I have seen you post this before. Could you please elaborate on your reasons a bit more. The dealer I uses has recommended a leak down test in the past so it was my understanding it was standard operating procedure when determining the condition of an engine.
It also seems the same dealer no longer does leak down tests so perhaps they came to the same conclusion as yours.
Thanks for your help.
Sure! But it's going to require a lot of reading.
Where to begin.... ?
What is a leak down test?
So a leak down test is supposed to determine where the inefficiency within the engine. Let me back up. If low compression or a weak cylinder is suspected, the first thing you would do is a compression check. Simply put, you use a gauge to measure cylinder pressure while cranking. There is a threshold for a compression check and typically an engine manufacturer will list a spec for it in the service manual. Typically a range. In most gasoline engines in good condition, cranking compression (compression produced with the engine not running using the starter motor to crank it) is above 150 PSI. Diesels are typically double that. An engine will still run perfectly fine all the way down to around 120 psi. Below that you start to notice problems. Below 100 psi starting becomes very difficult, but I've measured engines all the way down to around 60 psi that could still start and run. Now they didn't run well albeit, but they would run.
A compression test only reveals that there is a loss or inefficiency in the engine, but it does not reveal where. It could be any number of things that cause it. Things as simple as a plugged air filter all the way to a bad cam lobe, worn valves or a worn cylinder. You don't know. To know where the problem is you have to do more test, but you can't see inside. The only reason you need to know where the problem is, is because it may or may not be something internal to the engine. You may not need to take it completely apart to fix it.
About fifteen years ago (or so probably longer ago than I think) I started hearing about leak down tests and the tool people were advertising leak down test kits. All they are is a manifold gauge set similar to an air conditioner gauge set. You have a manifold with a two hoses. One screws into the spark plug hole and the other attaches to an air hose. One gauge is connected to each hose with a valve between them. (I don't even own one so I'm going off memory of looking at one.) So, you set the piston of the engine on TDC and put air pressure in the cylinder to a specified pressure (like around 100 psi) and you listen to hear where air is escaping. (Just for a bit more clarity, TDC is Top Dead Center. At TDC the valves are closed and the piston is in the top most position it can be in. This is when the spark plug would spark and ignition would occur.) We listen for air to escape in three places: 1.) Out of the crankcase vent or dipstick tube 2.) out the intake or carburetor and 3.) out of the exhaust. If air comes out of the dipstick or crankcase vent, it reveals and inefficiency in the cylinder. Piston rings or worn cylinder. If the air comes out of the carburetor/intake, it reveals a problem within the intake valve. Chipped valve, worn valve making a poor seal, bent valve, over tight valve train (valve adjustment needed). If you hear it out the exhaust it reveals the same, only with the exhaust valve. Now, you could possible also hear air escaping out of the head gasket, which of course reveals a blown head gasket.
If the cylinder and valve train reveal no major leaks during the leak down test, you would roll the engine 180 degrees and test again with the piston on the bottom of the stroke. Cylinders do not wear evenly. Typically, the cylinder wears in an egg shape pattern. More in the middle and typically more at one end or the other. Gasoline tends to wear a cylinder more at the top and less at the bottom, diesel engines exactly opposite. If the test now reveals a leak with the piston at BDC, then it indicates a worn cylinder.
The other test that you are "supposed" to do with the leak down test is letting the air leak down. So, you put the piston at TDC and lock the engine so it can't turn. (Harder than it sounds.) Then pressure it up to a specified pressure and leave it sit for a specific time. Like 3-5 minutes. You record the loss of pressure over the given time. This would be repeated for all cylinders. This test is supposed to reveal variance between cylinders or to indicate cylinder wear if the pressure drops "X" amount over the given time.
Why this test is a waste of time:
When you do a compression check, it reveals pretty quickly if there is or if there is not a problem within the engine. Most generally, this test is a go/no go. You either have good compression, or you don't. It isn't often you have a "meh" compression reading when you have an engine that won't start and run. Single cylinder engines are a bit more difficult in that you don't have any other cylinder as a basis to compare to. In the example of a V8 if you have a misfire, you can do a compression check and you have 8 cylinders for comparison. If 7 test at around 150 PSI, and one tests at 90.... well. That's easy to know where the problem is. Even in a 2 cylinder. One is high, one is low, you know which hole needs attention. But single cylinders.... it's a guess. Say compression is 100 psi. Certainly high enough to run, but is the engine just getting tired? Or is there a problem like the valves are out of adjustment? You just have to start taking it apart and looking until you find it. In the case of a single cylinder, a leak down test won't reveal much because there is no comparison to make. No companion or other cylinders to compare it to.
Typically the leak down test will reveal the problem right away. You will hear air escaping out the carb, or dipstick or exhaust really quick. Most generally it is a pretty large leak by the time the engine won't run. Now, I'm going to just say that this test requires you to be able to hear well enough to hear the air. As I lost my hearing, I had a lot of trouble hearing the leaking air (but not using a leak down test tool, which I will explain in a moment). I had to come up with another way to determine where the air loss was since I couldn't hear it. Well, for me it was pretty easy, because I also smoke. So, I would light a cigarette and hold it above the dipstick hole, the carb and the exhaust. How fast the smoke blew away revealed where the leak was, and how bad it was. Obviously you don't have to smoke to use a similar method. A blown out match or a smoke machine would also work. Anything that steams or smokes can be used.
Now to the meat and potatoes part. Basically a leak down test is looking for a loss in efficiency in the places mentioned. As I stated, most generally the problem becomes pretty obvious right away. In very few cases, you even make it to the portion of the leak down test where you air it up and time it. Even if you do make it to that test, it is going to mimic the compression test and basically reveal nothing. If a cylinder has a bit lower compression than other cylinders in the engine, it will also have faster leak down times: thus, the leak down test is a waste of time because you already knew that cylinder was weak when you did the compression check. If the air leaks out slow enough, you can't tell which of the 3 reasons is the cause, so you have to take the engine apart anyway. You didn't need to waste the time doing the test.
If you put the leak down test tool on the engine and it immediately (and this is most generally the case) reveals a leak in one of the three places mentioned, intake, exhaust and crankcase, then the test was also a waste.
Before leak down test tools were available, we would simply roll the engine to TDC and use a rubber tip blow gun, or a hose and fitting made to fit into the spark plug hole and put compressed air into the cylinder. You can't use too much air pressure, but air pressure around cranking compression is best. Some guys would use a regulator so as to put only a specific amount of pressure in the cylinder. I always just used a rubber tip blow gun the spark plug hole and barely let air go in and start listening for the leak. If I didn't hear it, I just put more and more pressure in until I could hear a leak. Eventually, even in a brand new engine, you will at some point get air out the dipstick tube. Piston rings are not perfect seals. They do leak some, if nowhere else but in the ring gaps. Just as a note, diesels engines hold air pressure far better than gasoline engines do because they are designed for much higher cylinder pressures, and thus have much better rings.
So you read all that to hear that you can do the same test with a rubber tip blow gun in the spark plug hole without the expense of another tool and without the time it takes to hook it all up and go through all the tests. So, why do they make them? Because young and inexperienced techs don't know how to determine things as well and they needed a way to teach them. Someone somewhere came up with that tool and it made sense to a lot of guys and they used it. For an older experienced tech, it was just a pretty thing to make an old trick look more "technical". Plus, you can make a bigger/better "story" to your customer with numbers from a leak down test to "prove" the engine is worn. It's really nothing more than a gimmick tool, or a tool for the inexperienced. Besides that, if you know little about engines it isn't going to tell you very much anyway because you won't know what to do with the information the tool gives you.
