Here we have our vehicle with 108,904 miles on it. It has the 6.8 liter V-10 motor between the fenders. The complaint is that it originally came in with a MIL lamp on and misfire codes, three cylinder specific misfire codes. The shop replaced those specific coil on plug units and figured they had it fixed. A quick road test revealed it still had a misfire. Not as severe as before, but still an issue. With no misfire codes set the shop wanted to pinpoint the misfire. That is where I come in.....
There she is a big V-10. That is ten Coil On Plug Units to go wrong, ten spark plugs to go wrong, ten cylinders to have mechanical issues with. Get the idea. Well first thing I do is take it for a road test and make sure I put it through its paces. Idle, cruise, hard acceleration. This test drive serves two purposes. One it allows me to get a feel for the misfire and two I drive the car to make sure that the learned correction for misfire detection has been satisfied. On most Fords this is hard acceleration a couple of times making sure you get it up to 55mph. If you dont do this you may get false misfire cylinder results. After the road test I am pretty confident from feel that this is an ignition misfire. I check Mode 6 and sure enough cylinder #2 is showing some value where the other cylinders have nothing. I check fuel trims just to make sure my hunch about ignition is correct. An ignition misfire will not affect fuel trims as much as a fuel related item such as an injector. Sure enough my fuel trims are pretty tight. Out comes the low amp probe.
Here I have my low amp probe around the #2 COP (Coil On Plug) unit power feed. Low amp probes are an excellent tool. They are unintrusive, fast, and concise. They can tell whether you have a shorted coil, poor connection, weak coil, etc. There is a learning curve here. you have to look at a lot of known good patterns before you can tell a bad one. I use the low amp probe frequently on COP units, fuel injectors, and fuel pumps. The uses for a low amp probe are endless.
Well there is the waveform from #2 COP unit. There are three distinct patterns. This Ford vehicle utilizes what is called Multi Strike technology at an idle and low speeds. It was an idea that was supposed to create a more stable and smoother idle and help out with emissions. When the vehicle is raced up passed approximately 1500 rpms we only have the one pattern. Lets dissect the pattern.
This first ramp is the initial coil on plug activation. What we are looking for is a nice sloping consistent amp charge. It shouldn't go straight up. A straight up pattern is an indication of a shorted coil on plug unit. Things look good here so far.
The next two patterns are the multi strike patterns. These should have upward ramps. Reason being the coils resistance changes after the initial charge. This sloping pattern on the multi strike indicates an issue typically with the coil on plug unit. Here again this is where experience pays off. Scope patterns are usually a matter of interpretation. You have to see plenty of good patterns before you can differentiate bad. Lets put some values on the scope capture.
Here I added some cursors with amperage values. I normally see around 6.5 amps on Ford coil on plug units. So, this is a bit high. I have enough to say I probably have a bad coil on plug. Time to yank the coil on plug unit just to give it a visual.
There it is. No arc marks or discolorations. Nothing physically wrong with this coil on plug unit. I could do a primary and secondary resistance test. But, that is a static test where my low amp probe test is dynamic. One test that I didn't show was checking spark output with a registered 30kv open gap style tester. It failed miserably with inconsistent blueish spark. It should be a nice even white spark jumping across the tester. So lets get a COP unit in and retest.
Looks a lot better....Notice the vertical ramping on the multi strikes. What about my amperage. Lets add some cursors.
There we go. We have the normal 6.5 amps back again. A test drive confirms no more misfires and the performance is back. It feels like a V-10 again.