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So I was looking at a 06 Pontiac G6 2.4l with a complaint of low power on hills. Had been at another shop to have the transmission checked and they referred the customer to us.

Road test confirms car doesn't go anywhere fast.

I didn't really think this through when I was taking captures and failed to get a longer zoomed out view on cylinder 1, the ignition timing mark, was short on time etc...

All captures are in Park. Using a Snap-on EEPV302AT 500 PSI pressure transducer.

Cylinder 1 running compression idle to WOT:

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The end of the exhaust valve event where the intake valve opens looks weird.

Zoomed in:

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It doesn't look right but what do I know?

So onto cylinder two...

Cranking compression @ WOT:

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OK... how about running compression?

Cylinder 2 idle to 3000 RPM to WOT zoomed out:

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Let's zoom in a little...

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Still seeing a pressure spike at the end of the exhaust stroke.

Zoomed in on the WOT portion:

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Zoomed in at idle:

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At this point I ran out of time and had to pack up. I plan on taking more captures on Monday with the B1S1 O2S removed. And confirming exhaust backpressure with a transducer in the o2 bung.

So, in pictures 1, 2, 5, and 7 what exactly is happening when the intake valve is opening and the exhaust valve is closing? Is this because since both valves are open and the piston is at TDC, it's sucking more air out of the intake manifold and results in the pressure spike due to the exhaust being potentially plugged?

In picture 6 this is kind of what I expected to see with a plugged exhaust. Would there be another explanation for the pressure spike?

In pictures 3 and 7 why are the exhaust and intake events almost the same? It looks like it's slowly building pressure.

Update

Confirmed exhaust backpressure with a gauge, around 3 PSI @ idle and 8 PSI @ 3000 RPM.

Did another in cylinder capture on cylinder 2. Even with the o2 sensor removed the waveform was similar to picture 6. Very puzzling.

After repair @ 3000 RPM

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Bonus picture off a Mini

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  • Must_Own a EEPV302AT some day. But where does the transducer go for individual cylinder measurements? O2 bungs I get, but is there a spark plug feed through or something? How (more importantly physically where) does it measure cylinder pressure on a individual cylinder from idle to WOT?
    – SteveRacer
    Jun 4, 2016 at 1:01
  • @SteveRacer It's all through the plug well you're basically doing a compression test, but on a scope. Ground the coil/wire unplug the injector etc...
    – Ben
    Jun 4, 2016 at 1:03
  • Thanks. Got it. Seems to be huge cylinder to cylinder differences, yet your blocked exhaust theory better explains the odd transition shape to the next cycle. So B1S1 is the *only× " bank " on this ecotech? Doesn't play well together
    – SteveRacer
    Jun 4, 2016 at 2:13
  • @SteveRacer Yes The Ecotec 2.4 is a single bank. No split manifold or anything like that. The differences cylinder to cylinder do make me wonder. The cylinder captures on 2 were a bit more thorough though.
    – Ben
    Jun 4, 2016 at 2:17
  • And this is a chain too, correct? Nothing that jumped a tooth without shrapnel...(sorry I'm not a GM guy)
    – SteveRacer
    Jun 4, 2016 at 3:02

1 Answer 1

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I did some searching on IATN and found known good pressure waveforms that are identical to my after repair captures. Unfortunately I can't post them here. It turns out that hump is completely normal.

After doing more captures on different cars with VVT I've found that most waveforms are unique to some degree. I guess the same can be said about non VVT engines based on the camshaft profile. Though they do look pretty similar.

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