[FIXED] Looking for a second opinion, in-cylinder capture

Oldsmobile Alero, 3.4L, misfire complaint, audible compression problem during cranking :lol: I take the opportunity to break out the in-cylinder transducer and get a running waveform:





I *think* I'm looking at an intake valve stuck closed. Yeah, it's low compression, but it's also going into deep vacuum in the expansion pocket and the intake stroke. No way we're going into that kind of vacuum with a burned valve or a hole in the piston.

I also see a very gradual slope coming off the exhaust stroke and into the intake stroke. Usually, it's a pretty sharp change in pressure when the intake valve opens, and this is much more gradual.

What say you? Am I on the right track? I'm pulling the valve cover off tomorrow, so we'll find out anyway, but I'm mostly interested to know if you think my interpretation of the waveform is correct. Thanks!

I wish I had some feedback to offer, I don't get a chance to play with my transducer as much as I would like.

I see what you're saying about the massive vacuum, and that certainly supports a stuck closed valve. Just trying to tie the low compression into that..
What is the voltage to pressure conversion?
Curious to see how this one shapes up, keep us posted!

Don't own a transducer yet so hope I'm not wasting people's time too much but could it be a worn cam lobe or collapsed lifter? Is it feasible the lack of air in is enough to limit the compression pressure that much?

Sounds good to me.
I also don't own a transducer

On a rocker arm engine I have seen a push rod jumped out from the rocker, I have also seen a snapped camshaft.

I have also seen those engines lose a rocker, but the ones I have seen have made enough noise to let you know something was broke in the cover.

Actually, now that you mention it, I've seen these pull the threads for the rockers right out of the heads.
Saw a guy open his up many times, every time he fixed one, another one would pull out a few months later.
Might have been a 3100, but same thing as the 3400.

Yep, design flaw I suppose, but this may be a great visual of that.

Thanks for all the replies!

Noah wrote: I see what you're saying about the massive vacuum, and that certainly supports a stuck closed valve. Just trying to tie the low compression into that..
What is the voltage to pressure conversion?
Curious to see how this one shapes up, keep us posted!

Oh crap, my bad! The conversion is 1mV per PSI.

As for the low compression, I'm thinking along the same lines as Matt:

Don't own a transducer yet so hope I'm not wasting people's time too much but could it be a worn cam lobe or collapsed lifter? Is it feasible the lack of air in is enough to limit the compression pressure that much?

Agreed, I think it's the lack of an intake stroke that's preventing compression. Worn cam lobe or lifter problem is on my mind, but my money is on...

I have also seen those engines lose a rocker, but the ones I have seen have made enough noise to let you know something was broke in the cover.

Actually, now that you mention it, I've seen these pull the threads for the rockers right out of the heads.
Saw a guy open his up many times, every time he fixed one, another one would pull out a few months later.
Might have been a 3100, but same thing as the 3400.

Yep, exactly what I was thinking! I did hear a bit of abnormal noise from the engine at idle, but nothing like the marbles-in-a-tin-can racket you usually get when a stud pulls out. Maybe it's just found a home at the bottom of the valve cover? :lol:

On a rocker arm engine I have seen a push rod jumped out from the rocker, I have also seen a snapped camshaft.

Nice! This reminds me of another Chevrolet that shoved it's pushrod through the rocker arm, stuck valve. Amazing that the pushrod didn't bend :blink:

Sorry guys, I haven't yet had a chance to take this one apart yet I'll get to it ASAP and let you know what I find!

I love these little discussions.

I have never seen an olds so not familiar with the exact design.

Would it be wise to do a leakdown test before disassembly. If it's low comp but little leakdown then that proves our theory. Obviously if it has huge losses then there's other issues. Just typing that though a leaking cylinder may not be able to pull such a large vacuum though?

Right, I should have caught that :blush: . No intake, no air to compress, viola! Low compression.

matt.white wrote: I love these little discussions.

I have never seen an olds so not familiar with the exact design.

Ah sorry, I could have been more specific about the vehicle :blush: One of these fine automobiles .

Would it be wise to do a leakdown test before disassembly. If it's low comp but little leakdown then that proves our theory. Obviously if it has huge losses then there's other issues. Just typing that though a leaking cylinder may not be able to pull such a large vacuum though?

Nice thinking, sir! I actually did do a leakdown test after getting the in-cylinder waveform. Partly to confirm the theory, partly to get more ammunition for pulling the valve cover. Came out at 10-15% leakage.

If it turns out that it is an intake valve problem, then I'll feel more confident in connecting the high vacuum with low cylinder leakage

Right, I should have caught that :blush: . No intake, no air to compress, viola! Low compression.

No worries! I honestly didn't know what to make of this waveform at first. I was convinced there had to be a leak, but couldn't explain why the exhaust valve opening was so clearly defined. I'm still a long way off from feeling truly confident with the pressure transducer :blush:

Don't apologise man. I love a sheltered life on this little convict island. Haha.

So we're all on the same page on this. Rip into it then and report back.

Got some sweet cars on that convict island.
Thinking v8 Ford Falcon. I'd LOVE to own one of those over here!

Yeah they're ok. They did a turbo 6 which is sweeter. I prefer commodores which is the basis for your camaro from what I understand. Last falcon was made around a month ago. Last commodore next year. No more local production unfortunately.

matt.white wrote: Yeah they're ok. They did a turbo 6 which is sweeter. I prefer commodores which is the basis for your camaro from what I understand. Last falcon was made around a month ago. Last commodore next year. No more local production unfortunately.

Chevy had to go continents away to fight the mustang :evil:

The fleet only Chevy Caprice some of the police run here are I believe are also a Holden of some sort

Yeah forgot them. They're a statesman which is just a lwb commodore. Sorry to go off topic.

Yeah, right. It happens. Wonder what's happening (or not happening) under that valve cover.

How dare you guys derail my thread! Jerks!

Sorry for the delay, but finally tore into this one Friday morning...

drive.google.com/file/d/0BwIz-f3J815JeVl...UVk/view?usp=sharing



Yup, pulled those threads right out :lol: A thread insert and some Loctite later, and it's back to making compression. How long 'till another rocker gives up? Who knows.

Thanks for your thoughts and input, everyone!