Relative Compression Waveform
I recently picked up a Pico 2204a, Hantek 600a current clamp, Hantek HT30 leads, and a set of alligator clips. I tested relative compression on my 2015 Jeep Wrangler 3.6L, and would appreciate any input from the resident experts on the interpretation of the results. I wasn't expecting to find a problem, but it appears to me that I may have. I have attached screenshots of the traces below with red trace showing amperage, and blue showing voltage. I have applied a 1khz filter to all of the traces. The firing order is 1,2,3,4,5,6 with 1,3,5 on RH bank and 2,4,6 on LH bank.
In analyzing the traces, I am noticing that there appears to be a short duration and magnitude drop in amperage that is regularly showing up near the top of the compression stroke. I first did the test on a cold engine which resulted in this drop showing up on every cylinder, then I redid the test with the engine at operating temperature, and this drop appears to show up on just about every other cylinder. Another thing I notice is that the amperage peaks are not equal cylinder to cylinder, however, there doesn't seem to be enough of a pattern to say definatively which cylinders are low on compression. The last item of concern is in relation to the starter solenoid because I notice a short duration of amperage dropping to zero between key on and starter drawing full current when I thought this should look like a gradual current ramp.
The engine itself has 120K miles, and the LH cylinder head as well as all rockers/lifters were replaced by a dealership at 85K miles due to ticking from the valvetrain but no misfire code. The only symptoms of an engine problem currently are that it will ping while running 87 octane fuel, and it feels like it might be very slightly down on power compared to when it was new, although that could be my own bias. There are no current fault codes, however, I do occasionally see 1 or 2 misfires pop up in either cylinder 2, 3, or 6 on the scantool live monitoring when I first start the vehicle that eventually go away when cruising. Long term fuel trim Bank 2 shows -7% when cruising, and Bank 1 shows +.8%. Vacuum test with a mityvac showed a pretty steady 20 inhg.
Can anyone comment on what might be causing these poor looking waveforms? Could this possibly be caused by the test equipment/software setup, or am I definately looking at a mechanical problem with my engine?
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I look forward to hearing from some of the other forum members. (Matt T, Weycraze, Tyler...)
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Do you think these traces warrant further investigation with a compression and leakdown test, especially since the peaks are not even cylinder to cylinder?
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This is a VVT engine. So, the possibility of overlap, or not, is there.do you know if this engine is designed that way, or would this be considered a malfunction?
My best guess would be a valve sealing/timing difference between a cold and warm engine.Any idea why this seems to occur on every cylinder with a cold engine, but then move to every other cylinder with a hot engine?
Whether they warrant further investigation, or not, is for you to decide. They, certainly, spark curiosity. As a minimally invasive test, I would be interested in seeing intake and exhaust pulse waveforms, added to the RC waveform.Do you think these traces warrant further investigation with a compression and leakdown test, especially since the peaks are not even cylinder to cylinder?
Since it becomes a bank-to-bank issue when warm, and you have had one head replaced, I would capture a running compression waveform from one cylinder, on each bank, when the engine is cold. And, then, capture them again when the engine is warm. Try to keep the RPM's the same during all captures. Then, you could overlay the waveforms on top of each other to look for differences.
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
I have learned more by being wrong, than I have by being right.
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Do you have any idea what would be causing the amperage to drop to zero for that short period before starter solenoid contacts close but after key on? I thought this was supposed to look like a gradual ramp on a healthy starter.
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It can be easy to go down the diagnostic rabbit hole even for the most experienced and best trained scope users.
This is exactly why you practice on known good engines, to become familiar with the variables you may see in known good patterns, which in my opinion is exactly what you have there.
Yes, it's not a textbook example of a perfect relative compression pattern, but you have no tangible symptoms of misfire, low power, trouble codes etc. By all means, continue scoping! Get all the data you can mine, but frame it in the perspective that the car is running well.
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My guess is that you are seeing a normal battery draw. Then, as you turn the key, the ignition switch momentarily interrupts power/current flow to a module? What position is the key, at the start of the waveform? Where is your amp clamp located? Again, play around, for fun and learning. But, don't go chasing the rabbit.Do you have any idea what would be causing the amperage to drop to zero for that short period before starter solenoid contacts close but after key on? I thought this was supposed to look like a gradual ramp on a healthy starter.
There are several people in the forums on facebook that sell them. Here is a link to one that has a website. Many people make them DIY, at home.Any recommendation on a pulse sensor so I can check the intake/exhaust pressure?
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
I have learned more by being wrong, than I have by being right.
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What would make that decision very easy would be to identify another developing valvetrain/head issue, or other major engine issue that will need to be addressed in the future. I guess the main goal is to attempt to determine if this engine is healthy enough to deliver another approximately 180K miles of operation without needing another major internal repair outside of warranty. I would plan to drive it for at least 12 more years if I decide to keep it, which would put the mileage at approximately 300K.
I wouldn't say that there are absolutely no symptoms of a potential problem though because it does ping on 87 octane when accelerating hard between approximately 1500-2500 rpm, has a couple seconds of valve clatter on startup, has a slight roughness at idle, and might be a bit down on power. Running 89 or 92 octane pretty much clears up the pinging, but the owners manual recommends 87 octane fuel. The idle and power issues have been fairly minor up to this point, and I'm not sure someone else would notice this without being told about it beforehand. It's hard for me to say exactly how bad this is compared to when the vehicle was new because of the tendency of a person to get used to a slow degradation in performance over time without noticing it.
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Purely because I already have the expensive tools and access is fairly easy if it were mine and purely out curiosity I would do all the fancy tests but the thing is running not giving you issues .
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