Solving Automotive Performance Issues
Ignition timing is sometimes controlled by the distributor position. Ignition timing is set up to produce a spark at the perfect time. The ignition sets off the combustion mixture just before the piston reaches TDC (top dead center) so you get maximum power from the combustion process. In essence, you’re turning the chemical energy of the gasoline air mix into mechanical energy. In order to do this effectively, you need to have proper ignition timing. The only time you should have to worry about ignition timing is if you have a distributor, and even then it’s in question. Just because you have a distributor, that doesn’t mean it’s adjustable or that you should adjust it.
I want to say this about ignition timing: Don’t adjust it. Unless you removed the distributor to perform some other repair, you should never have to set ignition timing. You can do more damage than good if you mess with ignition timing. If it’s not right, you can cause detonation or pre-ignition, both of which can cause engine damage and a loss of performance.
Each engine has a specific procedure that needs to be followed when setting ignition timing. This is so often overlooked. Don’t just grab the distributor and turn it till the engine runs well. Don’t laugh; I’ve seen people do it, a lot. If you have a coil on plug setup or you have a coil pack assembly, your timing is not adjustable, so don’t worry about it. If you think the engine is out of time on one of these engines, check the mechanical timing. These ignition systems run on mechanical cues. If the mechanical timing is off, it will also offset the ignition timing. This also means that if you find your ignition timing off, it could indicate a mechanical problem like a loose timing chain or belt.
In summary, if you suspect an ignition timing problem, check the mechanical timing first. Don’t adjust the ignition timing unless you’ve done a repair in which the ignition timing was affected. If you do have to set ignition timing, make sure you follow the proper procedure. Improperly setting ignition timing can cause a loss of performance, or, worse, engine damage. Here is a video demonstrating how to set timing on a 1991 Acura Integra. This procedure will not apply to all vehicles, but you can use it as a guide to see how it is done and the tools that are used.
A compression test is a good way to get a baseline of the mechanical health of your engine. What you’re checking is the ability of the engine to bring in air, compress it, and expel the spent gas. To perform a compression test, you’ll need a compression tester.
You can purchase this tool rather inexpensively, or you might be able to borrow one from your local auto parts store. To perform the test, first remove all the spark plugs from your engine. Then find a way to disable the fuel system. You can pull the fuel pump fuse or unplug the main relay. In the video below I show putting the gas pedal to the floor when cranking the engine. This should put the fuel system into clear flood mode, which means it turns off the injectors while you crank. This might not always be the case, so to be safe you might also pull the fuel pump fuse as suggested. You might even go so far as to unplug the fuel pump if you have access to its connector.
Next, install the compression tester into one of the cylinders. It really doesn’t matter which one, but it’s a good idea to keep track of everything so you can identify the location of your problem cylinder. You might want to pick cylinder number one as your first cylinder when doing your testing to make keeping track easier.
Now that the tester is installed and you’ve disabled the fuel system, hold the throttle plate open. I often just hold my foot to the gas when I crank the engine to do this. Crank the engine over. I usually listen for about five revolutions and quit. You really don’t need to crank more than that. Then take your reading. What you should see will vary by engine, make, and model, but you hope to see readings that are within 20% of one another. Ten to 15% would be even better. The key here is to look for balance. Remember an engine needs to be balanced to run smoothly; if it’s not balanced, it will run rough and shake. So if you see a cylinder that’s down significantly compared to the one next to it, look to that cylinder for the problem.
If you see two cylinders next to each other with low compression, this could indicate a head gasket failure. Keep in mind that a compression test is just a general test. It can only tell you the problem cylinder(s). It can’t tell you what’s wrong with that cylinder necessarily other than that it’s low on compression.
Another test that can give you a little more information is a wet test. A wet test is when you insert a small amount of oil into the cylinder before you do the compression test. After the oil is in the cylinder, do the test again and take a reading. You should see a jump in readings by doing this because the oil will help seal the rings on the piston. If you see a significant increase, this could indicate that your rings aren’t sealing well and it might be time for a replacement or rebuild of the engine.
That’s about the limit of what a compression test can tell you. If you want more information you can do a leak down test, which I’ll get into next. In the meantime, here’s a video on how to perform a compression test.
A leak down test can tell you quite a bit more about an engine’s mechanical health than a compression test can. They both test the sealing ability of the combustion chamber, but the leak down can tell you how much is leaking and where it’s going.
The procedure is similar to a compression test, to a point. I might break out my leak down tester after I find a cylinder with low compression, or I might start with a leak down tester when I suspect a mechanical problem with the engine. To perform the test, you must first remove the spark plug on the cylinder you’re going to test. You can pull all the plugs to make turning the engine over easier.
In order to test a given cylinder, it needs to be at Top Dead Center (TDC). To get a cylinder at TDC, you need to turn the engine over by hand. This is much easier to do with all the spark plugs removed. As you might know, the piston goes to TDC twice during the four-stroke engine cycle: once for the compression stroke, and once for the exhaust stroke.
There are a few different ways you can determine this. The first is to follow the firing order of the engine, starting with cylinder one. To determine if you’re on the compression stroke on cylinder one, you can use a couple of different techniques. The first is to place a piece of paper or other loose material over the spark plug hole as you rotate the engine. When the object moves out of the way, you know you’re on the compression stroke. Another way is to install a vacuum gauge into the spark plug hole. You can actually use the hose that you thread into the spark plug hole and hook your vacuum gauge up to that.
I should mention that your vacuum gauge needs to be able to read pressure as well as vacuum. As you rotate the engine, look for the gauge to go into the positive. As the piston approaches TDC, the pressure will go to zero and then start to go to vacuum as you pass TDC. You can work the engine back and forth till you find the moment were TDC happens. Here’s a video that explains the process.
Now that you know you have the piston at TDC, it’s time to do the leak down test.
- Hook your tester hose directly into the spark plug hole.
- Hook your leak down tester up to shop air and zero the pressure. This will be important later so you know how much is leaking out.
- Then, hook the tester to the hose and take your reading.
Leakages above 20% are considered excessive. I like to see them closer to 10%, but as an engine wears, its compression and ability to seal will be lessened. But wait, there’s more! Not only will you know how much is leaking, but you’ll now be able to figure out where the pressure is going simply by listening for where the air is escaping.
- If you remove the oil cap and hear a lot of air escaping, the rings are worn and that’s where your compression loss is.
- If you hear it coming out of the intake, the intake valve(s) isn’t sealing.
- If you hear it coming out the tail pipe, the exhaust valve(s) is leaking.
- If you take the radiator cap off and see bubbles coming out, you have a combustion leak into the cooling system, which could be a bad head gasket.
I don’t often use an actual leak down tester when doing this test. In fact, I do a compression test and look for a problem cylinder, and if I find it, I take a little short cut. You can actually use your compression tester hose for leak down testing. You first have to remove the Schrader valve in the compression tester hose. Once you’ve done that you can then hook shop air directly into the cylinder. You don’t need to know how much is leaking at this point, because you’ve already determined that a particular cylinder has low compression. All you want to know now is where the leak is going.
Doing it this way is a personal preference, but it can save you from purchasing another tool, thus leaving a little cash in your pocket. If you’ve got mechanical issues with your engine, you’re going to need it. Here’s a video on leak down testing that can walk you through the process that includes this little trick of using the compression tester hose hooked up to shop air to find the source of a leak.
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