Welcome back to another edition of “The data doesn’t lie,” a regular feature in which I pose a puzzling case study, followed by the answers to the previous issue’s puzzle.
Driveability symptoms have varying root causes. But if you have committed to engine disassembly, you had better know what you are looking for.
Most of us have been there before, facing a vehicle with an intermittent misfire concern. We run the gauntlet of tests, but the results of the tests collectively don’t prove what’s wrong, only what seems to be functioning right. Everything from spark plugs to ignition coil swaps and injector testing. Everything seems just fine.
It’s situations like this that infer an engine cylinder mechanical fault...but intermittent? As unlikely as we’d like to believe this could be, with today’s engine technologies present on vehicles worldwide, these type faults are becoming more common each day.
Today's Subject Vehicle
A 2013 Jeep Wrangler with a 3.6L DOHC V6 Pentastar entered the shop with this very complaint and the approach to the fault played out very similarly to what was just described above. These circumstances allude to a potential intermittent engine mechanical fault.
Testing was carried out, first with a scan for DTCs showing only a P0303 “Cylinder #3 misfire” stored in history. A Relative compression test seemed logical and was carried out as a swift means to determine cylinder integrity. The results also verified that the correct cylinder was identified as misfiring by the PCM (Figure 1).
Headed Down the Correct Path
The results of the relative compression test justified further testing for an engine mechanical/single cylinder fault. Clearly, the cylinder lost compression, but where did the cylinder charge go?
A logical test to follow would be a cranking intake-vacuum test. This test will not only show if the cylinder is leaking past the intake valve, but it will also show if the cylinder is not leaking past the intake valve. The latter result would’ve justified repeating the test at a different location for another arrow I the target (like the crankcase, tailpipe, or even the cooling system).
Shown is the cylinder loss correlating with the degraded intake manifold contribution (Figure 2).
This easy-to-perform test proves that a valve is leaking. However, considering the variable valve lift technologies present on this Pentastar engine, the results shown simply tell us only that. It could be a failing lifter, a hydraulic control fault and even a cam lobe issue.
Survey Says...
The in-cylinder compression test allows us to determine which of the above possibilities is most likely. After all, it's much less expensive to replace lifers than it is to have the cylinder head reconditioned. The test was conducted, and the variation in compression between cycles can be caused by variations in engine speed, however, this was easily proven out with two vertical measuring cursors. The time between cycles was identical.
The cranking intake waveform above was then analyzed more closely, and several aspects were evaluated. The intake peak that correlates with the #3 compression loss is of lower amplitude than the others (Yellow dot). And the timing of the transfer point (When the intake peak changes direction, indicated by the colored dots) is occurring sooner for cylinder #3 than the others (Figure 3).
The Data Doesn't Lie
With all the information in front of us and the desired information not yet obtained, we are faced with deciding how to proceed. Here are some bullet points of what we know to be factual, and I will ask all of you, diligent readers, for your input on what they mean to you, collectively:
- Variation in compression from cylinder #3
- #3 intake manifold peak lower than the others
- Timing of #3 transfer point is earlier than the others
- Fault is intermittent
Given this information, what would you do next?
- Adjust the valve clearances.
- Replace the lifters only.
- Replace/recondition cylinder #3 exhaust valve.
- Replace/recondition cylinder #3 intake valve.
