Testing coil pack question 99XLT

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- Mikepier
  
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posted
16 years ago

Mon, Jul 9, 2007 1:32 PM

I recently did a tune up myself on my 99 XLT SOHC 63K miles. I replaced the plugs, wires, PCV valve, and fuel filter. I still notice an occasional miss at idle, other than that the truck runs fine. The coil pack is original so I decided to take it out and test it. The primary resistances checked fine. The secondary resistances (between shared coil towers) was about 13.2K ohms on all coil towers. This falls out of spec of the 6.5K-11.5K ohm range, but I find it odd that all 3 coils had the same reading, which is leading me to believe that maybe the coil is good afterall. I was wondering does anyone know if this reading is normal or should I replace the coil pack? This is not exactly a cheap part so I want to make sure if it needs replacing.

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- C. E. White
  
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posted
16 years ago

Mon, Jul 9, 2007 2:10 PM

Measuring the "low voltage" resistance of the coil pack is not necessarily telling you the whole story. Certainly if the coil pack read open (infinite ohms) or showed very low resistance, you could be sure it was bad. However, you don't know whether or not the insulation has broken down to the point that internal arching occurs under actually operating conditions. I've seen the Ford procedure for checking a coil pack, and to be honest, it would be cheaper to just replace it than to pay a Ford mechanic for all the time involved in properly checking one.

I am not sure what you mean by an "occasional miss" at idle. The PCM is very good at detecting miss fires and if the engine is missing it will set a code and turn on the MIL. If the truck is running OK, don't look for trouble.

Ed

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posted
16 years ago

Mon, Jul 9, 2007 3:24 PM

If I stand by the exhaust pipe, I hear what I perceive as a miss, anotherwords occasionally I would hear a "puff". But I guess if there was a real problem like you said, the MIL light would come on. Also I did a search and it seems that someone had apparantly purchased a new coil pack from the dealer and it had the same reading, so I guess mine could be good.