'15 Silverado 5.3L - In-cylinder interpretation

Wanted to check with the brain trust around here to make sure I'm on the right track with this diagnosis.

This L83 will start to misfire on #7 during a WOT run, and continue to misfire for 30-60 seconds afterwards, at which point #7 starts contributing again. No, the injector is not getting shut off. Ignition waveform during the misfire shows a very low firing line, suggesting low compression. An audible relative compression with no miss present sounds perfect. Same test with the miss present has a distinct problem.

After awhile, I get the timing right and pull into my bay with the miss present. Because of the intermittent nature of the problem, I don't believe I have time to remove the valve cover to see which rocker isn't moving. The problem will have gone away by then. But I feel like I can get a plug out and the PV350 installed.

Here's a clear flood crank, followed by running at idle. 1mV = 1 PSI:



Here's a closer view of the cranking pressures:



At this point, I feel confident there's an AFM problem. Very likely an intake lifter that's intermittently collapsing. BUT, I also wonder about a failing VLOM , causing both valves to stick closed. I've never seen it, but the intermittent nature of the problem has me worried.

My question to the brain trust is: Do I have enough evidence here to confidently say ONLY the intake valve is stuck closed during this capture? And therefore that the cylinder heads are coming off. My understanding is that, if both valves were stuck closed, I'd be seeing two (low) compression events. One like normal and one in the exhaust section. The fact that the exhaust section is going to atmospheric suggests that the exhaust valve did indeed open.

"Tyler, why didn't you get the Pico out with an ignition sync and an intake pulse waveform? And why didn't you use the kPa setting on the PV350?" :lol: I wish I could have. Time was short, and the Pico was on the other end of the shop watching a parasitic drain.

Tyler wrote: Do I have enough evidence here to confidently say ONLY the intake valve is stuck closed during this capture? And therefore that the cylinder heads are coming off.

I'd say you called it right. You can, clearly, see a defined pressure change in the "expansion pocket", before the cylinder reaches BDC, indicating EVO.

My understanding is that, if both valves were stuck closed, I'd be seeing two (low) compression events. One like normal and one in the exhaust section. The fact that the exhaust section is going to atmospheric suggests that the exhaust valve did indeed open.

I believe, you are right on the money, here, too. If BOTH valves were stuck shut, there would be no distinguishing between compression and exhaust strokes. Or, expansion and intake strokes.

"Tyler,...why didn't you use the kPa setting on the PV350?" :lol: I wish I could have. Time was short

If you want, I can teach you how to push that button VERY quickly. :whistle:

Tyler wrote: My understanding is that, if both valves were stuck closed, I'd be seeing two (low) compression events. One like normal and one in the exhaust section. The fact that the exhaust section is going to atmospheric suggests that the exhaust valve did indeed open.

Agreed on seeing some compression at the end of the exhaust stroke. But what really makes me confident the exhaust valve is opening is the sharp rise to atmospheric which appears to be occurring before power stroke BDC. I doubt leakage past the rings could cause that.

Regards the VLOM take a look at the following link. I'd also pull all the DoD spark plugs to see how they look before selling anything.

www.melling.com/wp-content/uploads/2018/...-Issues-3.1.18-1.pdf

Tyler wrote: "Tyler, why didn't you get the Pico out with an ignition sync and an intake pulse waveform? And why didn't you use the kPa setting on the PV350?" :lol: I wish I could have. Time was short, and the Pico was on the other end of the shop watching a parasitic drain.

Coulda shoulda woulda :lol:

A 720* screenshot would help. And if Snappy have fixed SSC post up the capture file.

Matt T wrote: A 720* screenshot would help.

Chad wrote: I'd say you called it right. You can, clearly, see a defined pressure change in the "expansion pocket", before the cylinder reaches BDC, indicating EVO.

But what really makes me confident the exhaust valve is opening is the sharp rise to atmospheric which appears to be occurring before power stroke BDC.

Duh. :silly: I dunno why I didn't realize that sooner. Even if the pressure during the exhaust stroke was elevated, EVO is still clearly taking place. Thanks guys.

While we're talking valve events, I also noticed after posting this thread that there is no real IVO after TDC-E.

If you want, I can teach you how to push that button VERY quickly. :whistle:

I'm sorry! I'm a creature of habit. :blush:

Regards the VLOM take a look at the following link. I'd also pull all the DoD spark plugs to see how they look before selling anything.

www.melling.com/wp-content/uploads/2018/...-Issues-3.1.18-1.pdf

From the same document:


Great suggestion. If we do the repair, it's getting guides and a VLOM.

That looks just like the captures I have from some hemis with stuck closed intake valves. Looks like you nailed it to me.
Considering it's intermittent it's probably not a (completely) wiped cam lobe.

Side note, the refrigerant pressure sensors have way more resolution than that pv350 set to psi.
If you have time to play I'd like to see the difference set to kpa.

Noah wrote: Side note, the refrigerant pressure sensors have way more resolution than that pv350 set to psi.

Side side note, did you ever manage to get one of those sensors to read a consistent voltage at atmospheric pressure?

@Tyler you ever play with the mask or as some call it the Masking feature on the Picoscope?

Sounds like the boss needs a 2nd Pico or go for broke & have him get the ATS E-Scope 4 or 8 with the IEA KIT.

If he's got the $$$ they say the 8 Channel setup is the way to go. Bernie makes it look like a Sunday cakewalk but he's been using for years and years.

Did you confirm the villain yet?

One more thing I would like to throw out there for anyone just learning in-cylinder waveforms. Since this thread has a stuck closed intake valve problem, I though I would post a waveform of a NORMAL/GOOD cranking in-cylinder waveform.


In a cranking in-cylinder waveform, the intake stroke event is, usually, not very defined. The slow speed of the crankshaft does not draw the piston down fast enough to create much, if any, vacuum. The result is a, near, flat-line between the expansion pocket and the compression stroke. Many people, mistakenly, think the intake valve didn't open because they see no defined intake event. Tyler's waveform proves that the opposite is true. If the intake valve doesn't open, the in-take vacuum in-cylinder will be DEEP, and round.

The fact that Tyler's cranking waveform has a very defined intake vacuum that exceedes the limit of the Scope/Transducer is a quick/clear indicator of no IVO.

Explanation is much appreciated Chad

Matt T wrote:

Noah wrote: Side note, the refrigerant pressure sensors have way more resolution than that pv350 set to psi.

Side side note, did you ever manage to get one of those sensors to read a consistent voltage at atmospheric pressure?

Yes! I stopped using AC coupling on the scope whenever I use the transducer now for consistent measurement.

Noah wrote: Side note, the refrigerant pressure sensors have way more resolution than that pv350 set to psi.
If you have time to play I'd like to see the difference set to kpa.

Ugh, FINE. :lol: I'll take another capture if the stupid truck is still there on Monday.

Don't let me twist your arm or anything

All the cool kids are using kpa.

Truck left last week. You'll have to suffice with my lame kid PSI capture.