Cylinder Wave Form Diagnosis

Nicholas Diagnostician Virginia Posted   Latest   Edited  
Unsolved
Pico Technology
Driveability
2011 Subaru Outback 2.5i Limited 2.5L (EJ253) (TR690) — 4S4BRCLC8B3375931
Bad Diagnostic Strategy?
Pressure Transucer

I very frequently use my pressure transducer in-cylinder, and have used it in-cylinder to look at exhaust back pressure. I have successfully diagnosed and repaired exhaust back pressure problems looking in-cylinder dozens of times, but today I was proven wrong. 

Attached is pico scope captures of the old converter in-cylinder on driver's bank and the new converter in-cylinder drivers and passenger's bank as well as pressure transducer in the upstream A/F ratio sensor port. Before and after the converter was replaced I can see exhaust back pressure of over 6 psi unloaded at between 2500 and 3000 RPM. But it is basically no pressure when observed through the 02 sensor hole. 

Can anyone shed some light on why this may be? I would really appreciate it as it has been a test that has served me well so many times (normally after the repair I will see less than 1 psi) Thanks again!

**I should add the reason I was checking exhaust back pressure was because MAF data was a little low and I was expecting to find a back pressure problem--I also cant help but notice that Subaru necks down the exhaust manifold after the collector right BEFORE the upstream 02 sensor…maybe the cause?

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Interesting
Darren Technician
Wisconsin
Darren
 

Is this a turbo?

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Nicholas Diagnostician
Virginia
Nicholas
 

No

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Eric Owner/Technician
Wisconsin
Eric
 

It is always a good idea to have a sync signal when capturing wave forms. I can't tell much about your captures when there is only one wave form, I can't tell when the different pulses in your exhaust capture are happening. When you captured the in cylinder trace was it all the same cylinder? It looks more like a valve problem than excessive back pressure. I would use a pressure sensor in the…

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Agree
Nicholas Diagnostician
Virginia
Nicholas
 

The capture labeled high exhaust back pressure and the capture labeled new OEM converter were both tested through cylinder number 2, the other capture was tested through cylinder number 1--- what I'm largely concerned with here is the discrepancy in the testing method/why when looking at backpressure in cylinder on both banks do I see over 6 psi, but when looking through the A/F ratio sensor…

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Eric Owner/Technician
Wisconsin
Eric
 

The sync is important, in this case when measuring in cylinder it would be nice to have at least a crankshaft sensor signal and maybe even a camshaft sensor signal. When I look at your “new oem converter” wave form at higher rpm I measure the exhaust valve opening at 180° or BDC of the power stroke. I measured the engine speed at 2696 rpm. When I look at your idle at the end of the same…

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John Educator
Ontario
John
 

Looks like it might be a worn exhaust cam lobe?? Exhaust valve opening late and closing early? With a stroke overlay you can see the valve is opening very late, right at BDC. It should be around 40 degrees BBDC

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Stephen Technician
Tennessee
Stephen
 

I see what you’re saying but, I don’t trust anything that I see in that capture. Because in a running engine, there is only 45psi compression yet there is 22”hg intake vacuum. On the exhaust stroke there is 6psi “backpressure” by mid stroke going up. That immediately goes to 10” vacuum before the piston reaches TDC. There is no exhaust plateau. How does a cylinder go into vacuum in the middle of…

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Eric Owner/Technician
Wisconsin
Eric
 

That is why a fixed sync signal is so important. I saw a case study that showed the compression peak doesn't have to be TDC and that relying on it to be TDC can make you chase valve timing issues that don't exist.

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Stephen Technician
Tennessee
Stephen
 

I agree. Having a sync signal takes the guess work out of it. I considered that the peaks were not TDC but the IVC looked normal enough that I didn’t think that false TDC was the reason for the exhaust anomally.

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