2002 Expy:
How can one tell if a COP is bad? Is there a way to test them. I replaced #4 in December and it fixed my misfire, then problem reappeared shortly afterward. Removed, cleaned and regreased the insulator and problem went away only to reappear yesterday. The problem seems to occur after it rains and with cold weather. I pulled #2, #3, and #1 just to look for evidence of water. Also, how much dielectric grease needs to be used on the the wire and silicone insulator?

 

https://www.ford-trucks.com/forums/9...-problems.html

 

Thanks for the link. I tested the COPs as per instructions. They showed the correct resistance on both primary and secondary. I moved the suspect coil and the miss appears to follow the coil. I'll purchase another COP tomorrow. I did find evidence of water on cylinder 3. Reassembled suspect COP on cylinder 1 and the misfire moved with the COP. Pulled all the plugs just to have a look. They appear to be fine since they were changed about 4000 miles ago. Given that the COP's resistances are fine, what can go wrong with the COP?

 

Could be the rubber insulator under the COP is allowing the spark to jump to ground without firing the plug.

 

I bought a new COP and installed it. The misfire is now gone. Even though the old COP had the proper resistances across both primary and secondary it was still bad. Previously, I had swapped boots and still a misfire. I'm still wondering about the failure modes of COPs.

 

Resistance testing a coil puts a minuscle amount of current through the coil windings. If the coil has a definite open or a short you will be able to detect it with a resistance test.

But for a marginal or intermitiant coil issue, you really need to watch while under load, the current flow through the coil primary. For this you need an amp clamp and a digital storage oscilloscope to capture the current wave form.

For example, suppose there is a clean break in the coil's internal winding. The resistance test would show no continuity through the primary side of the coil and you could declare the coil as bad. Now suppose there was a weak spot in the coil's primary winding. It may be able to handle the micro amps of current the ohm meter sends through the coil's winding and the coil would appear to be good. But if you put the normal amount of current, maybe five to ten amps through the winding, the weak spot would break down and you would see the fluctuation in the currents wave form.

So to put it another way - the resistance test can show a coil is bad, but it can't prove that it is good. <!-- / message --><!-- sig -->
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