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
Here you go Robby. I filtered the RC trace and added a piston chart per Chris Martino's request.
Thanks Brin, Can You post a zoom-out (2-3 Engine cycles) to show the repeatativness of the failure? Thanks 😊
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
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.
I'm so glad you asked Chris, yes I do.
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
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
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?
Also, where did you get that overlay? It really is quite nifty.
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. AESwave's website aeswave.com/software-amp-c… The Drivability Guys Website. driveabilityguys.com I would also
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
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
Firing order is 18.104.22.168 ... so 2 and 1 are low . According to the relative compression Results . So not too familiar with vacumm waveforms ,so looking at the manifold pressure waveform it looks as if cylinder 2 isn’t having a vacumm pulse for the intake . ? Idk if I’m right in that one
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
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
Your post influenced me to add more data when checking cranking vacuum, and using in-cylinder pressure as a sync.