Hi bvaldez81. Welcome to the forum. A compression test is a very basic test that is just a starting point. It is usually the first test a mechanic learns to do. You will need a compression tester. It's nothing more than a pressure gauge and a hose that screws into a spark plug hole. The hose has a check valve and the gauge has a pressure release valve.
The most important thing to find is consistency between cylinders so it's very important to maintain consistency during the tests. The engine should be warmed up, the throttle should be blocked open, all of the spark plugs should be removed, and a battery charger should be connected and set on the lowest setting.
Parts expand, fit, and seal the best when they are near normal operating temperature. Warm oil is thinner, flows better, an ensures hydraulic lifters will be pumped up and opening the valves fully. Each cylinder is an air pump. You want them to pump and squeeze as much air as possible. The throttle blade is a restriction to air flow, so blocking it open removes that restriction. With three spark plugs left in the engine, it will crank slower causing reduced compression readings. A weak cylinder will let the engine spin faster momentarily resulting in the good cylinders reading a higher percentage better than the weak one. That error is avoided by removing all of the spark plugs during the testing. Any battery will start to run down during the extended cranking. That will cause the last cylinder tested to read lower than the first one. The battery charger makes up for that. Use the lowest setting. The higher ones will provide a lot more voltage than the battery would by itself resulting in false readings.
With older cars with carburetors, it was simple to disable the spark, and holding the throttle open caused no fuel to be drawn in. That meant you could just use the ignition switch to crank the engine. With your car, disabling the spark will result in setting diagnostic fault codes and turning on the Check Engine light. The fuel injectors will still pulse too resulting in flooded cylinders and raw fuel washing the oil film off the cylinder walls. That film helps the piston rings to seal. Losing that oil will reduce the compression readings due to leakage and it will slow the engine cranking speed down due to lose of lubrication.
You can avoid these things by not using the ignition switch to crank the engine. Instead, you can use a remote starter switch connected between the small wire on the starter and the battery positive terminal, or you can pop the cover off of the starter relay and squeeze the contact. Either of these methods will crank the engine without turning on the electronics. It is very important to crank the engine the same number of revolutions for each cylinder. I prefer five because by that time, the maximum reading will have been reached and there's no point in cranking longer. By having all of the other spark plugs removed, you will be able to easily hear when the cylinder under test comes up on the compression stroke. That makes it easy to count the revolutions.
Now that you've done all that, I doubt it's going to help with your stalling problem. A single low cylinder can feel like a misfire but it would have to be very low. If all of the cylinders are low, that will not cause stalling of a running engine but it would cause lower than normal power. For an engine to run, you have to have five things. There must be fuel, air, spark, compression, and they must occur at the right time. A loss of any one of those will result in the engine not starting, but only the fuel and spark can be intermittent causing stalling after it has been running. Timing can change, typically from a jumped timing belt, but it won't magically jump back again to its proper setting. Loss of compression is caused by a failure of the valves or piston rings to seal properly. A leaking valve won't suddenly fix itself. Air flow can be restricted, but you've already identified the single problem cylinder. Everything in the fresh air intake system affects all four cylinders together, not just one.
That leaves a loss of spark, fuel, or compression for your dead cylinder. Most of the parts you listed, (water pump, thermostat, timing belt, and sensor), have nothing to do with a single cylinder misfire. The new spark plug and wire eliminate part of the system but you still have to look at the coil pack or distributor cap as a possible cause of loss of spark. Problems there can be intermittent. You can have a defective injector causing loss of fuel to one cylinder. You can switch it with the injector from a different cylinder to see if the misfire moves to that other cylinder. If the same cylinder is still misfiring, and it has spark, look at the compression readings. If it is low for that cylinder, suspect burned valves, but that alone won't cause stalling. Many engines will run just fine other than the puffing sound at the tail pipe. If the compression reading is normal for the problem cylinder, it may be necessary to remove the valve cover and inspect the camshaft lobes to see if one is worn down. That will cause that valve to not open fully. As long as it is still opening its valve just a little, compression readings for that cylinder will be normal.
The symptoms you described do not point to a compression problem. The hard-to-start / dying are caused by something affecting all of the cylinders at the same time. That would be fuel delivery, spark, and timing. Spark is usually there or it isn't. Hard starting due to a spark problem would typically be caused by weak spark, not missing spark. That would lead to a coil problem. The fuel system should maintain pressure for weeks after the engine is turned off. If it bleeds down due to a leaky injector, pressure regulator, or check valve in the pump, the engine won't start until pressure has had time to build back up. That happens slower than normal during engine cranking because battery voltage is drawn down by the starter. A clue to fuel pressure bleeding off is it will start right away if you cycle the ignition switch to "run", then "off", then back to "run" a few times before cranking the engine. Each time you do that, the pump will run for a second or two and build up fuel pressure. If the engine starts instantly when doing that, suspect a leaking injector as the most likely culprit.
A jumped timing belt will also cause hard starting and very poor engine performance. That may not show up with a compression test or it could result in low reading on all cylinders. The valves will be opening and closing a little late but not significantly so as to affect the readings, but during operation, the late spark will fire the fuel in the cylinders when the pistons are already on their way down. The fuel / air mixture won't be squeezed so tightly so it won't develop much power when it ignites, and some engine power is used to drag the pistons down before the cylinders produce any additional power. The best way to find a jumped timing belt is to remove the covers and line up the marks to visually inspect them.
A more informative test after performing the compression test is the cylinder leakage test, (leak down test). That involves placing each cylinder at top dead center on its compression stroke, one at a time, then forcing regulated compressed air in through the spark plug hole. The gauge on the tester will show the percent of leakage, and you can listen to find the cause of the leakage. Hissing heard at the tail pipe indicates a leaking exhaust valve. Hissing at the air intake system is due to a leaking intake valve. Worn piston rings will cause a hiss to be heard at the oil fill cap or dipstick tube. Bubbles in the radiator point to a leaking head gasket or cracked cylinder head.
After performing a compression test, a follow-up "wet compression test" can also identify worn piston rings. That involves adding a teaspoon of engine oil to the cylinder in question, then repeating the test. The oil will temporarily seal the rings so if they are the cause of leakage, the new compression reading will be higher. Failure to see an increase in the reading points to leaking valves.
Based on the symptoms you've found, you might have multiple problems. For the hard starting, I'd look very closely at the timing belt. If the tensioner pulley wasn't adjusted properly, taking up the slack in the belt could cause the camshaft sprocket to turn a tooth or two. If the timing was off far enough at one point during the repair process, turning the crankshaft or camshaft individually would cause the pistons to hit any open valves and likely bend them. That would cause a severe loss in compression. If the bent valves are the result of a jumped or broken timing belt while the engine is running, you'll usually have damage in all four cylinders. If the damage happened while turning the crankshaft by hand, the valves might be bent in only two cylinders. The compression test would show typically 0 - 30 psi with a bent valve where 120 - 160 psi is normal, depending on the engine. A cylinder leakage test would show near 100 percent leakage while less than 10 percent is desired.
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Sunday, September 5th, 2010 AT 5:44 PM