Bad spark plug or coil? - Page 2 - Ford F150 Forum

09-14-2014, 11:21 PM

#11

sigmanf150

Senior Member

Join Date: Mar 2014

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Quote:

Originally Posted by lewissa81

A word of caution when bench testing coils. While you may find the primary and secondary resistances to be acceptable, there may be deformations within the windings that only show when the coil is firing for a while. These can't be found with an average multimeter. While a bench test can tell you if a coil is definitely bad, it may not tell you if it is good.

100% spot on. I've been having the bucking and missing for a while under load and would seem to go away on its own. I just towed my camper to a campsite this weekend. When I got there, the CEL finally came on for cylinder 3, and it was missing even idling. Took the COP out and did the primary and secondary test and it passed. Noted the corrosion on the spring connector. Cleaned it off with what I had, filled it up with dielectric grease, put it back on and it helped, however the missing was still there. Drove 60 miles to a parts store, paid $73 for the Motorcraft DG511, swapped it out and now all is well! I don't buy cheap aftermarket parts for critical components such as plugs, coils and fuel pumps.

Since I have 295k on the other 7 COPs, I'm looking at close to $400 to replace all plugs and COPs from Rockauto.com. Yikes, but I don't want to be stranded. Luckily I had my tools with me.

So in conclusion, a volt meter is an extremely useful tool, but it can't find all faults.

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