It is unfortunate that today misfire analysis can be a
hit-or-miss concept on many of today’s engine families. If one views several
misfire failures, and documents the behavior of fuel trims, it is reasonable to
state that ignition misfires do not have a significant impact on them, whereas
fuel-related issues create a significant impact.
There are many traditional techniques for determining the
cause of misfire activity in a cylinder. A few traditional tools that come to
mind are the compression gauge, cylinder leakage tester and vacuum gauge. It is
also interesting to cite a few non-traditional tools for reference as well. An
in-cylinder transducer can be used today to look at cylindrical pressure and
vacuum in a cylinder to evaluate engine mechanical integrity. A transducer can
also be placed in the exhaust to look at exhaust pulse activity as a means to
evaluate engine mechanical integrity. I was also amazed to find that there
existed computer software to evaluate misfire activity on an engine.
These non-traditional tools represent the move toward more
efficient ways of obtaining an accurate diagnosis in less time. The modern-day
engine has components that are very difficult to access in reference to
analysis and repair. The traditional testing takes more time to gather the data
needed to make a logical decision.
This article represents the third in my series of
investigations on the top emission-related codes as indicated by many states.
Our goal in this report is to discuss the characteristics of the P0302 code as
it applies to many manufacturer products today.
A suggested strategy for misfire analysis is to first
perform engine mechanical tests. This will be defined by three sub-tests:
mainly relative compression, cranking vacuum and running compression
(cylindrical pressure). This was a great experience to witness the results
taken from two subject vehicles as a comparison. The traditional compression
gauge was compared against the modern-day in-cylinder transducer. The subject
vehicles -- 2008 Chevrolet Uplanders -- were located at a local fleet, and we
started with the known-good vehicle first to set up our baseline for analysis.
A relative compression test was performed first; the scope
pattern showed consistent activity. See Figure 1. There was no abnormal
activity noted. The second test was a cranking vacuum test; the scope pattern
showed good consistent activity. The final test performed was a comparison
between the compression gauge and in-cylinder transducer; the static
compression reading per the gauge was 200 psi. The running compression reading
per the gauge was 90 psi. See Photos 1 and 2. The same tests were run with the
transducer and similar readings were obtained as noted per the gauge. The
running compression pattern was analyzed for cam timing issues; the pattern
showed no abnormalities. We have now obtained a baseline to work with in terms
of test numbers.
Relative Compression Test -- Known-Good Vehicle
• We are synched on cylinder #4.
• We are looking for good consistent activity per the humps.
• A good peak-to-valley measurement should also be noted: 30
amps minimum.
We are now ready to test our second vehicle, which is our
suspect vehicle. A code P0302 was stored in its memory, indicating possible
misfire activity on cylinder #2. The first test performed was a relative
compression test. The test results did indeed show that there was a problem
within the #2 cylinder (see Figure 2); it is clear from the test results that
this is a mechanical issue.
Figure 2: The test results did indeed show that there was a problem within the #2 cylinder
A cranking vacuum test is now performed and it also shows a
slight issue in the scope pattern. See Figure 3.
Figure 3: A cranking vacuum test shows a slight issue in the scope pattern.
A final test is performed
using the compression gauge and in-cylinder transducer. The compression gauge
shows a cranking compression of 200 psi and a running compression of 90 psi.
See Photos 3 and 4. Does this sound familiar?
The transducer is now placed in the cylinder and a strange
cranking pattern is noted. The running compression pattern is also recorded and
it appears to be normal.
Final Analysis
The cranking pattern is an example of a cylinder with an
upper end sealing issue. The most likely cause is the valve(s) are not seating
properly. See Photo 5. The compression gauge shows that the cylinder
compression cranking and running is good.
Photo 5: The exhaust valve was not seating properly.
It is interesting to note that the compression gauge showed
the same values as the known-good vehicle. The transducer showed that we had a
true mechanical issue with our suspect vehicle. The cylinder head was
disassembled from the engine and a close visual inspection was made. The
inspection revealed that we had a burnt exhaust valve (Photo 6).
Photo 6: A burnt exhaust valve was our guilty party.
Why did the compression gauge show that we had good
compression on both vehicles? I think this can open up lines for discussion.
Please feel free to e-mail me your thoughts. This Pulling Codes case is now
closed.
About the Author
Carlton Banks
carlton196100@yahoo.com
Technical Contributor for TechShop, Carlton is an emissions trainer for three Midwest states. He's made it a mission to find the truth in emissions-related codes.
More Articles in Emissions