2010 Sentra S 2.0L misfire cylinder #3 low compression in cylinder pressure transducer explaination
Got this one today and wanted to share my in cylinder cranking compression capture of cylinder #3 using pico pressure transducer. Removed intake and spark plugs had spark and injector pulse. so decided to use the in cylinder pressure transducer for further analysis. Found low compression in cylinder #3. captured the waveform will share that file with other results. Did a leak down test on cylinder #3 and found one of the two intake valves was leaking badly on cylinder #3 you could feel is and hear it coming out of that port with plenum removed. My question is the captured waveform of cylinder #3 the downward swing just past 360* was that my clue ? cylinder #2 had the same result and that cylinder had a bit more compression but leak down showed that same signature at the same point. Cylinder #1 the good cylinder has a similar downward bump but much more subtle and it happens a few degrees sooner. Can someone explain what i am seeing here im still a greenhorn with this type of testing thanks in advance.
Hi, After that bumb there should be the intake pocket. As you can see there is no vacum in this cylinder... Bad news for the owner :-)
This was cranking compression not running compression i took these with the intake plenum off the vehicle.
Hi Robert, I would not be worried about that hump. Take a look at the leaning compression tower and deeper vacuum pocket at the point of exhaust valve opening.
Does cranking compression not show valve timing very well? I'm still pretty new as well, but the suspect cylinders show up with retarded valve timing. Both seem like the EV is not opening until BDC, while #1 is showing EVO much more advance where you would expect to see it. Also those downward spikes are where I would expect to see IVO, and they are also retarded in the suspect cylinders…
Error when looking at file I had rotation ruler degrees set improperly. sorry cant figure how to delete my response
I don’t think that missed anything. The lack of intake vacuum during cranking makes seeing the beginning of the intake difficult. I find the intake closing point and step back the degrees of duration of the cam to find where the valve should have opened. The exhaust valve is late compared to the other cylinder and to usual. The cam can’t be out of time on one cylinder and not on another so there…
The small bump is both valves closed on the intake stroke. The large one looks like a valve was open during the beginning of the compression stroke and not having the same volume of air during the ignition stroke.
Robert, There are 3 big clues that I see. 1) Lower compression 2) Leaning compression tower 3) A much deeper exhaust pocket in the bad cylinder These 3 data points are hard evidence of a compression leak. The deep exhaust pocket happens because the cylinder had lower pressure in it when the intake stroke started so it's able to pull a deeper vacuum than the good cylinder.
Thanks for the waveform files, Robert! Are you saying this “wandering dip” phenomenon was not mentioned in pressure transducer classes you took? Does anyone know which classes cover this topic?
I was looking at that dip again and here is what I think is happening. When the piston starts to descend after the 360 mark the cylinder starts to pull a lower than normal vacuum because the pressure in the cylinder was lower than normal at the start. When the intake valve opens the vacuum returns to what is in the intake manifold, which is very little while cranking. I believe it's just an…