Scope capture diagnostic game

Hans Diagnostician Salt Lake City, Utah Posted   Latest   Bounty  
Bounty
Driveability
2009 Chevrolet Cobalt LS 2.2L (H LAP) 4-spd (4T45-E)
P0300 - Random/Multiple Cylinder Misfire Detected

I'd like some feedback on these captures I took last night if you're bored. 2009 Cobalt 2.2. Misfires at 2 and a few on 3 at idle with ac on and in gear. Other tech used the escan and it said cyl 2/3 had low vacuum. Tech swapped plugs, coils, injectors, crank relearn, and injector balance test. He mentioned the valves had no carbon build up when he had the injectors out when I brought up the TSB on it.

Assuming cyl 1 is the good one and 2 is the bad one.

I know what's wrong, but I couldn't get that information from the scope captures I took. I knew for sure cyl 2/3 had low compression based on the RC and the cranking cadence, and the intake pulses weren't pulling evenly. But what I now know was wrong, doesn't show up for me!

Firing order is 1342.

The bounty is for being able to explain to me what happened, and how you determined that. I really need more training in this area!

+4
Phillip Diagnostician
Anchorage, Alaska
Phillip Default
 

Well the in cylinder wave form shows to that their is a valve issue of sorts on the exhaust valve. Let's see if the smarter people can confirm my thoughts cause I am by no means an expert.

0 Default Ð2 Bounty Awarded
Caleb Technician
Mishawaka, Indiana
Caleb Default
 

So I am no expert but ill play along. I completely agree that it looks like the exhaust valve is closing too soon on #2. However because you have 2 cylinders side by side I would like to see RC with transducer in the radiator and crankcase before I make any kind of call. But I am mostly here too learn.

+2 Default Ð2 Bounty Awarded
Doug Owner
Midland, Michigan
Doug Default
 

There are several things that can cause low compression and with just one sensor providing data the diagnosis will be difficult to pinpoint to one specific cause. We use a frequency analysis of the waveform to help identify various conditions, so if the valve is not seating fully, we will detect a high pitched "squeal" or "whistle" as the air is forced out through a small opening. We also place…

+3 Default Ð2 Bounty Awarded
Hans Diagnostician
Salt Lake City, Utah
Hans Default
 

That is an awesome technique I'm going to have to look into!

+1 Default Ð2 Bounty Awarded
Brin Diagnostician
Melbourne, Florida
Brin Default
   

Many would see the rise in intake manifold pressure and the weak pull following as a sign of an intake valve sealing concern but Harv Chan once called me out explaining that I have to consider the change in crank speed during and after a weak contributing cylinders compression stroke. Matt Fanslow explained this phenomenon during a DN video with Scott Brown. He said that due to the reduced…

+2 Default Ð2 Bounty Awarded
Jj Technician
Quincy, Massachusetts
Jj Default
 

Do you suspect the power stroke of the problem cylinders because on the intake stroke any pull on the exhaust should be less considering the intake valve is open if there is a leain exhust valve? I only ask because I'm still pretty green with this stuff and like to make sure that what I'm thinking is at least remotely correct. Also, what could cause the rise in the in cylinder exhaust pressure…

0 Default Ð2 Bounty Awarded
Brin Diagnostician
Melbourne, Florida
Brin Resolution
 

Hans, Thanks for the post. I maintain what I said about the slight anomaly in the intake trace. I'm very confident that these slight anomalies have to do with the varying crank speed that occurs when there are low contributing cylinders. What I neglected to consider initially is the unlikeliness of having exhaust valve concerns with both cylinders as Albin mentioned. I was so focused on…

+1 Resolution Ð2 Bounty Awarded
Brin Diagnostician
Melbourne, Florida
Brin Default
 

There are signs of volume loss in the suspect cylinders compression waveform's but not as obvious as one might think. I have two thoughts here. I think that the indicators for cylinder sealing concerns might be more noticeable with a cranking compression capture due to the slower crank speed but I'm not sure that would be the case this time. Those indicators that I'm alluding to are leaning…

+1 Default Ð2 Bounty Awarded
Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Your relative compression test shows 2 cylinders with low compression. Both those cylinders are side by side. What is common between those two cylinders with low compression?

0 Default Ð2 Bounty Awarded
Hans Diagnostician
Salt Lake City, Utah
Hans Update
 

He did do a block check (the blue fluid to check for combustion gasses in the cooling system), and it passed. He had done that since he was running out of things to do. A standard compression and leak down test was next on his list before he asked me to swing by. The coolant level was good when I looked at it, with no signs of over pressurizing. Didn’t matter if it was hot or cold, it misfired…

0 Update Ð2 Bounty Awarded
Albin Diagnostician
Leavenworth, Washington
Albin Default
 

The thing that jumps out to me is the low compression on two adjoining cylinders, 3 & 4. This rules out cam timing, since that would effect all 4 cylinders. It is possible that two valves could be leaking, but not probable. If leaking valves were the case, you would easily see that in your intake pressure waveform. The most likely cause of your problem is a leaking head gasket between the…

0 Default Ð2 Bounty Awarded
Hans Diagnostician
Salt Lake City, Utah
Hans Default
 

All of that went through my head too, and I may have approached it different if I was the first one to look at it, and I wasn't standing in front of students I'd never met, trying to diag a car I've never worked on with a failure I've never seen! I initially did think it was some sort of valve issue, but couldn't prove it right then. I've been thinking about how I could have read the waveform…

+2 Default Ð2 Bounty Awarded
Albin Diagnostician
Leavenworth, Washington
Albin Default
 

When it comes to upgrading our minds to the use of electronic testing methods, it is a good thing to use them first, then possibly back them up with a test we have used for years and are comfortable with. By using the electronic test first, you will hone your skills and get to the point where the mechanical compression gauge will stay in the drawer.

+2 Default Ð2 Bounty Awarded
Tom Owner/Technician
Boise, Idaho
Tom Default
 

The head gasket is failing between the two cylinders. The exhaust appears to close early, but is actually pressure induced into the cylinder from the neighboring cylinder?

+1 Default Ð2 Bounty Awarded
Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
   

#3+2 are companion cyes...when one of them is on induction stroke, the other is on expansion. With a compromised headgast (between 2+3), both contribute to the intake simultaneously...creating the “deep-pulls”

+1 Default Ð2 Bounty Awarded
Randy Diagnostician
Glendora, California
Randy Default
 

Brandon great explanation. wounder boy ha ha

0 Default Ð2 Bounty Awarded
Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
 

Thanks, I hope that makes sense

0 Default Ð2 Bounty Awarded
Richard Technician
Stony Brook, New York
Richard Default
 

I would say that 2 and 3 both showing lower relative compression would indicate a head gasket failure between the two cylinders. Your scaling on channel C was turned down to .1. Is there any reason why you had it this way?

0 Default Ð2 Bounty Awarded
Hans Diagnostician
Salt Lake City, Utah
Hans Default
 

I had the intake capture on that channel and it was way high when I took it, so I scaled it down instead of recapturing.

0 Default Ð2 Bounty Awarded
Richard Technician
Stony Brook, New York
Richard Default
 

Aahh, Ok. I thought you were just trying to make it more challenging. Lol

0 Default Ð2 Bounty Awarded
Caleb Technician
Mishawaka, Indiana
Caleb Default
 

I would love too see you swap those 2 transducers to the radiator and crankcase with RC and see what you get.

0 Default Ð2 Bounty Awarded
Stephen Technician
Gallatin, Tennessee
Stephen Default
 

The way I read it; 1-the intake pulls of all 4 cylinders are the same, 2-cyl 2 is not leaking compression. That is backed up by the even floor of the compression tower and that the intake pull is the same as the others. 3-the valve timing of cyl1 and cyl2 are equal. The only thing I can think of to account for the lower compression is that cyl2&3 have bent rods. If so, I'm not sure why…

0 Default Ð2 Bounty Awarded
Alex Owner/Technician
Longueuil, Quebec
Alex Default
   

Im confused a bit here. Judging by the rpm on the cylinder presure wave form wich is about 990 ish rpm. This is a dynamic pressure capture. Memory serving dynamic compression should be 50% of static compression. That being said it would but static compression of cyl2 around 200psi. And cyl1 around 300psi. So my reasoning is could it be possible that the issue is not a mechanical one but rather…

0 Default Ð2 Bounty Awarded
Hans Diagnostician
Salt Lake City, Utah
Hans Update
 

Well I still have a lot to learn! Thanks to Brin for the in depth explanation. It does always seem to help analyze the issues after the fact, and after you know what's wrong. One major thing I've learned I need to do is take a cranking in cylinder waveform as well. Doesn't take much longer to do, and can add more information. I also want to start using the WPS more to capture actual pressure…

+2 Update Ð2 Bounty Awarded