Basic backpressure scope test in cylinder
Here is an extreme example I diagnosed this last week. The car was a slow crank and stall shortly after. Even quicker stall if you gave it any throttle at all. I wanted to confirm what I witnessed on scan data for the short moment. I went in cylinder for a quick second finger pointing to the same problem. Sorry I didn't save the scanner data. I really wasn't thinking I would be sharing the diag. It is really a basic engine performance diagnosis jobs. Here we see during cranking the pressure was positive numbers during the exhaust valve open section of the waveform. 3 psi. I want to see nothing here. It clearly doesn't look like the correct flat line look of a good cranking waveform. So I measured the speed of the waveform by using the rulers. In the bottom right of the screen it displays the RPM of the engine. The cranking was indeed slow at 53 RPM. I expect around 170's range to be the expected-ish range so this was around 1/3 of the speed I want to see. This was captured with ign coil fuses removed.
Reconnecting the coil fuses and a minute to reset the scope and we capture this seconds after the first test. This is after the engine started but not all the way wound up. So another slow as we can get it capture of the starting pressures. The 21 psi is way off the chart of the theoretical, should be zero. This one was captured at 250 rpm, give or take a few points that don't matter on this scale. My job is to get the answer as quick as possible and move on. This was the quickest way I could, without a doubt that even the shop owner, who doesn't know anything about scope pattern, can be shown. That's the only reason they are saved. I take moment explaining the basics of the waveform and he can clearly see, as we do, why the exhaust is without a doubt plugged. Quick tests and decisive action, get in and get out. This is not the most glamorous diag tests to make case studies on ,but it is back to the basics first, most of the time. That is a known good coaching practice, "Practice like you Play".
Sorry if you came and spent your time reading this and I didn't wow you with the big mind blowing reveal case study.
It feels somewhat unusual to see camshaft rpms rather than engine/crankshaft rpms... Could it be helpful to denote it as camRPM?
Great diag! Your rpm measurement needs to be multiplied by 2 due to a crank spinning 2x on a 720⁰ cycle. This doesn't affect the results but something to keep in mind
If you feel you need to to come to the conclusion you need to then sure go ahead.
You mentioned "rpm of engine" that's all. This is half of actual engine rpm. No biggie. I just was in a situation playing with math channels for hours and couldn't figure out why my math was always half. I brain farted forgetting a crank is 2x per cycle lol.
Thanks Justin 👍 ....not many people think about a hard-start to be due to a breathabiltiy fault unless timing is shifted. It’s good to be reminded That an engine is an air pump and if it can’t exhale, it certainly can’t inhale
Nice job. I think a lot of guys would benefit from case studies that you might call a basic diag. Im assuming MAP was the scan data that gave you your ”first finger” pointing to the problem. How high did it go? or was there something else that jumped out at you on the scan tool?
It was MAF that I was watching. But more than that it was my experience and feel. It was either fuel or breathing. I did test the low pressure fuel pump. the scan data on the fuel rail sensor wasn’t convincing enough for me to rule it out.
Justin, thank you for posting! Excellent waveforms. Ray
Quick and accurate diagnosis is what being a mobile diagnostician is about. I was in and out of this shop in 30 minutes. When doing my diagnosis I’m not trying to build a case study. Where are the guys that preach about knowing what good is so you know bad quickly when you see it? I know what numbers to expect because I run this test regularly. Not just to find extreme restrictions in exhaust. I