'11 GMC Acadia - Mechanical weirdness!

I could use some fresh eyes on these waveforms.

2011 GMC Acadia was brought in with severe engine noise. I don't have a video, but it's bad. :ohmy: Rod knock or something close to it. Oil level is full. No external leaks. Stored codes look like this:



History miss counts on #1 and #6. The shop has already stuck a fork in it and ordered a long block to replace it with, BUT I wanna know what I can determine from some scope testing. Red is relative compression, green is a Cody's sensor in the intake and blue is a secondary ignition sync on #4:



Here's the same thing zoomed out for whatever it's worth:



For me, the relative compression is the really goofy part. Firing order is 1-2-3-4-5-6.



It's almost like the #1 compression stroke began too soon? :huh: Intake valves closing early? Or, is the rod bearing spun and the piston isn't going through it's full stroke? Just speculating.

Let me know your questions/comments/thoughts. Yes, I can redo these captures with a Pico if you insist. :silly: It's just a lot harder to get the Pico out into the parking lot than the Autel.

I'll be doing a post-mortem on this engine before the core gets sent back, so place your bets on what's wrong now.

Do the peaks on the green trace indicate low pressure (suction) and the valleys higher pressure?

juergen.scholl wrote: Do the peaks on the green trace indicate low pressure (suction) and the valleys higher pressure?

Positive voltage is positive pressure (changes). Negative voltage is vacuum. Sorry, didn't include that. :blush:

Engine will speed up from lower compression cylinders and with 4 of them low the next two cylinders seem to get the compression increase. Looks like head gasket went or warped head perhaps both. Any pressure buildup in coolant tank or radiator?

The amperage draw around 265A for #1 and #6 looks really high (too high indeed). It does not look like these two cylinders are binding, otherwise the companion cylinders would show higher amperage draw as well on their respective compression strokes while #1 and #6 are on the exhaust stroke.

I'd like to see an RC with a pulse sensor on the dip stick tube.

Hardtopdr2 wrote: Looks like head gasket went or warped head perhaps both. Any pressure buildup in coolant tank or radiator?

Possibly? I'll get a radiator pulse waveform when I go to get the next round of waveforms.

The amperage draw around 265A for #1 and #6 looks really high (too high indeed). It does not look like these two cylinders are binding, otherwise the companion cylinders would show higher amperage draw as well on their respective compression strokes while #1 and #6 are on the exhaust stroke.

Good thought, sir. I hadn't considered companion cylinders.

I'd like to see an RC with a pulse sensor on the dip stick tube.

I'll grab that on the next go-round, too.

Do you have a piezo sensor to sync the engine noise?

Tyler wrote: Yes, I can redo these captures with a Pico if you insist. :silly: It's just a lot harder to get the Pico out into the parking lot than the Autel.

Can't really tell what's going on without cursors.......

If you did a *.MSdata save of the waveform post it and I'll see if I can open it.

I looked up that EBCM code and it's related to intake vacuum. Might be a rabbit hole??

Geez, y'all are demanding. :lol: I'll see if I can get back to this tonight or tomorrow. I'm trying to avoid running it more than needed - it sounds that bad.

Tyler wrote: Geez, y'all are demanding. :lol:

You know how we are :lol:

I put a ruler on the trace and it looks like RPM starts dropping on #5 compression stroke. If you get the Pico out I'd like to see RC, Ignition sync, CKP and piezo in the tailpipe. Psdata not just a screenshot.

Finally got back to this one for some more waveforms. Had to grab these while we were shutting down the shop Friday night, so I didn't have time to fill all the requests. Didn't have time to get a crankcase or exhaust pulse waveform.

Couldn't cut the .psdata files down enough to get them under 5 MB for Kunena. PM/email me and I'll send them.

Cranking intake, radiator, relative compression and sync on #2:



Then there's the running waveform with a Honda single wire knock sensor held on the valley of the block, plus intake pulses and #2 sync. First, zoomed out for perspective:



I zoomed in and added some Pico phase rulers:



Then I added ETO for good measure:



My eye sees the knock happening when #6 is transitioning between exhaust and intake. OR, #3 starting its power stroke. Thoughts?

Tyler wrote: My eye sees the knock happening when #6 is transitioning between exhaust and intake. OR, #3 starting its power stroke. Thoughts?

It does appear to be right at TDC for #3 compression and #6 exhaust. #6 is also the first high relative compression cylinder. Maybe #6 rod is banging when it's unloaded at TDC exhaust and binding when it's loaded on compression??

Matt T wrote: It does appear to be right at TDC for #3 compression and #6 exhaust. #6 is also the first high relative compression cylinder. Maybe #6 rod is banging when it's unloaded at TDC exhaust and binding when it's loaded on compression??

I like that. I might try unpluging the #6 coil and seeing if the knock doubles in frequency.

Tyler wrote: I like that. I might try unpluging the #6 coil and seeing if the knock doubles in frequency.

It looks to be happening right at TDC so if it is #6 rod it's probably compression pressure preventing the noise, not combustion pressure which occurs during the power stroke. So I don't think unplugging the coil will change anything. Removing the spark plug might.

Looks like some pressure pulsing in radiator for the low pressure cylinders. Not looking good with the knocking sound if its a wrist pin or a bearing it seams like head damage has happened either from piston hitting head or o overheating

So, who had their bets on the #6 rod bearing? Because you just won... well, bragging rights, mostly. :silly:

drive.google.com/file/d/1rpWxq-f__8YQowm...y5n/view?usp=sharing



Stick a fork in it, 'cause it's done. :lol: Sorry about the video quality. Something about Streamlights really screws with my phone camera.

Thanks to everyone that replied! The old block will be hanging around for a few days if anyone else has questions/suggestions for further inspection.

Appreciate the feedback!

Did you ever repeat the knock sensor test with spark disabled on #6 or spark plug out?

juergen.scholl wrote: Appreciate the feedback!

Did you ever repeat the knock sensor test with spark disabled on #6 or spark plug out?

No. :blush: Sorry, sir. Stuff has been bananas at work since the initial diag happened. I was thrilled that we finally got around to this job at all. :silly: