Relative Compression With Cranking Vacuum; A Modern Engine Mechanical Testing Techniques Exercise
I love when vehicles with mechanical faults come into my bay but that hasn't always been the case. There was a time that I would regret it because I had a hard time efficiently proving these mechanical faults to myself or my clients.
Having the opportunity to attend classes and learn from our industries top diagnosticians and trainers in this topic like Bernie Thompson, John Thornton, Scott Mana, Brandon Steckler, Eric Ziegler and Scott Shotten definitely set me up for success. The rest was up to me. I had to put this newly acquired knowledge to the test and I did just that. After a few years of reading conversations on forums, taking online and traditional leader led training classes on this subject, I had my chance to practice these techniques on real broken cars. This was less than two years ago. It was bizarre to say the least. I rarely had vehicles with mechanical concerns until this point but all of a sudden, we seemed to get one right after the other.
I really enjoyed being able to prove the failure with these mechanical test accurately, efficiently and without getting invasive ( what I call time consuming engine disassembly for verifying mechanical failures). I'll go as far to say that I love being able to do these test but I also love sharing these test and the results with others.
I have a decent amount of captures from broken cars that I'd like to share. Some easier diagnostic cases than others. This is an kind of exercise that my friend Brandon Steckler would challenge us with from time to time and I really enjoyed it. A lot of us learned a quite a bit from the exercises and I hope that we can recreate the learning opportunities right here on Diag.net.
Here's how it will go. I share the capture, give the vehicle information, the clients description of the concern and a few details. I'd hope that we can discuss what we see in the capture as opposed to what we guess is the cause. After some conversation gets some steam, I'll ask or answer questions, guide the discussion if needed and share my findings. Please be patient with me. I may not be able to respond to comments as quickly as I'd like but I will do my best.
I'd encourage others to share captures for discussion as well. Let's share our war stories and learn from one another.
This was a 2009 Toyota Corolla with a 1.8L. This is a cut and past from the repair order detailing the clients concerns.
"Mike states that the check engine light is on and sometimes flashes during acceleration combined with a loss of power. Mike states that Toyota performed a tune up attempting to resolve the concern but it did not help. Toyota then wanted to perform a compression test but Mike declined any further testing. J.P. at Intercoastal Auto refereed Mike to us. Mike request that we check and advise. Mike did state that the temperature gauge has been fluctuating but have never gone into the red. He has added coolant."
I started the vehicle and pulled it into my bay. It appeared to be misfiring on more than one cylinder.
See the screen capture of a relative compression test with cranking vacuum and an ignition sync attached and let's talk! The blue trace is a ignition trigger for cylinder 1 used for sync, the green trace is starter current used for relative compression, and the red trace is intake manifold pressure.
Hi Brin, these are great exercises. I've learned a lot from doing these in the past too.
These are my observations:
- It appears #2 has the lowest relative compression.
- There's also a significant rise in manifold pressure on the #2 compression.
Do you think the mechanical fault is causing a filling issue in the other cylinders?
These are great observations. Relatively speaking, cylinder # 2, the third cylinder in our relative compression capture is the lowest and the red trace, intake manifold pressure, is rising during cylinder # 2's compression stroke.
Do you mind rephrasing your question? I think that I understand what you're asking but I want to be sure.
The relative compression is different for each cylinder. I was looking for yours (or anyone's) thoughts on that.
The RC is a little hard to read because there's a lot noise in the amp clamp. I generally put a lot filtering in the starter current channel to help with that.
I agree Robby. I like to turn filtering up and down on starter current traces to better visualize what's going on but I usually have the signal filtered right about where it is in this capture. I certainly do understand why you would've filtered this signal more. I guess I like to average the area that has the heaviest concentrated noise if that makes since.
Would you like me to upload this capture with more filter?
The relative compression is different for each cylinder. Thank you for bringing that up. In my experience, there is more starter current needed to push a piston upward in a good mechanically contributing cylinder following a cylinder that is contributing less due to a mechanical failure. I know that you know that but I appreciate you bringing it up. This allows me the opportunity to make sure others understand this.
Can You post a zoom-out (2-3 Engine cycles) to show the repeatativness of the failure? Thanks 😊
I think this will give those that are new to this type of testing, a better understanding of what it is you see in the waveform
Thanks for bringing that up Brandon. It can help others better visualize what is happening but it also reminds me of our conversations. You always reminded me that we needed to verify that the anomaly was repeating. This would keep us from investigating a weird one off anomaly unnecessarily.
Awesome exercise, My Friend. We all learn a lot from other Diagnostic Technician's Case Studies. THANK YOU for sharing,
Brin, In no way do I want to "Hijack" this AWESOME thread, I'm waiting very anxiously for responses of this discussion. I just thought this would be a Great time to add some information to an earlier version of the "Running Compression" test. As recent as yesterday, A tech brought to me that he thought he had heard about Running compression testing years ago ? For me personally, I was taught running compression way back in the 60's in Detroit Diesel when I worked for a construction equipment manufacturer, I also was reunited with this test at one of Jim Linder's awesome training sessions. This test was basically for seeing if the "Individual" cylinders were breathing correctly or not, as Jim Linder taught us, The manifold vacuum gauge was used to see if the ENTIRE engine was breathing and the Running Compression was used to pinpoint individual cylinder concerns.
The big difference in this running compression test is that the tech can perform this test with a conventional compression gauge. My illustrations were captured using my pressure transducer in order to understand the test better.
This is the description of this test: misterfixit.com/dyncompr.htm
basically when the throttle is SNAPPED open the cylinder should be able to INHALE approximately 80% of the cranking compression. If the snap throttle reading is less then the 80%, there is an Intake restriction and if the reading exceeds the 80%, the cylinder was able to inhale but not able to exhale properly indicating an exhaust restriction.
As you can see, at idle the running compression on this good cylinder was running at 77.03 psi and when the throttle was snapped the pressure increased to 136.3 psi. The cranking compression was about 150 psi.
As I indicated, for those without transducers and a scope, they can at least get started with in-cylinder diagnostics.
I hope this shades some light on the confusion of a running compression test that some are asking me about.
Thanks Jim! I'm all about utilizing what is in your tool box. I never heard the 80% rule. That's interesting. Thanks for the tip.
Hey Brin, do you have a piston chart you could place over this? This way I can see what's happening during every stroke.
Given the coolant consumption and the low starter draw on cylinders 1 and 2, I would have to think the rise in manifold pressure is being caused by compression from number 2 bleeding into cylinder number 1 while cylinder 1 is on its intake stroke and cylinder 2 is on its compression stroke. I don't think it's a valve sealing issue, but rather a head gasket sealing issue.
Nicolas is paying attention. I did add our clients description of the concern in my post. The last statement in his description says that the temperature gauge has been fluctuating but has never reached the red area. He also stated that he has added coolant.
This information combined with the fact that we have two cylinders with low compression located next to each other would be a good indicator that we might have a cylinder head sealing issue.
During the cylinder # 2 compression stroke, we are seeing a rise in pressure in the intake. Like Chris states, this could indicate cylinder volume leaking past cylinder # 2's intake valve during cylinder # 2's compression stroke. That part of this waveform is a textbook intake valve sealing issue but why do we have low compression on cylinder 1? Well, if you consider all of the puzzle pieces, the answer is there. If we had a cylinder to cylinder leak, meaning that volume was leaking from cylinder 2 to cylinder 1 between the cylinders during cylinder # 2 compression stroke, the volume would then make it's way into the intake manifold because the intake valve is open on cylinder 1 when cylinder 2 is on the compression stroke. This is, in fact what we found.
I want to thank everyone for their involvement in this post. I enjoyed talking through this capture and it's obvious that you guys have a great understanding of these testing techniques. I hoped for a little more participation but it was fun all the same.
Nicolas, is the winner of the chicken dinner. I'm impressed with your ability to take all of the information into account and your ability to reason through this waveform.
Thanks again everyone! I encourage others to post more thought provoking case studies, conversations, questions etc. Our powers combined will allow us to be unstoppable.
Awesome Brin and Nicolas, I totally missed that.
I was expecting an intake valve sealing issue on #2. What made me leary of that was -
- If the #2 intake valve was leaking, I'd expect to see the manifold pressure rise like we did
- I would also expect to see a deeper intake pull on #3 too
What do you guys think?
Nice catch Robby! I agree. If the intake valve was leaking on cylinder #2 the companion to cylinder #2, cylinder # 3, would potentially have a deeper intake pull. Cylinder 3 would be pulling on the intake during it's normal intake stroke but cylinder 2 could be pulling on the intake at the same time during it's power stroke through the leaking intake valve.
I guess it would depend on whether the leak would take place under negative pressures (expansion stroke). In my experience, you would have a deeper intake pull right after the rise in manifold pressure as you indicated. With that said, consider in cylinder waveforms from engines that are experiencing valve sealing faults. The reason that we often have deeper exhaust pockets than the intake pocket is due to the fact that a volume is leaking past a valve during the compression stroke with no leak past the valve during the expansion stroke.
I'm seeing pressure go into the intake during number 2 compression stroke. I am not seeing an intake pressure rise during number 1 compression stroke. I would like to say that it is the number 2 intake valve, but I really would like to see more repetition on the screen. do you have that?
Yes Chris. I did attach a screen shot with more sweep time. The attachment was in my reply to Brandon's request above.
I would be inclined to go with a head gasket issue, with the compression leaking between cylinders, however this doesn't explain the single pressure rise in the intake, I could go with a burnt valve coupled with a head gasket. How'd I do?
Great question Chris! AESWave, a Diagnostic Network Corporate Partner, sells everything needed to perform these testing techniques. Aeswave and the Drivability Guys sell the Piston Chart/overlay that you are referring to. The software is called TDC 2.
The Drivability Guys Website.
I would also encourage anyone that wants to learn more to attend training from Brandon Steckler, John Thornton, Eric Ziegler, Scott Shotten and Scott Mana. These are some of our industries heavy hitters when it comes to teaching Engine Mechanical Testing Using Oscilloscopes.
There are also some great classes put on by Corporate training companies like CTI and ATG.
John Thornton has some videos on the subject through Automotive Video Institute and Automotive Seminars, The Trained By Techs have at least one case study video using these techniques and Bernie Thompson's Automotive Testing Solutions website has a few nice videos featuring these testing methods as well.
You did great!
This failure would only cause elevated intake pressures during cylinder # 2's compression stroke. Cylinder 1 would of course leak into cylinder 2 during the compression stroke but cylinder 2 would be on the power stroke so there wouldn't be any volume leaking into the intake. Does that make since? Am I understanding you right?
Makes perfect sense. This is why a piston chart is a great idea. It also takes some getting used to so you can realize which position the camshaft is in. My leg jerk reaction is to blame a valve only. But looking at the cam position makes me see that it can't be. great one Brin!
Awesome I love these exercises as I believe they are invaluable to learn from. And why does it have to be Mike lol kidding. Now on to the waveform that you shared. With number 2 being low and the rise in your intake manifold pressure as it goes up on compression would indicate a valve sealing issue.
I'm sorry Mike. I thought that I read your entire reply but I apparently missed your last sentence until now. Your statement is correct. More specifically, it could indicate an intake valve sealing issue but as Robby stated, there would usually be a deeper intake pull immediately following the rise in manifold pressure. Read his comment and my reply and let me know if that makes since.
This would also not explain why we would have low compression on cylinder 1.
Yeah I missed some facts in your post thats my error as I failed to notice the vehicle also was using coolant. See told you I like these I learn from them
Adrian! It's great to see you on DN!
You're understanding this capture well. Cylinder 1 & 2 are low and there is an anomaly in the cranking vacuum trace during cylinder 2's compression stroke.
Remember that cylinder 2 can't be on the intake and compression at the same time. Th
I added an piston position chart in my response to Chris Martino's reply above. This chart will help you visualize what stroke each cylinder is on. Knowing what stroke each cylinder is on in relation to the relative compression waveform is necessary to pinpoint the cause of the concern.
"The pull" (descending part) in the cranking vacuum trace indicates the intake pull four each cylinder. We have a 4 cylinder engine so we should have 4 pulls and we do. There is a pull in that anomaly but we see that the trace rises more quite a bit before the pull. A rise indicates an increase of pressure and a fall indicates a reduction in pressure. Think about what could cause a rise in pressure on the intake manifold during cylinder 2's compression stroke.
A different type of failure would cause a missing intake pull.
Thanks for chiming in. The answer is in the comments but take your time trying to analyze this capture. After you're satisfied, read through the comments and let me know start you think.
Thanks Brin. Glad to be aboard . Thanks for clearing my head even more on this subject . your explaining helped my mind picture what is going on ,just still not there . So during the rise in pressure It could be from its adjecent cylinder leaking into it ?? Because from what I’m reading in the cylinder position chart while number to is in compression number one is in intake ?
Yep, cyl 2 is on compression, both valves are closed. As pressure rises, it leaks through the head gasket into the next cylinder that is on the intake stroke. This leaks out the open intake valve and creates the pressure rise in the intake manifold. When cyl 1 is on its compression stroke, the pressure leaks past the head gasket again to cyl 2, which is on the power stroke, filling that with air. This is then expelled during the cyl 2 exhaust stroke. You can see a slight current rise there as well. It may be due to the pressure (instead of a slight vacuum) in the cylinder. Or not. I could be crazy. A transducer in cylinder would be able to tell for certain.
Your post influenced me to add more data when checking cranking vacuum, and using in-cylinder pressure as a sync.