P0302 Cylinder 2 Misfire Detected
This was a new customer to our shop. Vehicle was at another shop and they advised the customer he was in need of an engine. The engine is running very poorly with a flashing CEL.
During initial diagnosis technician audibly heard compression leaking from vehicle when the engine was running. An injector seal was suspected to be leaking creating a density misfire. A smoke machine was used in the cylinder to identify a potential compression leak external to the cylinder. After the smoke machine was hooked up, smoke was present in the intake manifold on tdc compression. A mechanical problem was suspected. The customer authorized the technician to tear the intake manifold off to identify the potential problem. Before the tear down was done I wanted to prove another way what the other technician was seeing with the smoke machine. These tests and analysis were performed by me in order to prove what was wrong with this vehicle before tear down was done.
As a technician that works on a lot of vehicles that have been to multiple shops, we have to stay centered on our diagnostic routine. Being centered on a routine first means we need to have one. So many technicians out there just jump around trying to eliminate everything they can until they find out what it is by ultimately finding out what it is not. Don’t get me wrong there is nothing wrong with deductive logic, it is part of my routine too but we need a direction to funnel us into additional testing. Many times technicians want to prove what the other shop said is wrong because they think they’re better than others or that’s what they think the customer needs to hear/wants. That is not a routine….that is failure. After all, the customer is usually at our shop because the car hasn’t been fixed or they want a second opinion. All the customer really needs to hear from you is an accurate diagnosis with being able to back your results two ways. If the other shop had a logical analysis that made sense to the customer they’d probably be the one fixing the vehicle. You’re wasting your time by focusing on what somebody else says. My routine is not influenced by what another shop says. My routine is not influenced by what the customer’s mechanic says or another technician. I follow a systematic approach that leads me to the cause of failure without influence. This technique is key to mastering diagnostics. I’m not a master by any means but I believe in the tools and tests that I know. Too many times in the past I have used someone else’s input or ideas that have let me off a path that either has taken too much time or led me to replacing parts that I wasn’t able to prove to myself needed to be replaced. The best way to make any diagnostic profitable and accurate is to prove to yourself not only once but twice that part/parts you’re are trying to condemn are faulty. If we are only looking at something one way we lose the ability to fact check our reasoning. In a world where technicians are expected to be correct every time this will help us avoid that unwanted trip up to the service writer with our tail between our legs. My goal is that I want to provide the best service our industry has to offer 100% of the time. Having the best service does not always mean that you are correct 100% of the time. It means to me that 100% of the time I use techniques to funnel me into a testing plan to condemn whatever is bad two different ways.
As I continue to grow as a diagnostic technician I’m constantly learning new techniques to better understand/diagnose the systems that I’m working with. Luckily for misfire concerns I think there is a fundamental first test that we all should think about using to gain a direction as to what our next test should be. This first test for me is secondary or primary ignition analysis. Secondary and primary ignition analysis has not changed much from the beginnings of the internal combustion gasoline engines. Certainly obtaining these waveforms have gotten a little more difficult over the years with the invention of the cop coil and transistorized coil packs. However the fundamentals of all ignition systems work the same, there is power and ground provided to a primary coil with few windings, once the ground side is released a low voltage field surrounding the primary coil is induced into the secondary windings which has a lot of windings. Since there are more windings voltage is multiplied so now a high voltage in the secondary winding follows the easiest path to ground which we all hope is in the cylinder near tdc compression across a spark plug gap. That is how the internal combustion gasoline engine has worked since the beginning. So how much training is out there regarding secondary/primary analysis………….in my opinion not enough. I can’t count how many times I’ve talked to other technicians about this training and they look at me like I’m crazy. They say things like “that technique is outdated”, “there are way better testing methods out there”. Certainly I continue to push to learn the new techniques but I’ve spend a lot of time over the past year breaking ignition waveforms down to give me a diagnostic direction. I say direction because like I stated earlier in the article I’m not willing to condemn any one part/parts off of one test. I’m going to use my results from the secondary pattern to make a hypothesis on what is going on and what my next tests should be. So I grab my newly purchased cop wand that hooks to my Pico scope and start looking at this misfiring vehicle. As I go down the line of coils the pattern on cylinder 2 has a repeatable event in the waveform at park idle no load. (ref 1)
Performing secondary analysis is tricky sometimes. You really have to trust your equipment and hope what you’re seeing isn’t noise from a multitude of other contributors. One of the best ways to trust your pattern is to get primary ignition. With primary ignition you are essentially hard wired into the primary circuit. You can trust that what you’re seeing is true. However we don’t always have the time or ability to get there with transistorized cop designs. If we are going to use secondary for analysis, get to know the good cylinders first before going after the one you suspect to be bad. As we look at the pattern I’ve attached above (ref 1) we see the point of primary turn on. We see some coil oscillations and then a rise in voltage in a triangular shape to the point where primary turns off. When the primary driver is released that is where the secondary current flow begins. We have a firing line and then a burn line. As voltage continues across the plug we see little blips /rise in voltage. This can happen because of the presence of turbulence. When at idle the combustion chamber is rather stabilized compared to high load/high cylinder volumes. When we see this sort of behavior at idle we can draw some conclusions as to where our next testing should go. In order to make these oscillations in the burn line there needs to be movement of airflow across the spark plug. When a cylinder is sealed the volume is being condensed but it is not blasting past the spark plug because it has nowhere to flow. When you have an existing path for the movement of air such as a leak then it can interact with the electron field and create higher resistance for the path of ground momentarily. If the voltage goes up and down like this it means that the energy to cross the gap is going up and down. The spark is quite literally getting blown apart. This type of analysis needs to be done at park idle no load. It also should be a repeatable pattern with a dead misfire like I have. As of right now I’m starting to suspect a sealing issue with the valve train. This test does not identify what is leaking in the cylinder but it does provide me with a direction for additional testing.
I think at this point my number two cylinder has a problem with the inability to seal compression due to an audible noise and a secondary ignition pattern. But I still do not know what or where the leak is with definitive results. One of the ways I can back up my leak hypothesis a second way, and create another hypothesis on where the leak is located, is to perform an in cylinder cranking compression test with an in cylinder pressure transducer. So I remove the number two spark plug hole and crank the engine over with fuel disabled. The quickest way I disabled fuel on this vehicle was pulling the injector harness connector located near the throttle body. My results are on the following page ref 2.
In this waveform above you can see very low compression of the vertical column to the left. Not even 18psi of cranking compression. We know there is a problem here. But what if we had a manual gauge. Would a manual gauge tell us we had a leak? What if we had an intake valve not opening? Since there is no air flow into the cylinder how can the cylinder fill to make compression? It cannot fill the cylinder and we could have just as low of compression with a manual gauge with no intake valve opening. This test proves to me that there is a leak in this cylinder and it’s not a sole issue with a valve not opening. The reason I know there is a leak is due to the deep pocket developed on the expansion stroke. During a normal cranking compression test a vacuum pocket is not made due to low rpms and the throttle plate not being a complete restriction to air flow. When this pocket is developed the best way I learned to analyze this anomaly was seeing a video of this with a homemade experiment. The experiment involved
hooking a syringe up to a vacuum gauge. As the syringe was pushed in and drawn out there was a proportional movement in the vacuum gauge between pressure and vacuum. However if you released some of the pressure in the hose externally before the syringe stroked all the way closed when the syringe was drawn back open, vacuum occurred much earlier. This happens because of a loss in volume. So as we see in the waveform we have low compression because it leaked out. We also draw a deep vacuum because there is less volume in the cylinder than when we started the stroke. Now I know with audible, and two visuals waveforms that there is an internal leak in this engine affecting compression and I need to prove where the leak is at.
At this point I know beyond a reasonable doubt there is leak in the cylinder. I can show this to the customer visually two different ways and back up my previous hypothesis. But I don’t know where the leak is. Another way to prove where the leak is located is to perform a running compression test. This test does not always yield the results I need but in this case it did. During a running compression test depending on where the leak is I may have to manipulate the rpms, doing a snap wot test or a decel from higher rpms. This leak was present in the waveform at park idle no load. On the next page we will see how the waveform represents where the leak is located (ref 3).
In the waveform above there is a couple of differences I see from a known good cylinder. All data publishes the compression pressure to be between 160 and 204 psi. I’m thinking on a newer engine with low miles we should easily be hitting 180 psi. As I stated before running compression should be half to one third of cranking. So at this rate we would expect our running compression to be around 55 on the low end and 100 on the high end. Were a little bit below 55, around 53psi. So from that aspect alone that’s not telling us much. However during the cranking test we only got about 18psi so there is a substantial increase in compression during the running event. This is due to the nature of the leak and engine rpm. When engine rpm increases the leak doesn’t have as much time as cranking to push the volume out of wherever the leak is. The key indicators for identifying the leak in this circumstance is looking at the level of vacuum on the expansion pocket vs the intake pocket. Normally expansion pockets and intake pockets should be very similar if not identical in reference to the level of vacuum. There are a few engines out there that this is simply not the case but for this analysis there is a clearly defined problem. When we look at the expansion stroke we have a deep level of vacuum around -11psi or 22 inch of hg. One may think that with that kind of vacuum level it may indicate that the valve is in fact sealing. We need to know that sometimes things fail under vacuum and do not fail under pressure and sometimes things fail under pressure but not vacuum. In order to pull that kind of vacuum on this engine we would have to be on a fuel cut decel with no boost in the intake. Sitting here at idle is not going to give us those readings unless the car is broken. One way that we can identify what valve is leaking is to focus on the exhaust valve leaking. If the exhaust valve is leaking what is the exhaust valve connected to when it is open on the exhaust stroke? Well of course it is connected to the exhaust manifold. And what kind of pressure is in the manifold? Well on a normal vehicle none or atmospheric pressure. Adding fuel to the fire this vehicle is turbocharged. Turbo chargers by design need back pressure to spin the turbine. So one might theorize if the exhaust is leaking it may lower the vacuum pocket due to it drawing in positive pressure relative to the normal negative pressure during expansion stroke. So with that theory in my head I don’t think the exhaust valve is leaking. What about the intake valve??? What is the intake valve connected to when it is open? As John Thornton would explain it is connected to the intake manifold which is a temporary storage device of manifold vacuum. So if we are on the piston on its way down on the expansion stroke and the exhaust valve hasn’t opened and intake valve should be shut we start to build vacuum. But if the intake valve is leaking it’s connected to manifold vacuum which is already negative. If you add negative and negative together it becomes more negative. This is why the expansion pocket is so deep on this stroke. It’s not normal it’s because the car is broken. We also need to focus on the intake stroke vacuum level. The vacuum level there is higher than the expansion or said in a different way it’s not as deep of a vacuum. There is a reason that is the way it is as well. Sometimes when we have such a large leak in the intake when the piston is on its way up on the exhaust stroke the intake can essentially be siphoning air from the combustion chamber in to the manifold lowering the amount of negative pressure in the manifold. The piston can also push air into the intake manifold during the exhaust stroke. This is not always the case but this is what is happening here because of how bad the leak is. When you add positive pressure to negative pressure the result is a rise in negative pressure closer to 0 psi. So the differences in these pressures are because of the nature of the leak. So now I know there is a leak and I’m pretty confident I have a leaking intake valve. So for my final test I want to prove again that I have an intake valve sealing issue before the intake manifold is removed.
I think I have a leaking intake valve but I need to prove it another way to have sufficient evidence to back up my theories. The next best test to verify a grossly leaking intake valve is to perform a cranking vacuum test with an ignition sync. In the next test I gain access to a central port on the intake manifold, disable fuel and sync up to an ignition event and analyze the vacuum pulls in the manifold (ref 4).
In the above waveform I have provided a piston position chart; a program created by Scott Shotton with The drivability guys. When we think about the four stroke cycle and the ignition firing event we know that the spark plug is always trying to fire near top dead center. So to use this kind of analysis we are using the ignition event as a reference. If we are using the known bad cylinder’s secondary ignition as a sync than we can deduce that somewhere around 360 degrees later the intake valve/valves for the cylinder should open up. Using vertical cursors built into the Pico software we can identify the vacuum pulls related to each cylinder. Then we will use the piston position chart to understand where the piston is for each cylinder in the four stroke cycle. Typically when this analysis is done the sync cylinder is the number one ignition firing event. I used the number two ignition firing event because I believed that this would show me the results I wanted to see more quickly. There are two things in this waveform that stand out to me immediately. One is that the number 3 intake pull is much deeper than the rest, about twice the vacuum. The other is that a pressure pulse is created above the 0 psi line near the top dead center mark of the number two cylinder. One way to create a positive pressure pulse in the intake while cranking the engine over is to have a leaking intake valve which we have previously theorized as leaking. Another way to build a deeper vacuum on another cylinder’s intake stroke is to add additional vacuum to that because the number 2 piston is on its way down on the power/expansion stroke with a leaking intake valve. Additionally the crankshaft speed momentarily speeds up due to a lack of compression in the previous cylinder. If you recall earlier we thought that the reason why we have a deeper expansion pocket on the cranking compression test was because we lost volume in the cylinder right before the piston changed directions going from compression to expansion stroke and then started pulling a vacuum sooner than normal. We see this identically mimicked in the cranking vacuum waveform. The pressure rises in the intake manifold during number 2’s compression stroke and then the number 3 intake stroke becomes a deeper vacuum because number 2 cylinder is on its way down on the power stroke/expansion and creating a vacuum adding additional vacuum to the number 3’s intake pull event coupled by the increase in crankshaft speed. Are the other strokes effected? What about cylinder 1 and 4 intake pulls? I think they are affected. If we look at the piston position chart for number 1 intake pull what is the number 2 piston doing? Well the number two piston is beginning the compression stroke leaking pressure into the intake manifold lowering the negative pull of the number one intake stroke closer to atmospheric pressure. What is the number 2 piston doing on the number 4 intake stroke? The number 2 piston is going up on the exhaust stroke pushing pressure into the intake manifold raising the pocket of the number 4 intake stroke closer to 0 psi. All of these events relate exactly to my in cylinder cranking compression test. I have now backed my theory another way that the intake valve is leaking.
The first test with the secondary ignition analysis matched what I found with the last test, that there is a leak. The last test and the second to last test accurately identified which valve was leaking and backed each other up accordingly. The tear down on the intake ultimately identifies why we see what we see
The technician working on the vehicle removed the intake manifold and the engine was cranked over. As compression blew past my face from the stuck open intake valve I knew we found the problem (ref 5). Only one of the intake valves is opening and the other one is seized in the head and has a broken valve spring and the rocker is off the valve and lifter. I imagine the intake valve stuck due to carbon build up, over working the valve spring till it broke. The piston probably hammered the intake valve into the head and therefore probably causing substantial damage to the valve seat. Weirdly enough after the valve cover was also removed the intake rocker is missing entirely from the engine. We managed to find most of the valve spring and one keeper still laying in the head. Does this need an engine? Maybe we can get away with a cylinder head but who knows what happened to the piston and cylinder walls. Was the other shop correct? Yes they were correct because they took the rocker arm out of the engine! I do not usually see a car in for a second opinion and was correctly diagnosed. Usually the car comes in with its fifth pcm and they want us to program the sixth and we find a blown fuse! What I’m getting at is follow your routine to diagnose the problem without worrying about what someone else said/says! The tests you pick first will lead you to additional test that you may or may not need to do. If you structure your tests to give you the best value out of your time you can find the problem faster, easier and more accurately more times than not.
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