Engine mechanical issues are becoming more common in our repair bays today, and they can be difficult to diagnose. Engines are built with more valves, camshafts and complex variable valve timing systems. All of these components fail, and there are very easy tests that a technician can do with a DSO to confirm the issue. Guessing is not an option for these failures, if you are wrong it could mean a lot of expensive parts and many hours of lost labor.
A 1998 Ford Expedition 4.6L came to me with an expired license plate, it needed to pass a state emission test to be renewed. The MIL was illuminated at one point, the customer said the vehicle had lean codes. The vehicle sat for a while, so when I got the truck, it had no codes and none of the monitors were complete. I noticed that the engine had a pretty noticeable misfire at idle that got better as I raised the RPM.
Some of the early Fords are notorious for not setting misfire codes. This vehicle was no exception, it had no pending codes and the owner has never retrieved misfire codes in the past. I used the Ford IDS Power Balance to quickly identify the misfiring cylinders. This test is now available on many aftermarket scan tools. (Fig. 1)
Cylinders 1, 2, 3 and 4 are all misfiring at idle, and the misfires clear up when the RPM is raised. These cylinders are all on the same bank which typically points to camshaft timing or a clogged catalytic converter on these vehicles. Since the misfires clear up when the engine is loaded, I am leaning more towards cam timing. There are other issues that could cause a single bank to misfire, but cam timing issues are relatively common on Ford modular engines.
So time to tear apart the front of the engine, right? That is a lot of work for a guess, so it is time to do more testing. Using a DSO and a high-amp current probe around the starter cable, I do a relative compression test. The engine compression is not consistent for half of the cylinders. A cam timing issue would cause uneven compression bank-to-bank, further pointing to a timing chain issue. (Fig. 2)
The last nail in the coffin for the timing chains on this engine will come from an in-cylinder analysis with a pressure transducer. If you are unfamiliar with this technique, watch my short YouTube video here:
First we will look at cylinder 1, which is on the suspect bank that is misfiring. Using the tools within the PicoScope software, I am able to break the waveform into 180 degree segments. Using time cursors, I can check events as they relate to the 720 degree four-stroke cycle. Cylinder 1 has an exhaust valve opening (EVO) of 26 degrees before bottom dead center (BBDC). EVO will vary on engines, but this particular engine should be around 45 degrees BBDC. This waveform quickly allows us to confirm the diagnosis without being too intrusive. (Fig. 3)
If you find yourself unfamiliar with an engine doing this test, you can compare bank-to-bank. I used the transducer in cylinder 5, which is not misfiring. Cylinder 5 has an EVO of 45 degrees BBDC. This confirms that we absolutely have a cam timing issue on this engine, time to pull the front cover. (Fig. 4)
We pulled the front cover to find a broken timing chain guide, which caused the timing chain to jump. Unfortunately, pieces of the guide ended up in the oil pan. We replaced the timing chains, tensioners, guides and oil pickup tube.
After getting the engine back together the monitors, all ran and the vehicle passed the emissions test. Using some DSO techniques allowed me to quickly and accurately diagnose the vehicle. Instead of tearing the engine down on a guess, I was confident of the diagnosis going in. This vehicle was diagnosed in about 45 minutes, once you practice these tests the process is quick.