Keep in mind each ignition coil fires two spark plugs. To have only one spark plug misfiring, it would need to be fouled or shorted, then the other plug would have a really strong spark. If the coil was partially-shorted so it was developing less than normal voltage, there would be no spark at either of those two plugs. To have spark at one plug, you have to have current flow, and to have current flow, it has to have a complete circuit, meaning through both spark plugs, both wires, the cylinder heads, and the ignition coil. A break anywhere in the circuit would cause loss of spark at both plugs.
Where I would start next is with a cylinder balance test. Years ago, with carbureted engines, we simply grounded one spark plug wire at a time and watched how much the engine idle speed dropped. If it was the same for all cylinders except one, that one cylinder was diagnosed further. Today, we can unplug an injector, which is easier, but when idle speed drops, the Engine Computer compensates by raising the speed back up. We can't use idle speed as the indicator, but we can look at the idle "steps" on a scanner.
The engine computer pulses the automatic idle speed motor to one of 256 positions corresponding to the desired idle speed. For a properly-running engine, step 32 is typical. With a single-cylinder misfire on a V-8 engine, you can expect to find it at around step 50. That means the computer opened the air passage around the throttle blade a little more, and it increased the amount of time the injectors are pulsed open, to bring idle speed back up to where it should be. We can use this for the cylinder contribution test, aka, cylinder balance test.
You will need a scanner for this so you can view live data. Look for the "AIS steps" and observe the step number currently requested. For this sad story, lets say it is on step 28. Now unplug one injector, then watch what step number is needed to get the idle speed back up with only three working cylinders. Suppose you find step 62. Reconnect the injector and verify it goes back to step 28. Do that for the other cylinders. You should see pretty close to step 62 for each one. If one cylinder only goes to step 40, it was not developing as much power as the other three. Now we have to figure out why.
You already replaced the spark plugs and wires, and you switched two injectors. That leaves compression and timing. Timing is not going to change for just one cylinder, but compression can. A compression test can verify it is low on a cylinder, but it will not tell you why. That is where the cylinder leakage test comes in. This involves forcing compressed air into the cylinder, through a tester with a built-in pressure regulator. Its gauge will show the percent of leakage, then you can observe where the leakage is showing up.
You place the piston at top dead center on the compression stroke. There is a whistle you can screw into the spark plug hole to make this easier to find. It will whistle when you turn the crankshaft by hand and the piston is coming up with the valves closed. It will stop whistling when you near TDC, then you have to watch the timing mark on the vibration damper and set it right on TDC. Remove the whistle, then screw in the hose for the tester. It is similar to the hose for a compression tester, but it does not have the check valve. Connect the compressed air, then observe the gauge. Normal leakage for a cold engine is perhaps around ten percent. If there is a problem bad enough to cause a misfire, you will find the leakage much higher, like 25 to 40 percent. If you find high leakage, there are four places to look. Listen for a noticeable hiss at the tail pipe. That indicates a leaking exhaust valve. Listen at the throttle body. Leakage there is due to a leaking intake valve. Listen at the oil cap or dip stick tube. That relates to excessive leakage past the compression piston rings. Look for bubbles in the radiator. Those point to a leaking head gasket.
If the cylinder leakage is acceptable, the only thing left is a valve train problem. Some engines were noted for worn camshaft lobes due to inadequate oiling. That is not a common problem on your engine. Instead, I would be looking for a collapsed valve lash adjuster or even a broken rocker arm. Unlike hydraulic lifters that make a clicking noise when they are not pumped up, the lash adjusters in your engine may not make any noise, but they can prevent a valve from opening fully.
For my last comment of value, the flashing check engine light means too much unburned fuel is going into the exhaust system where it is going to overheat and damage the catalytic converter. You are supposed to stop the engine right away to prevent that expensive damage. Most people suspect a spark-related problem, and they are usually right, but if there is a big enough vacuum leak, the front oxygen sensor will see that as an excessively lean condition, and the computer will respond by adding fuel to all cylinders. No matter how much fuel it adds, there will still be that unburned oxygen in the exhaust. You would need to watch what the front oxygen sensor is reporting to know if a vacuum leak is suspect. The additional clue is a vacuum leak will cause engine idle speed to increase. The computer will respond by reducing the number of AIS steps to bring the speed back down to normal. If you see a low step number, start by looking for a vacuum leak.
Tuesday, December 27th, 2016 AT 3:11 PM
