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Engine Problem Diagnostics - When S*** Goes Down, Don’t Freak Out

Aaron Bonk
Sep 4, 2013
Break down compression test Photo 1/7   |  
When performing a compression test, be sure to remove the spark plugs and disable the ignition coil and fuel injectors. Hold the clutch in and turn the engine over at wide-open throttle for an accurate reading.

Your engine is simpler than you think and only four things separate it from becoming a good-for-nothing lump of aluminum: fuel, air, spark and compression. Smoke billowing from the back of your hoopty almost always indicates some sort of problem with at least one of those components. Luckily, figuring out exactly what’s to blame isn’t as hard as you think.

Smoke Signals

Sheets of smoke bilging from your exhaust will never be good. The results are lost power and can even foretell a dying engine. There’s more than one type of smoke than can stream out of your exhaust ports, though. Fortunately, their colors make differentiating them easy.

Break down compression tester Photo 2/7   |  
A compression test can quickly tell you how bad off your engine is. By introducing compressed air into the cylinders, damaged or worn pistons, rings, cylinders, valves, and headgaskets can all be determined.


Here, a rich condition caused by excess fuel wasting away in the combustion chamber is happening. The results are lost power and an increased chance of you getting nabbed for mobbing around in a gross polluter. An overly rich condition can wash fuel past the piston rings into the crankcase. Mix enough of it with oil and you can say sayonara to important parts like rod bearings and piston rings. The culprit could be mechanical, like leaking fuel injectors or a damaged fuel pressure regulator, fuel pump or return line. It could also be electrical, like a bum ECU or airflow sensor.

What to do: Verify fuel pressure using a fuel pressure gauge. With the vehicle running, pressure should increase when the regulator’s vacuum source is disconnected and plugged. (If fuel leaks out of the vacuum connection, replace the regulator.) A pressure drop of no more than 20 psi once the ignition’s killed is normal. If more, a faulty pump, regulator or injector is likely. A stethoscope will help you determine if the injectors are pulsing properly; they should all sound similar when idling.

Break down vacuum tester Photo 3/7   |  
A vacuum test is a fast way to determine whether or not your headgasket’s failed. Once hooked up to any vacuum source, an erradic or bouncing needle generally means there’s trouble.


Blue smoke’s almost always the most expensive. Here, oil’s burning, which means there’s something wrong underneath the valve cover(s). Blue smoke indicates oil that’s escaped into the combustion chamber, smoldering along with the air/fuel mixture. This can’t be good for performance. Extreme cases can foul the spark resulting in misfiring but, more importantly, point to a larger problem: a doomed engine.

What to do: There’s more than one place to look when determining the source of burnt oil: piston rings, valve stem seals, and valve guides are all possibilities. Smoke upon startup often indicates worn valve stem seals or valve guides. Over time, their clearances increase, allowing oil to escape into the combustion chamber, which generally subsides once the engine reaches operating temperature. Excessive smoke while accelerating or at idle is usually caused by poor piston ring sealing. Smoking when decelerating is more than likely caused by worn valve stem seals or guides.

Break down leak down tester Photo 4/7   |  
The leak-down tester intoduces compressed air into the cylinders and measures how much is potentially escaping past the piston rings or valves. Leak-down results in excess of 10 percent are generally not acceptable for performance engines.
Break down leak down test gauges Photo 5/7   |  
It’s important to use both gauges so that the appropriate amount of air is introduced into the cylinders when performing a leak-down test.


White smoke can be traced back to coolant that’s burning inside of the combustion chamber, resulting in steam. It sounds harmless, but steam points to a serious problem, like a cracked cylinder, blown head gasket or damaged intake manifold gasket. Once enough of it enters the combustion chamber, it’ll seep into the crankcase at which time you can forget about your oil doing whatever it’s supposed to be doing.

What to do: Inspect the oil cap and radiator cap for signs of cross-contamination. If they’re mixing, the oil will appear caramelized and foamy. Connect a vacuum gauge to any vacuum source. A leaking head gasket will cause the gauge’s needle to vibrate erratically. A cooling system pressure test also isn’t a bad idea. Most pressure testers connect to the radiator cap inlet and can be done with the engine cold. Verify what your vehicle’s pressure is supposed to be and monitor the gauge with the vehicle running.

Performing a Compression Test A compression test can further verify the mess you’ve gotten yourself into by measuring how well an individual cylinder seals. The results can tell you a lot, like how well the pistons, rings, cylinders, valves and head gasket(s) hold compression. For an engine to run smoothly, all cylinders should produce similar readings. Make sure the engine is warm and that the battery is charged. Remove the spark plugs and connect the compression tester to a single cylinder’s spark plug opening. Disable the ignition coil(s) and fuel injectors by unplugging their connectors. Depress the clutch and turn the engine over at wide-open throttle until the gauge’s needle stabilizes. Repeat for each cylinder. Depending on the engine’s compression ratio, numbers will vary. A high-compression Integra Type R will read a whole lot different than a low-compression turbo engine. Instead, pay attention to major differences between cylinders. If one reads low, squirt a small amount of oil on top of the piston and redo the test. If results improve, it’s likely the rings or cylinder walls that are to blame. If they stay the same, a failed head gasket or valves are more likely.

Break down tools Photo 6/7   |  
Tools of the trade: compression tester, leak-down tester, and vaccuum gauge. Almost any mechanical engine problem can be diagnosed using them.

Leak-Down Test

The leak-down test is the most conclusive, is the most complicated, and will make you look the most important in front of your friends. The results reveal how well an engine’s pistons, rings, cylinders, valves, and head gasket(s) are doing their jobs. The higher the percentage of loss, the worse off you are. Leak-down equipment connects to the engine much like a compression tester but relies on compressed air from an outside source instead of turning the engine over. Rotate the desired cylinder’s piston to TDC so that its valves are closed, thread the tester into the spark plug location, connect the compressed air source, and monitor the gauge. A properly running engine shouldn’t exhibit more than a 10-percent leak. To determine what’s leaking, simply follow the sound. If you remove the oil cap and hear a howling inside of the crankcase, rings or cylinders are likely to blame. Check for damaged intake or exhaust valves by listening for leaks at the throttle body (when opened) or exhaust tip. Finally, be sure that the amount of air introduced is the same for each cylinder for consistent results. If you’re getting readings of 100 percent, double-check that the cylinder is at TDC on its compression stroke. If you’re off by just a few degrees, the forced air will push the piston down the bore, giving a false reading, and otherwise ruining your day.

The More You Know: Spark Plugs

Your spark plugs are the window to what’s going on inside of your combustion chamber. A quick inspection of their center insulators can reveal the cause of serious engine damage or give you the information you need to prevent it from happening in the first place. A healthy engine’s spark plug’s center insulator should be light tan-colored or even gray. Look for these signs the next time you inspect your plugs:

Break down spark plug Photo 7/7   |  
A healthy spark plug: Notice the plug’s light-gray-colored insulator and relatively clean electrodes. A black- or white-colored insulator and rounded or corroded electrodes are common signs of a worn or damaged spark plug.

Black insulator (dry): Carbon deposits from fuel can be traced to an overly-rich air/fuel mixture or weak ignition. Other less harmful causes include extended idling or slow-speed operation.

Black insulator (wet): Oil deposits are a sign of damaged piston rings, valve guides, or valve stem seals.

White insulator: A white or speckled insulator is a sign of detonation caused by an overly lean air/fuel mixture.

Rounded center electrode: Normal wear and tear can round out a plug’s center electrode, but excessive or preemptive roundness can be blamed on a weak ignition system.

By Aaron Bonk
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