[updated:LAST EDITED ON 04-Oct-02 AT 07:19 PM (EST)](My appologies to those who may have seen this post appear and disappear over the last couple of days. I've been working with the FTE guys to find a problem. If we've nailed it, this post may hang around. If not, it may disappear again (Sorry, Eric).)

Has anyone had experience using/buying/building a leak-down tester?
I've seen them for sale (pricey) and plans to build them (inaccurate?) and was wondering what opinions I could collect here.

For those who may not know, as I understand it, a leak-down tester is used to get an idea of an engine's health, sort of like what a compression test does. But, instead of using the engine's own compression stroke to put pressure in the cylinder, you put 100psi (I think) into the cylinder through the spark plug hole with the piston at TDC and the crankshaft locked to avoid spinning the engine. The tester measures the difference between input pressure and output pressure, letting you know how much of a leakage problem you have.

You can hear air rushing past the rings and/or the valves to let you know where the problem is, I'm told.

-Mark

 

I've never used a real one, but I did build something like one. I first made an "air hold" adapter by brazing a coupler to a spark plug extender. Then I got a T fitting, screwed a pressure gauge into the top, and put a short hose with a female coupler to match the spark plug doodad on the output side of the T. The inlet side of the T got a male quick connect. This connector was plugged with a chunk of metal with a single small hole drilled through it (I'll call this the aperture). I feed air to this from an adjustable regulated air supply.

Here's a page on how to build one;
http://www.xs11.com/tips/misc/misc3.shtml

 

I have necer used a leakdown tester in an automotive application. But while I am going to small engine school I have used them often. They work great for that. You were right about how they work, the piston has to be at TDC on the compresion stroke. You should be able to hear if you have any leakage or the guage will tell you. I have never used one on a multi cylinder engine though

 

Angus,

Yup, that link (among others) is what I'm using for a plan. I've got all the parts I need except the damper valve, or what you called the aperture. I still need to get a #60 drill bit to make it.

I've bought a "valve lifter" for about $10 at Checker (or Kroeger or whatever it's called where you live) instead of making one out of an old spark plug. It has two different spark plug sizes (14mm and 18mm, I think) and the other end has 1/4" NPT, so it screws right into pneumatic fittings.

I'll try to post some pictures and info when I get it done. Meanwhile, anybody have any tips for using this beast?

Thanks,

-Mark

 

I've had a home built leak-down tester for years. I used an adjustable pressure regulator with gauge on it to set the input pressure at exactly 100, which then means that the output gauge reads the % of air retained in the cylinder being tested. As stated earlier in another post, the piston in the cylinder has to be at tdc (top dead center) at the end of the compression stroke, and if the vehicle has a manual trans, put it in high gear and set the parking brake. If the trans is an automatic, then trial and error is necessary to finally get a reading that is reliable. A 95% reading equals a very good cylinder. A reading above 80% means it is a resonably decent engine and still has some miles left. If it is only 20%, then the air is getting away via several sources. Bubbles in the radiator fluid means the worst problem such as cracked head or block, or blown head gasket. Air out the exhaust means an exhaust valve problem, air out the intake means the intake valve is bad, air out the oil fill tube means a problem with either the cylinder wall, or the piston. It is advisable to do a compression test first to get a general idea of the engine's condition, and if one or two cylinders are bad on that test, then use the leak-down tester on just those cylinders to identify the specific problems. My understanding is that FAA required annual aircraft inspections include a leak-down on the piston type engines to evaluate them. It truly is the best means to get a specific cylinder problem identified, short, of course, doing a tear down and physical inspection. Hope all this helps. I've used mine on many prospective purchases, and it always gets me past the seller's idea that the "engine was rebuilt just 10000 miles back". I usually don't buy if only 65% is the reading.

 

A good regulated 100 psi could be a little difficult to maintain on some compressors that cycle 90-120psi. Could a person use 50 psi and just double the cylinder pressure reading? What size orifice did you use with the 100 psi and what size would you have to use for 50 psi?

 

robertr,

That's the kind of advice I'm looking for! I'm a little confused about your reference to the transmission, though. I assume you mean that a manual tranny can hold the piston at TDC, but an automatic can't, right? Maybe it'd be a good idea to block the flywheel with a crowbar or something...

I'm building mine to use to test the engine I bought from a junkyard and plan to put in the truck. It's currently got a broken timing chain, sitting on an engine stand in my garage, so I'll have to hold each piston at TDC by some other method.

This may be a dumb question, but... Can you do a leak-down test on a piston that is at the bottom of the power stroke? Perhaps the valves are already opening at that point...

I've finally got all the parts I need, now I just need to find some time to put it all together! I'm planning to put up a web page about it, if I get my act together.

Thanks,

-Mark

 

You can do a leak test at the bottom of the stroke but it won't tell you much. The greatest cylinder wear occurs at the top of the stroke and the rings will be at their greatest expansion point.

I would suggest a wrench on the front damper to hold the engine. Just remember there is a lot of potential force involved.

 

If you get a bad reading on 2 adjacent cylinders the problem is usually a blown head gasket section between the cylinders. Air noise though the dipstick hole could be worn rings

 

I have used a leak down tester several time over several years, and I can swear by their results, When purchasing an engine that has "just been rebuilt" the engine will show its true colors and you may be able to be in a better position to determine a fair price or to make the purchase at all.
What I have been using to determine TDC is a whistler device that you screw into the spark plug hole and turn the crank over and then listening to the whistle on the compression stroke. I found this item at a tool store on one of those cardboard tool racks.
I didn't make the tester that I have now, I found it on "e-bay" and there are several of them available at a very reasonable price, as alot of people purchase them and don't want to go through the trouble of the proper testing proceedures.
I personally think that this is a handy and valuable tool to use before sinking money into an unknown part or vehicle or boat purchase.

 

If you are makling your own tester and using 50psi air, try a 1/16" orifice. This SWAG is from checking some air flow tables to produce about the same airflow that is produced by a 0.040" orifice at 100psi. You can check it with a couple of known good engines or check it against the reading obtained from using 100psi with 0.040". 50psi with 1/16" is a good starting point for experimentation.

 

Well, I've finished assembling it. It only took me, what, 3 MONTHS!

Here's some pictures I took of the pieces. I tried to buy things where I could, without spending a lot of dough. I also tried to make things modular, so that I could assemble this, but use some of the parts elsewhere.

So, to make the aperture, I bought a brass 1/4" pipe thread plug from Home Depot and a #60 drill bit from a hobby shop. Drilled through the plug lengthwise, and ground down the square head so that it wouldn't interfere with screwing it into an air fitting:

http://www.clubfte.com/users/mcgyver4/LeakDownTester/ApertureCloseup.JPG

What I failed to realize, though, was that these things tend to have tapered threads! The head end wouldn't screw into anything, since that end was too wide!

So, I bought a 1/4" die and ran it over the plug, cutting even threads across the whole length. I then assembled it with a couple of compressor fittings and a gauge, wrapping all the threads in teflon tape:

http://www.clubfte.com/users/mcgyver4/LeakDownTester/ApertureParts.JPG

I also assembled another gauge with some fittings, and put a fitting onto the end of a "valve holder".

http://www.clubfte.com/users/mcgyver4/LeakDownTester/Parts.JPG

Now I can put together a leak-down tester, and re-use parts of it for other things, like spray-painting.

-Mark

 

Thanks for the great pictures Mark!

Have you tried it out yet?

 

 

Finally tried it out. However, I used it on a cold engine; I know that's not recommended, but it's on an engine stand in my garage, so I can't run it. I wanted to get an idea of the health of the thing, however inaccurate.

The leak-down tester worked pretty well. I was disappointed with the two gauges; they didn't agree very well. I decided that one gauge downstream of the orifice would have been sufficient. Since I put a quick-connect on the valve holder, I could pop it off, and the downstream gauge would read full pressure. I'd position the engine, pop on the valve holder, and the downstream gauge would drop to some level, easily compared to the previous reading.

Since the engine is upside down with the oil pan off, it was easy to see TDC. However, it took me a bit to figure out how to tell the difference between TDC between the compression and power stroke, and TDC between the exhaust and intake stroke. Each cylinder appeared to be quite tight at the valves and head, but the rings let lots of air pass. I couldn't get any better than 70/100 psi on any cylinder. This depressed me for a while, until I decided that cold rings couldn't be expected to hold much pressure.

So, I'll try it again after I get it in the truck and running.