2007 Yukon Misfire

vjsmedlo

I have a 2007 Yukon XL with the 5.3L Flex Fuel engine with AFM. It's 10 years old this May, and has 205,000 miles. This engine has always been an oil burner, and I've solved ignition problems before by changing fouled plugs (usually #6). My occasional oil-burning and -leaking problems seemed to have gone away since I switched back to conventional oil from synthetic oil. Until now.

This time is different. The problem started abruptly at highway speed and three hours into a 4-hour drive. The Check Engine light came on. The DIC now complains about traction control. And the scan tool shows a #4 misfire code.

I've think I'm only getting an occasional spark on #4. In my troubleshooting, I discovered and replaced a fouled #4 plug that was very wet with fuel. I've replaced the plug wires, the #4 ignition coil pack, and even the right-bank wiring harness that feeds the coil packs. I also inspected/replaced all four O2 sensors (the right, pre-converter one was sooty). I've concluded that the injector and valves seem to be operating, since I'm getting smoky exhaust and the smell of incomplete fuel burn. I've put an (antique) inductive timing light on the spark plug wires, and can see that #4, while it does produce an occasional strobe it flashes a helluva lot less than the other wires.

Has anybody else seen this problem? Does anybody have any suggestions on how I can further troubleshoot what I think is an ignition problem? Could the computer malfunction in such a way that it stops sending ignition pulses to only one cylinder? Could my symptoms be caused by an AFM malfunction, a faulty valve seal or spring? A rounded out cam? A compression problem like a broken ring or cracked head?

thecardoc3

One of the tests we would run uses a pressure transducer and a digital oscilloscope. This lets us test the compression on a given cylinder over multiple events. If there is a problem such as a worn roller on one of the lifters, you will see variations in the cylinder peak compression as the affected valve does everything from hang open to close erratically. Laptop based scopes such as PICO scope allow us to capture a lot of compression events and then zoom in onto any that show a variance.

As far as the random ignition events go, if the cylinder loses compression it will take very little energy to fire the spark plug. Depending on how your timing light works that low spark demand "might" be too low for the light to sense the ignition pulse, so you don't really know if it is occurring or not. Again the PICO scope would allow for the command to the coil, as well as the coil's current draw to be measured and confirmed. http://www.techshopmag.com/analyzing-cylinder-pressure-waveform-running-engine-part-1/

thecardoc3

Here is a tutorial on testing which demonstrates an engine where the compression is different on subsequent events. http://www.autoinform.co.uk/caution-v-common-sense/