Confirming a Timing issue with In-cylinder and Cam/Crank correlation waveforms

2012 Chevy Malibu 2.4

Complaint: Rough Idle, Low Power, and CEL (P0016 and P0017) Customer stated that a friend installed a new timing kit, about 1 1/2 years ago.


In-cylinder pressure waveform shows an abnormal pressure spike in the exhaust stroke, indicating early EVC. Running compression is 190 psi.





Next, is a capture of Cam And Crank Correlation





Known Good Cam And Crank Correlation




Subtracting the Known Good values from the Suspected Bad values gives us a difference of 27.75° on the Intake Cam. And, 27.5° on the exhaust.

Reading the waveform from left to right, the Suspect timing events are happening BEFORE they should. This timing is advanced, confirming our suspicions of the In-cylinder pressure waveform.

The customer "OK"ed the repair. I pulled the valve cover off and saw one of the most sludged-up engines I have ever seen (this one ranks, probably, #3). You can, also, see the wear marks on the cam lobes.

The picture doesn't, really, do it justice.



I called the customer in and voiced my concerns about oil pressure/circulation, and recommended that he consider other options. He wanted me to continue with the timing kit, and accepted that there would be no guarantee/warranty.

Continuing on, I saw that all of the timing components were, in fact, new with the exception of the phasers. However, both guides were broken, and the crank shaft was off by two(?) teeth.

360°/23 teeth = 15.652° per crank sprocket tooth.


Everything back together, and this is the In-cylinder waveform:



Notice there is, still, a small hump at the end of the exhaust stroke. I believe this is due to the cam lobes being worn. ( I checked 3 out of 4 cylinders. All of them had this hump.) Worn cam lobes will cause a valve to open late, and close early.

To confirm timing, I re-captured Cam and Crank correlation.



Exhaust cam matches the "Known Good", exactly. The Intake cam has a few degrees difference. I have checked multiple "Known Good" files. They all have a few degrees of variance.


In-Cylinder - BEFORE





In-Cylinder - AFTER

That thing looks sad! Short trips, not making it up to temp and lack of maintenance?

I've noticed a similar rise in pressure at the end of the exhaust stroke on engines with no apparent issue, but never really had the time to investigate the cause.
I suspected VVT making minimum (no) over lap at idle to minimize EGR effect, but don't have any proof. Worn cam lobe makes sense.
Did you notice if the other cylinders had the same rise in pressure?

Thanks for sharing Chad, quality work as always!

Did you notice if the other cylinders had the same rise in pressure?

I checked Cylinders 1, 2, and 3. The Hump was there in all 3. Then, laziness got the best of me, and I assumed cylinder #4 was the same.

I, also, captured Intake Manifold pressure, and Tailpipe Exhaust. Relatively, all cylinders look good? The overlap "M", in the Exhaust waveform, is there. But, it's smaller than I am use to seeing. I will capture a Know Good, sometime, and compare.

I suspected VVT making minimum (no) over lap at idle to minimize EGR effect

That's good reasoning, and makes since. In this case, though, the solenoids are disconnected.

I suspected VVT making minimum (no) over lap at idle to minimize EGR effect

IIRC one of the engines you found that TDC exhaust hump on did have enough phaser travel to cause zero overlap. Difficult to say that is the cause without knowing what the PCM is commanding though.

That's good reasoning, and makes since. In this case, though, the solenoids are disconnected.

The phasers may move to their minimum overlap positions as a default when control is removed.

That exhaust pattern does appear to confirm that overlap is occurring, based on what I have read any way. I haven't used my pulse tester in the tailpipe to confirm an issue yet.