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Relative Compression Waveform

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1 year 5 months ago - 1 year 5 months ago #58588 by splat2030
Hello All,

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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Last edit: 1 year 5 months ago by splat2030.

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1 year 5 months ago - 1 year 5 months ago #58593 by Chad
Replied by Chad on topic Relative Compression Waveform
That is a great question. I have seen that "M" shaped pattern in Relative Compression waveforms, before. I can't offer a definitive answer. But, I have theorized that this is caused by valve over lap. It reminds me of an Exhaust pulse waveform, where it has been proven to be caused by valve overlap. However, I have never set out to prove that this pattern, in an RC Waveform, is caused by the same thing. This one, definitely, seems to be a bank-to-bank occurrence. I theorize that as a cylinder comes up on DTC compression, its companion cylinder has the intake valve opening. I suspect that has something to do with it. But, have not proven it. This one, being a Bank-to-Bank example, would be fun to play with. I would break out the Pulse Sensors, WPS, and a Piston chart. I'd just start playing to see if I could see some kind of correlation, somewhere. Adding a sync to your RC would tell us which bank is producing the "M" pattern.


I look forward to hearing from some of the other forum members. (Matt T, Weycraze, Tyler...)

"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. :-)
Last edit: 1 year 5 months ago by Chad.
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1 year 5 months ago #58595 by splat2030
Thank you for the reply. That is an interesting theory about valve overlap, do you know if this engine is designed that way, or would this be considered a malfunction? 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?

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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1 year 5 months ago #58597 by Chad
Replied by Chad on topic Relative Compression Waveform

do you know if this engine is designed that way, or would this be considered a malfunction?

This is a VVT engine. So, the possibility of overlap, or not, is there.

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?

My best guess would be a valve sealing/timing difference between a cold and warm engine.

Do you think these traces warrant further investigation with a compression and leakdown test, especially since the peaks are not even cylinder to cylinder?

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.

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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1 year 5 months ago #58599 by splat2030
Thank you for the input. In order to do further testing with the scope I will need to purchase additional accessories. Any recommendation on a pulse sensor so I can check the intake/exhaust pressure?

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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1 year 5 months ago #58603 by Noah
Replied by Noah on topic Relative Compression Waveform
It is an interesting waveform, but I am reminded of something I heard an instructor say before a class. Possibly John Thornton. To paraphrase, "these tests are performed for a reason, to determine the cause of an issue."
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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1 year 5 months ago #58604 by Chad
Replied by Chad on topic Relative Compression Waveform
I, absolutely, agree with what Noah says. If it ain't broke, don't fix it. I would encourage you to get the scope out and play. I would, even, encourage you to strive to understand these particular waveforms. But, as Noah said, Don't fall into a rabbit hole. I would do these tests with a goal of learning and understanding. Do not approach it as "I am trying to fix my Jeep".

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.
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.

Any recommendation on a pulse sensor so I can check the intake/exhaust pressure?

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.

"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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1 year 5 months ago - 1 year 5 months ago #58607 by splat2030
I generally agree with the "if it ain't broke" sentiment, but one of the reasons I wanted to get into scoping was to be able to identify developing problems so they could be addressed before they result in unplanned downtime or a breakdown. This vehicle is 8 years old and still retains pretty decent resale, so I am trying to decide whether to keep driving it or to sell it now and get something else.

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.
Last edit: 1 year 5 months ago by splat2030.

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1 year 5 months ago #58659 by Noah
Replied by Noah on topic Relative Compression Waveform
All tests aside, to answer the question "will my Jeep make it to 300,000 without needing major engine repairs?", the answer is NO, 100% of the time.

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1 year 5 months ago #58676 by Andy.MacFadyen
Coming from left field I wonder if this could be a starter solenoid contact issue ? but as all the peaks are the same height and it isn't giving any issues I am firmly in the if it ain't broke camp
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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