1257.html) but it doesn't seem like it would be very suited for just basic compression testing though I'm pretty ignorant to all this stuff right now so I could be way off.

I saw a reference to one from JC Whitney that had 2 guages on it one for precentage and one for compression. This seams like a more complete setup and it was about the same price I believe. I did a search on their site and came up empty :confused:

Any one know of a descent tester for under 100 bucks.

Demetri

23rd March 2004, 17:33

RADOne

23rd March 2004, 17:45

Well you don't have to use a leakdown tester for a compression test. But if you're going to do it alot I'd get one that measures leakdown too. If you're just the average shade tree a $15 cheapo compression tester will do. Any autoparts, probably even a Walmart would have one.

The main differance between a leakdown tester and just a compression gage is the leakdown tester has a way to hook up a compressor to pressurize and test the cylinder.

Gilligan

23rd March 2004, 18:52

Well, I spun a bearing and will be swapping the lower end and while I had it all apart I figured I might as well test my head out. OR I will be getting another engine and would rather do the leakdown/compression test myself vs. trusting the vendor/person supplying the engine.

Peace of mind I guess.

TimMullen

23rd March 2004, 19:18

A compression tester and a leak-down tester are two totally different tools.

A compression tester measures the highest pressure that is reached in the compression stroke of the cylinder. It gives you a high reading (hopefully) of the pressure inside your cylinder. Worn rings and/or valves will cause the reading to be lower. Things like carbon buildup in the cylinder will cause it to be higher.

A leak-down tester does not measure compression at all. You apply a fixed amount of pressure (supplied by an air compressor) to a cylinder and the leak-down tester measures the amount of leackage (as a percentage). The way it works is that air is applied to the leak-down tester, and measured on a guage (the first guage is not absolutely required). The air passes through an orifice that restricts the flow and "past" the second guage, then to the hose that screws into the cylinder. Without screwing the hose into a cylinder, you adjust the pressure so that you get a zero reading on the second guage (0% reading) - total leakage with zero air pressure being retained. Then plug the hose, and the reading that you get it is the 100% reading (which should be the same as the first guage if so equipped).

Now that you have the leak-down tester calibrated, you screw the hose into a cylinder (with piston at TDC) and measure the amount of leak-down. Since some air will always leak past the rings and/or the valves, the second guage will be somewhere between 0% and 100% - if it reads 85% of the way between the zero pressure reading and the high pressure reading, then you have 15% leakage (some of the second guages are actually marked in %).

You also listen for the hissing sound - if it's coming out the exhaust, you have a leaking exhaust valve; the intake indicates a leaking intake valve, and the oil cap (dip stick or breather tube) indicate leaking rings... You can have any/all of the conditions.

You actually need to be able to perform both compression and leak-down tests, and they are separate tools...

Tim Mullen

Gilligan

23rd March 2004, 20:34

Ahh, thanks for clearing that up Tim!

seams like you could "seal" of the compressor side of the leak down tool and be able to read compression through the gauge though? I know that would be ghetto and basically not worth the 20 bucks for an el cheapo compression tool but just a proof of concept idea. Is there any reason to stay away from el cheapo tools in this department?

Thanks for the knowledge it is most appreciated!

TimMullen

24th March 2004, 13:54

Well...

Compression testers have a one-way-valve in the inlet (a tire valve usually). Pressure can go "in", but not "out" (until you press the release button). That's why you crank the engine (with the coil packs disconnected on a Miata) a few revolutions to get the maximum reading. Otherwise, the guage would go up and down as the piston came up and then back down. Even if you could stop the piston at exactly TDC, the guage would start dropping as the pressure leaked out of the cylinder.

A leak-down tester on the other hand, does not have the one way valves - it's guages read the current pressure, not the peak pressure due to the compression.

By the way, I figured this all out the hard way, buy trying to make interchangeable hoses for my compression tester and my leak-down tester... :O I still have separate hoses for the two tools... ;)

Oh yea... You don't really have to spend a lot of money for a "good" leak-down tester - a cheap one works just as well as an expensive tester...

More or less the same for the compression tester - you are not really looking for an absolute compression value, but more of a comparison of the values between the cylinders. I.E. If the book says the reading should be 180 PSI and you get 150 PSI, that really doesn't tell you much (the guage could be off, the engine isn't warmed up enough, you live in Denver, etc.). But readings of 150, 152, 99, & 151 does tell you a lot - there's something wrong with the third cylinder)...

For both tools, it's overall quality (good, but not necessarily Snap-On quality) and how well the hose can screw into the spark plug hole is the important thing.

Tim Mullen

photon

24th March 2004, 14:51

Gilligan

24th March 2004, 16:58

Tim, thank you again! A very educational and well written post! I was not aware of that valve either but now seeing how the tool is used it does make sense!

Also, thanks photon for your link, even if I don't build it, it still has value as a trouble shooting manual... good tips in there.

Hey Tim, if you know of any good deals on these tools feel free to shout them out for me ;)

Thanks guys, I understand a whole lot more then I did just yesterday!